DEVBIOL: Developmental Biology

Lecs 1-4: Intro, Gametogenesis, Fertilization, & Cleavage to Blastulation

Dr. Gliceria Ramos
Term 2 AY 2022-2023
Transcribed: Berana, Capistrano, Dalapo, Delos Angeles, Juachon, Narbonita, Paclibar, Sartorio, Silao, Soliman

now more focused on

processes
Outline
● Embryo: early stage when the
1. Introduction developing animal does not yet
1.1. Concepts in resemble the adult of the species
Developmental Biology
1.2. History and Persons DEVELOPMENTAL BIOLOGY
Involved ● Embryology evolved into
1.3. Approaches in the Study
developmental biology, the analysis
of Embryology
2. Gametogenesis of biological development
2.1. Phases of ● The study of how a (simple) single cell
Gametogenesis becomes a (complex) multicellular
3. Fertilization embryonic stage
3.1. Major Events in ● Study how (simple) individual cells
Fertilization
differentiate into specialized cells (in
4. Cleavage to Blastulation
4.1. Functions/ structure and function)
Accomplishments of
Cleavage Division
4.2. Factors influencing
Cleavage Pattern

LECTURE 1: INTRODUCTION
TO DEVELOPMENTAL BIOLOGY

DEVELOPMENTAL BIOLOGY
AND EMBRYOLOGY
● Developmental Biology: came from
embryology SIGNIFICANCE OF
DEVELOPMENTAL BIOLOGY
● Embryology: study of embryos
● To understand the normal and
○ traditional definition:
abnormal development
(descriptive) study of
structural changes in
embryonic development
(from a zygote to 2, 16, 32,
etc. cell stages to a body
form)
● To understand better the mechanisms
of such development

The points above serve as foundations for

○ contemporary definition: the ff.:
study of developmental ● Development of new techniques for
processes of integrated prenatal diagnoses and treatments
complex phenomena; it’s ● Therapeutic procedures to circumvent
problem of infertility
● Interventions to prevent birth defects ● Gametogenesis: generation of
and to address abnormalities specialized cells involved in
● Get involved with stem cell research! fertilization – gametes
● Oogenesis: generation and
All the foundations above then leads to: maturation of oocytes
● Improvement of prenatal development ● Spermatogenesis: generation and
● Improved long-term postnatal effects maturation of spermatozoa

DEVELOPMENTAL BIOLOGY SCOPE MAJOR ACCOMPLISHMENTS OF

ONTOGENETIC DEVELOPMENT
● Generation of cell number
(growth)
● Cellular diversity within generation
(differentiation)
○ From embryonic stem cells
(early cleavage cells) giving
rise to different cell types in
the body
● Cellular order within generation
● Usually, embryology and (morphogenesis)
developmental biology starts with ○ Giving rise to the general
fertilization and ends with birth body form of an organism
(mammals), hatching (avian and specific to the species it
reptilians), or metamorphosis belongs to
(amphibians). The ends are ● Continuity in life
convenient landmarks in the
continuing process of development MODES OF ONTOGENIC
● Development: series of uninterrupted DEVELOPMENT
correlated events MOSAIC DEVELOPMENT
○ Types: ● Where the fate of a cell depends
○ Ontogenic - development upon the specific cytoplasmic
of a new individual determinants in the zygote galned during
■ from a fertilized ● Cytoplasmic determinants are cell division
oocyte in the sexual unequally or asymmetrically
reproduction distributed or apportioned to
■ from the budding off daughter cells during cell division
from a parent ● Example in Molluscs (snails,
organism in asexual gastropods, and bivalvia)
reproduction
○ Phylogenetic: evolutionary
development of a species
■ development from a
simple life form to
more complex forms
of life
● Usually, in embryology and
developmental biology, we start with
gametogenesis
 MOSAIC DEVELOPMENT: ● If a part of the embryo is removed,
HOW IT WORKS certain cell types would be lacking
in later stage of development
● If a blastomere is isolated, it
cannot develop

REGULATIVE DEVELOPMENT
● Where the fate of a cell depends
upon interactions with neighbor
cells, not by what piece of
cytoplasm it has acquired during
cell division
● Cell to cell interactions - involve
● In vertebrates like molluscs, they signaling factors which have an
use mosaic mode of development influence on the development of a
● Orange cells are the embryo. Cells cell, its developmental pathway
1, 2, 3, 4 vary in the cytoplasmic ● The fate of the cells are not limited
determinants they gained during early. Cells can give rise to any cell
cell division type to the body, they have
● Cell 1 was able to acquire totipotential or unlimited potential
cytoplasmic determinants that will ● Characteristic feature of
guide its development into a vertebrates, but in combination
muscular foot with mosaic development
● Whereas, Cell 3 was able to REGULATIVE DEVELOPMENT:
HOW IT WORKS
acquire cytoplasmic determinants
that will guide its development
pathway into a visceral mass
● If all were intact (Cells 1-4), this will
result in the formation of a normal
larva
● But, if one of these cells were
excised early on, so three were
left. This will not grow into a normal ● In this, one of the blastomeres was
larva, but into a defective larvae. excised. The excised blastomere
● It might lack the muscular foot became a normal larva, just the
because the cell for it (Cell 1) is same with the three remaining.
excised. ● Removal of one is compensated by
● Each cell was marked to follow a the remaining cells so it can still
certain development pathway. form complete normal larvae or
Cells have a limited development embryo
potential. ● When a blastomere is isolated
early in cleavage, it can form a
● Remember, mosaic development new complete individual
is where the fate of a cell depends
primarily on cytoplasmic
determinants that are unequally
distributed to the cleavage cells!
 COMPARING AND CONTRASTING neurulation is the laying
MOSAIC DEVELOPMENT AND down of the body axis or
REGULATIVE DEVELOPMENT pattern formation
● Morphogenesis - body formation
in creating the general body plan of
the embryo characteristic by the
species it belongs to
● Cell differentiation - pattern
formation and morphogenesis
● During mosaic development, happens because of this
cleavages produce cells that are ○ By this time, the embryonic
limited in their developmental cells are already in their
potential. This can be destined locations in the
accomplished by asymmetrically embryo
distributing molecules that regulate ● Growth - the result of cell
development. Daughter cells then differentiation, pattern formation,
inherit different sets of these and morphogenesis
determinants during cell division. germ layers
○ Fate of daughter cells are SCOPE OF EMBRYOLOGY X
micromeres
limited macromeres
○ Relies on cytoplasmic
determinants that are
unequally distributed
among embryonic cells
● During regulative development,
cleavages produce daughter cells
with equivalent and relatively ● Gametogenesis
unlimited cell fates. Interactions ○ Spermatogenesis and
between cells regulate oogenesis
development. ● Fertilization
○ Fate of daughter cells are ○ Steps and prevention of
unlimited polyspermy
○ Relies on cell to cell ● Cleavage
interactions, involving cell ○ Patterns and influences
to cell signals and its fuel for onset
● Blastulation bodyaxes or
KEY PROCESSES OF DEVELOPMENT ● Gastrulation ~patternformation
● Cleavage division - precedes
● Neurulation/Organogenesis -not yet
gastrula and neurula functional
○ Precursor cells of an organ
● Pattern formation (body axes
● Histogenesis (tissue formation)
formation, starts actually
and Differentiation 2
specific functions
gastrula between gastrula and neurula)
● Pattern Formation and specialization
stage
↓
○ Anterior-posterior axis
pattern formation Morphogenesis due to cell differentiation
Ibody formation) ○ Dorsal-ventral axis
● Fetal Growth
axes

○ Left-right axis
↓
neurula
stage
○ The transition from
gastrulation to
 FUNDAMENTAL QUESTIONS structure of the organism
ADDRESSED IN DEVELOPMENTAL and its component parts.
BIOLOGY PROBLEM OF GROWTH (CELL
● How does the fertilized egg gives DIVISION)
rise to an adult? ● How are cell division and growth
● How does that adult provides yet tightly regulated?
another body? ● What dictates an embryonic
THE QUESTIONS SUBDIVIDED INTO cell…?
GENERAL PROBLEMS OF PROBLEM OF REPRODUCTION
DEVELOPMENTAL BIOLOGY: ● How are reproductive cells set
● Problem of differentiation
apart during embryonic
● Problem of morphogenesis
development?
● Problem of growth
● Only the germ cells pass chicken
● Problem of reproduction
characteristics on to the offspring before
● Problem of evolution
○ “Mother hein is only the egg
● Problem of environmental
egg’s way of making
integration
another egg”... Samuel
Butler
PROBLEM OF DIFFERENTIATION:
PROBLEM OF EVOLUTION
● How does the same genetic
● How do changes in development
information result in different cell
create new body forms? And what
types?
changes are possible?
● How can the fertilized egg
● Why is the distinction between
generate different cell types?
analogous and homologous
* structures important?
THE QUESTION OF ENVIRONMENTAL
INTEGRATION
● How is the organism’s phenotype
influenced by the environment?
● The differences in the color of
butterfly wings is related to the
● Zygote becomes blastula which exposure, during their caterpillar
has now micromeres and stage, to varying temperature and
macromeres. Blastula becomes day length. More intense the
gastrula, where germ layers temperature and exposure to day
(ectoderm, mesoderm, endoderm) time, the more vivid colors they
exist. become.
PROBLEM OF MORPHOGENESIS:
● How do cells form ordered IMPORTANT BASIC CONCEPTS IN
structures? EMBRYOLOGY/DEVELOPMENTAL
● How are cells positioned in the BIOLOGY
right place? At the right time? ● The ff. was not explained. Just
● How form and pattern emerge from enumerated (for now):
the simple beginnings of a fertilized ● 1 Concept of guidelines
egg? ● Concept of 2 fate, 3 potency,
4 determination
○ Morphogenesis be

investigates how this ● Concept of capacity and

regulation of cell fates competence
contributes to the form and
 5

● Concept of Embryonic
induction 7
● Concept of 8Regulation
● Concept of Inevitability
10

● Concept9 of Differentiation
● The Hox (homeobox)
genes
I

CONCEPT OF GUIDELINES ● Above are line drawings of

● Guidelines - directive influences
amphibian/fish oocytes
on embryonic development
● Maternal factors are the maternal
-come-off as:
mRNA. But these are silenced or
○ Preformed Guidelines
not being expressed as proteins,
■ present right at the
waiting for their activation.
start of ontogeny,
● Oocytes (amphibian and fish
even before
oocytes) have distinct poles:
fertilization
animal and vegetal
■ refers to maternal
○ Vegetal pole is the one with
factors as a result of
more yolk
activation of
● The maternal factors are
maternal
color-coded (like VegT mRNA is
genes/maternal
purple)
effect genes
● The red boxes are the expression
○ Progressively-formed
of proteins at the and signaling
Guidelines
factors
■ appear gradually in
● The unequal distribution of vegetal
every step of
pole having more maternal factors
ontogeny
and the animal pole having more
■ these pertains to
proteins – these are preformed
cellular products as
guidelines
a result of zygotic
genes
PERFORMED GUIDELINES:
MATERNAL EFFECT GENES/FACTORS
● Zygotic Genes - fusion of IN AMPHIBIAN AND FISH OOCYTE
maternal and paternal genes that
resulted from the nuclear fusion
during fertilization

PREFORMED GUIDELINES
● Maternal genes/maternal effect
genes
● Oocyte cytoarchitecture

● Balbiani body (at the vegetal

pole) c reformedat
I
the 00cyte
 ○ accumulation of ● Egg Cytoarchitecture - study of
mitochondria and the cellular structure of the egg,
cytoplasmic granules (germ like its yolk distribution
granules) containing ● Vegetal pole - end with the highest
silenced mRNAs concentration of yolk
○ Mitochondrial cloud + ○ All animals have different
cytoplasmic granules (germ amounts of yolk though
granules) ○ Yolk - affects the cleavage
● Maternal mRNAs are silent and are pattern of the oocyte
organized in cytoplasmic granules
together with several regulatory PROGRESSIVELY-FORMED
proteins responsible for their GUIDELINES
post-transcriptional processing and
thus translational regulation
● Maternal mRNAs are silent until
the oocyte is activated by the
sperm, in other words it is
activated by fertilization.
● Protein expression occurs
immediately ● Guidelines that appear gradually in
● In the image above, sybu and wnt6 every step of ontogeny
are now translocated from the ● At the cleavage stage, the pattern
vegetal pole closer to the animal is under maternal control. The
pole where it will influence the maternal effect genes and
establishment of the future dorsal cytoarchitecture is more on
side of the embryo preformed guidelines
● In the late cleavage stage,
Remember… transitioning to the blastula stage,
● BB (Balbiani Body) - vehicle for maternal factors become depleted.
transporting and localizing When it is depleted, it switches to
maternal factors to the vegetal activation zygotic genes which
cortex during oogenesis by means contains the paternal genes
of microtubule network and motor ● New proteins are expressed
proteins (yellow arrows) progressively in the gastrula stage
● At egg activation and fertilization, and in the next stages
Sybu and Wnt8 (other maternal ● Guidelines come in the form newly
factors are expressed and) are synthesized proteins and signaling
translocated to the future dorsal factors
axis through microtubule-mediated
transport (blue arrows) PROGRESSIVELY-FORMED
GUIDELINES: ILLUSTRATION
● Anterior-posterior axis is coupled
EGG CYTOARCHITECTURE
to gastrulation
○ Gradually in gastrulation,
anterior-posterior axis is
being laid down
○ This is guided by
developmental potential
 and inducing properties of ○ Where in zygote undergoes
cells in the dorsal lip of the lineage decisions to form
blastopore (DLB) change placenta, yolk sac, or the
with time. fetus
○ Early cells in DLB
becomes anterior
mesoderm which then
becomes neural tissue
○ Latter cells in DLB ● Dependent on:
becomes posterior ○ Cell asymmetries
mesoderm which induces ○ Unequal cytoplasmic
posterior neural determinants
structures towards the ○ Inductive information
lower part of the spinal cord (signaling factors)
Fibroblast growth factor ○ Morphogens (chemical
Bone signators substances in the
morphogenetic developing embryo that
proteins
guide body formation)

-
● Below is an illustration that shows
how cell asymmetries resulting to
blastopore different formation of cell lineages
in the embryo
Interleukin and insulin-like
growth factor
● The image above is in
gastrula stage, cut sagittally
● The opening closed by a
L

yolk plug (where endoderm

is at) is called as the
blastopore

DORSAL BLASTOPORE LIP

● Has inducing property but cells ● Cell with a black nucleus is a polar
must have the ability to respond body
○ It synthesizes signaling ● Cells are color coded. Red
factors that expressed at expresses Oct4 gene at a low
different areas and at times level, orange expresses Oct4 at
which denotes high level
progressively-forme ○ Oct4 - gene required for
guidelines maturation of inner cell
○ Wnt signal activity: high in mass (ICM)
posterior, low in anterior ○ Cdx2 - required for the
2 maturation of
FATE trophoectoderm (TE)
● “What cells would become”
● The range of cell types that a ● Four cells become eight cells then
particular embryonic cell can give 16 cells then the blastula stage. In
rise to mammalian development, blastula
stage is called as the blastocyst
 early
cleavage
● In here, you can see two distinct ● Totipotent - has total potential to
populations of cells ICM and TE give rise to any cell type
(trophectoderm) ○ Blastomeres are totipotent,
● Red, which expresses Oct4 at a if one is excised it has the
low level, divides asymmetrically. potential to any cell rise up
While orange divide to a complete embryo
asymmetrically (regulative mode of
● What happens with Red is that it development)
has small and large cells which ● Pluripotent - inner cell mass (ICM)
compose the ICM. Large ones on cells
the outside, small cells inside. ○ ICM cells cannot create
● Cell asymmetries influence the fate yolk sac and amnion and
of the cell! other extraembryonic
3 membranes as it is
POTENCY pluripotent; however, is
● The ability of a cell to follow a capable of self-renewal
developmental pathway ○ Can differentiate into any
● Embryonic stem cells - body tissue, but cannot
unspecialized; can undergo support full development of
unlimited self-renewal (totipotent) the entire organism/embryo
● Multipotent - cells that can
differentiate into cell types within a
given lineage
○ Hematopoietic stem cells in
the bone marrow can give
rise to blood cell lineages
such as RBCs and WBCs
● Unipotent - fully specialized, can
generate its own specific type
○ Examples:
■ Gut cells’ epithelial
layer, they renew
itself because of
stomach acid
■ Stratum basale,
inner layer of the
epidermis,
undergoes active
mitosis to generate
new skin cells but
cannot be liver cells
4
DETERMINATION
● Determination - gradual
commitment to a certain cell fate;
geared to follow a certain
developmental pathway
 and on the other hand, the
ectoderm has the
competence to respond to
the evocative influence
○ Illustrating primary
induction

● Pathway 1 (and 2), primary germ

layers are generated from inner
cell mass. ICM cells are also
embryonic stem cells as they are
self-renewal ● During embryonic development,
● Pathway 3, primordial germ cells primary induction is not the only
are embryonic stem cells as they type of interaction among cells;
are also self-renewal. These cells primary is followed by secondary
can become embryonic germ cells, type of induction, followed by
so they are pluripotent reciprocal interaction, followed by
5 epithelial mesenchymal interaction
EMBRYONIC INDUCTION – which will be discussed
● Evocative influence of cells, within eventually
a cell or a cell with a neighboring
cell, and between cells
7
6
CONCEPT OF REGULATION
CAPACITY & COMPETENCE ● Regulative vs mosaic development
L
○ Regulative development
ectoderm
= the potential of a cell is
much greater than what is
C

chordamesoderm
indicated in its normal fate;
in other words, has
● One of the developmental unlimited potential (if
landmarks, most distinguishable controlled exclusively by
feature of a gastrula, is the inductive signals)
formation of a dorsal lip of ○ Mosaic development =
blastopore, which will have embryonic cells can
eventually the chordamesoderm develop only according to
(CM), which has very powerful their early fate; their fate is
inducing properties determined very early (if controlled exclusively
by
● If this were to be traced, this 8
cytoperminants(
chordamesoderm (CM) can act on CONCEPT OF INEVITABILITY
the overlying ectoderm to form the ● Phenomena that will take place
precursor cells of the central and the embryo cannot evade from
nervous system, starting out as the this
neural plate stage ● Ex.:
○ This goes to say that the ○ Apoptosis (programmed
chordamesoderm has the cell death)
evocative influence on the ○ Biological clock
ectoderm; or the mesoderm ○ Intracellular clock
has the capacity to induce,
 ○ Presence of normal (Muscles), MG
homeobox genes (midgut), HG
○ Positional information (hindgut) (purple)
○ There are undifferentiated
FUNCTIONS OF PROGRAMMED CELL embryonic cells collectively
DEATH (PCD) DURING DEVELOPMENT termed as imaginal discs
■ Novel structures
raised from
undifferentiated
cells termed
imaginal discs
(various colors)
○ Development fates and
locations are shown in the
adult
● PCD also controls cell number by
deleting cells which fail to partner
○ A mechanism that operates
during embryonic
development of the
peripheral nervous system
(neurons to target cells)
● PCD eliminates dangerous and
● One of the functions is to regulate abnormal cells such as
sculpting of the structures autoreactive lymphocytes
○ Formation of sculpting the PROGRAMMED CELL DEATH (PCD):
ILLUSTRATION
heart, creating the auricles
and ventricles
○ Sculpting the innear ear
with the ossicles and the
labyrinth structures inside
● PCD regulates proper structure
sculpting by eliminating
interdigital-webbings.
○ In adulthood, the ● PNS development:
interdigital-webbings are ○ Neurons are overproduced
already carved ○ Survival depends on
○ The cells in between the competition for limited
digits were eliminated by amounts of
programmed cell death survival-promoting factors
mechanism produced in target tissues
● Drosophila melanogaster (2nd ○ Quantitative matching of
example of the image) neurons with their targets
○ At metamorphosis: ● Apoptosis adjusts the number of
■ Larval structures nerve cells to match the number of
are destroyed: SG target cells that require innervation
(salivary glands) M
BIOLOGICAL CLOCK: THE SOMITE
SEGREGATION CLOCK
● Somites - repeated structures in
embryogenesis
○ Paired muscles on the
dorsal side of the body
○ Ex. multifidus spinae,
coccygocostalis(?),
● Oligodendrocytes - one of the
iliocostalis
group of neuroglial cells that form
○ Generated sequentially
the myelin sheath of nerve cells
■ First pair somite,
outside the CNS
second pair somite,
○ Components of its
third pair somite
cell-intrinsic timer:
○ Generated because of the
■ Signaling factor
expression of a specific
(PDGF-platelet-
group of genes that
derived growth
oscillates in a 2-hr cycle
factor) serves as
(works as the biological
timer component
clock)
and measures
7
oscillates in a elapsed time
2-hr cycle ■ Effector
(TH-thyroid
hormone) stops cell
division and initiates
differentiation at
appropriate time;
also controls spatial
● Notch signaling via a transcription temporal gene
factor Hes7 is the central expression
mechanism for generation of (expression of
oscillatory gene expression genes at the right
○ Notch signaling is a location at the right
signaling involved in time)
fine-tuning formation of ■ p27/Kip1 - a
somites, activating specific cell-cycle inhibitor
group of genes accumulates in the
precursor cells as
CELL-INTRINSIC TIMERS they proliferate
● Intracellular developmental ● It seems likely that similar timing
programs that change precursor mechanisms operate in other cell
cells over time lineages of the embryo
● Ex. those that stop mitotic division
9

HOMEOBOX GENES
● The master developmental control
genes
● Act at the top of genetic
hierarchies regulating aspects of
 morphogenesis and cell ● Homeobox occur in clusters
differentiation animals (clusters of genes; vary in number)
● The homeobox has a DNA ● Conserved from invertebrates to
sequence containing about human
180-nucleotide sequence ● In invertebrates and vertebrates,
(contained like a box, hence the there is a correlation between the
term) position of Hox genes in the cluster
● Encodes DNA-binding proteins (60 and their expression pattern along
amino acid homeodomain) the anterior-posterior axis of the
○ Homeodomain act as body
transcription factors that 10

can control and regulate CONCEPT OF DIFFERENTIATION

the activity of the genes
underneath them
○ Regulate gene expression
○ Control aspects of
morphogenesis and cell
differentiation

Hox genes in normal development

1) Patterning embryonic structures,
with segmentations, such as the
axial skeleton
2) Patterning of the limbs, the genital
and digestive tracts
3) Craniofacial morphogenesis and
the development of the nervous
system – prosencephalon,
mesencephalon, rhombencephalon
might occur from posterior axis

● Homeobox genes are responsible

for specifying cell identity and ● The single cell is a zygote which
positioning during embryonic embarks into a series of mitotic
development – craniocaudal division forming the cleavage or
morphogenesis morula
○ Morula: solid ball of cell
● Since it is an ordinary mitotic division,
the daughter cells are genetically
equal (contains identical copies of
DNA)
● Following the developmental pathway
of the two daughter cells, the left
daughter cell proceed to form neuron,
and the other proceeds to form
epithelial cell
 DIFFERENTIAL GENE EXPRESSION development are DNA
methylation and Histone
acetylation
● Affects the differential expressions of
Pluripotency-associated genes &
Developmental genes
○ Pluripotency-associated
genes are highly expressed
during the pre-implantation
stage
■ Cells are pluripotent
■ Pluripotency genes
must be turned on
and must be
expressed at a
● Differential Gene Expression: They higher rate
vary in the subset of gene they compared with
express developmental
○ The expression of genes
neuron-specific and ○ Developmental genes are
epithelial specific gene highly expressed during
○ For a cell to become a later stage of development
neuron cell, only a specific ● Genes are expressed differently in a
subset (upper portion) of specific timeline
the genome is expressed ○ In Pluripotency-associated
(neuron-specific gene) genes’ timeline, cells are
○ For a cell to differentiate pluripotent
into an epithelial cell, it ■ Pluripotency genes
needs to express the lower must be turned on
subset of the gene and must be
(epithelial specific gene) expressed at a
higher rate
compared with
developmental
genes
○ Since developmental genes
are not needed during the
Pluripotency-associated
genes’ timeline, they are
● Regulated by mechanisms
turned off
○ DNA methylation
○ In developmental genes,
○ Histone modifications: can
since embryonic cells have
be methylation or
their determined fate, the
acetylation
pluripotency genes are
■ H3K27 methylation
turned off
■ H3K4 methylation
● The turning on and off of genes rely on
○ Most common mechanisms
the three mechanisms (DNA
during embryonic
 methylation, H3K27 methylation, and
H3K4 methylation)

EARLY HISTORY
ARISTOTLE

● Harvey contributed to the idea “omne

vivium ex ovo” promoting the
existence of ova
○ “omne vivium ex ovo” = all
life comes from an egg

HAMM & LEUWENHOEK (1677)

● 1st systematic study of embryos

○ He was the first one to do
the systematic study of
embryos
● Recorded different stages in the
development of the chick embryo
● Recognized that there are multiple
ways that organisms reproduce

HISTORICAL BACKGROUND ● Observed human sperm

● Important people in the history of
embryology ANTONI VAN LEEUWENHOEK
Reinierde (1632-1723)
WILLIAM HARVEY & GRAAF (1672)

● Discovered animalcules in semen

● Argued that a tiny preformed human
was already present in these
Cat ovary
animalcules
● 1st to describe the ovarian follicle ● His discovery supposedly led to the
○ “Graafian follicle” prereformanist period
● Coined the Theory of Preformation
 LAZZARO SPALANZANI & CASPAR unfertilized chick embryo to the
FRIEDRICHWOLFF (1733-1794) formation of the tiny entity with
proliferating blood vessels

● Spalanzani: The presence of both

male and female sex products, and TWO EARLY VIEWS HOW ANIMALS
both of these products are necessary DEVELOPED FROM AN EGG
for the initiation of ddevelopment ● Theory of Preformation
○ There is no profound entity ● Theory of Epigenesis
inside the sex cells
● Wolff: Embryological development THEORY OF PREFORMATION
occurs thru progressive growth and ● All parts of the future embryo were
development imagined to be already in the egg
○ The organism (human) but these were transparent, folded,
goes through progressive small and cannot be seen
growth and development ● The embryo is preformed in the
○ Laid the Epigenetic concept egg
● Spalanzani and Wolff coined the
Theory of Epigenesis

LAZZARO SPALLANZANI (1729-1799)

● Successfully performed the first
artificial insemination (using frog
eggs)

THEORY OF EPIGENESIS
CASPAR FRIEDRICH WOLFF ● The egg does not contain a
(1738-1794) preformed embryo but only the
● First person to demonstrate materials of which the embryo is
morphogenesis formed
● Saw the development of structures ○ Stressing the importance of
out of structureless materials sex products
● He made use of chick embryo, ● The embryo is formed by
tracing the formation of structures progressive series of events
out of structureless materials from
 ○ Structureless, gradual
acquisition of morphology
○ Then, formation of the
embryo with the blood
vessels

● Amnion- one of the

extraembryonic membranes,
enclosing, protecting the
developing embryo
● Karl Ernst Von Baer (1828):
"Father of Embryology" laid the
Von Baer's Law
● Von Baer's Law: The more
general features that are common
to all members of a group of
animals developed earlier than the
more special features which
distinguish the various members of
the group.

VON BAER’S LAW

1. Embryos look similar during early
stages; all have gill slits, tail,
somites, notochord which forms
KARL ERNST VON BAER (1792-1876) the scaffolding for the formation of
the vertebral column
2. Uninterrupted series of correlated
events
3. Differences become gradually
more distinct

● Made significant strides in

descriptive embryology searching
for the VITAL FORCE
● the first person to note the many
similarities between the embryos of
vertebrates particularly amniotes
 ERNST HAECKEL (1834-1919) ○ If you hate being a chick,
blame your mother for
being a hen.

● Leading authority in embryology

during the late 1800s
● Laid down the concepts of :
"ONTOGENY RECAPITULATES
PHYLOGENY"
○ Individual development
follows the development of
the ancestor species
● Haeckel with Johannes Muller
re-interpreted Von Baer's Law in
the light of evolutionary theory

LAW OF BIOGENESIS ● There is only every living thing in

● Law of Biogenesis: Ontogeny is a its individual development passes
shortened/ modified recapitulation through series of constructive
of phylogeny stages like those of the
evolutionary development of the
"ONTOGENY RECAPITULATES race to which it belongs.
PHYLOGENY"
● Individual development progresses CHRISTIAN PANDER
through the adult stages of the ● Christian Pander: existence of
organism's ancestors. germ layers
○ Meaning, there is only one ○ Known for identifying the
way to upbuild a given primary germ layers:
organism and that endoderm, ectoderm,
organism will do it in mesoderm
essentially the same way ● Nucleus of chicken egg = nucleus
as his ancestors did. of Pander
○ An individual can do this
more quickly, more
economically than his
ancestors by making use of
something nature has
provided for it.
○ There’s no other way.
 Rathke's
pouch
HEINRICH RATHKE which future parts of a developing
● Affinity between embryos of higher organism are determined
and lower vertebrates (because of the ● 1880 - He began a program which
presence of pharyngeal pouches) experimented on frog eggs to
○ every species forms gill elucidate mitotic cell division
slits at certain development
stages INTEGRATION OF GENETICS INTO
● Difference between a pouch and a EMBRYONIC DEVELOPMENT
cleft (pharyngeal)
○ Pouch - side lined with AUGUST WEISMANN
endoderm; outpocketing of ● The Germplasm theory
the oral/pharyngeal cavity ○ Self-reproducing
○ Cleft - side lined with determinants as guiding
ectoderm force for morphogenesis
○ The offspring does not
inherit its characteristics
from the body (soma) of the
parent but only from the
ferm cells (egg and sperm)
○ The germ cells are not
influenced by the body that
bears them
● Chromosomes

OSCAR & RICHARD HERTWIG

Figure: Representation of organism with ● Oscar - 1st to observe sexual
pharyngeal pouch. Endoderm (green), reproduction
Ectoderm (blue), Mesoderm (lavender) ○ Demonstration of
fertilization (sea urchin)
SCHLEIDEN & SCHWANN (1839) ○ Existence of polar bodies
● Laid the foundation of Modern ○ Advances in the
Embryology and Histology with the understanding of meiosis
advent of the microscope
● Matthias Schleiden - 1838
○ German Botanist,
concluded that all plant
parts are made of cells
● Theodor Schwann - 1839
○ German Physiologist, close
friend of Schleiden, stated
that all animal tissues are
composed of cells
HANS SPEMANN & HILDE MANGOLD
WILHELM ROUX (June 9, 1850 - Sept. ● 1935 Nobel Prize for Medicine
15, 1924) ● Concept of Embryonic Induction
● Founder of experimental embryology ● Organizer effect (dorsal lip of
● He believed that mitotic cell division of blastopore)
the fertilized egg is the mechanism by
MODEL ORGANISMS THAT WERE THE
BASIS OF OUR KNOWLEDGE ON
EMBRYOLOGY
● Wilhelm Roux - Frog & Sea Urchin
● Thomas Hunt Morgan - Fruitfly

● ICSI: intracytoplasmic sperm

injection (1990)
○ The first ICSI baby was
born in 1992.
MILESTONES IN DEVELOPMENTAL ○ This is the technology of
BIOLOGY choice in cases of extreme
infertility.
○ Introducing directly the
sperm into the oocyte
cytoplasm.
○ They make use of a
micromanipulator.

APPROACHES IN THE STUDY OF

EMBRYOLOGY
● Descriptive Embryology
○ Involves detailed study of
structure and arrangement
of minute internal organs.
○ Concerned with
explanations of structural
features
○ Investigates when and how
a process is carried out.
● Comparative Embryology
○ Establishes relationships
between developmental
stages
● Experimental Embryology
ERA OF ART (ASSISTED
REPRODUCTION TECHNOLOGY) ○ Finds out why a process is
carried out at a specific
EDWARD & SLEPTOE (1978) time in a specific manner.
● In-Vitro Fertilization (IVF) ○ Ascertains which activate
● 1st test tube baby - Louise Brown or regulate the
development process
● Chemical Embryology WHERE DOES IT TAKE PLACE?
○ Involves biochemical ● Oogenesis and folliculogenesis:
investigations of the generation and maturation of the
embryo; ushered in oocytes which goes along with the
molecular biology. formation of investments around
● Teratology the oocyte. The increase in the
○ Study of embryonic follicle layers.
malformations ○ It takes place in the ovary.
● Reproductive Embryology
○ Techniques in fertilization,
implantation of embryos
○ Endocrinology of
reproduction and
embryonic development
○ Concepts on conception
and contraception.
● Developmental Biology
○ Broader approach from
embryonic development to ● Spermatogenesis: takes place in
postnatal development the seminiferous tubules of the
■ Normal growth testis.
■ Metamorphosis
■ Regeneration
■ Tissue repair

LECTURE 2: GAMETOGENESIS

SERIES OF CHANGES/PROCESSES
● Gametogenesis consists of a
series of changes/processes that
gets to transform the primordial
germ cells into specialized sex
cells.
● Primordial germ cells → primordial
follicles → spermatogonia →
● Frontispiece of William Harvey’s spermatozoon → oocyte
book on The Generation of
Animals ● Primordial germ cells:
● All living beings come from an egg. specialized generative cells,
(Omne vivum ex ovo) collectively called germplasm.
○ In the case of mammals,
there is no obvious
 readily identifiable Just one ovum from one
germplasm. oogonium.
○ In the case of animals other ● In the case of one
than mammals, spermatogonium that completes
germplasm can be easily the process, there are four sex
distinguished as a cells that are produced, the
cell-dense association of spermatozoa.
mRNAs and mRNA-binding *
proteins such as VASA, gonial
stage
DAZL, and MILI
homologues.
■ This can be stained. primary
sex cell
■ This can be easily stages
identified in other
animals' oocytes.
secondary
sex cell
stages

GENERAL OVERVIEW OF
GAMETOGENESIS
● Gonial Stage: Oogonium and
spermatogonium undergo one
more round of growth and
differentiation and beget primary PHASES OF GAMETOGENESIS
sex cells: primary spermatocyte 1. Generation of germ cells and migration
and primary oocyte. to the gonads
● The primary sex cells stages are 2. Multiplication of germ cells in the
the ones embarked in the first gonads (mitosis)
meiotic division (meiosis I). 3. Reduction in chromosome number by
● Primary sex cell stages that half (meiosis)
complete the first meiotic division 4. Maturation and differentiation
beget the secondary sex cell
stage: secondary spermatocyte 1.GENERATION OF GERM CELLS
and secondary oocyte. These cells AND MIGRATION TO THE GONADS
enter the second phase of meiosis ● Germ cells arise outside the
(Meiosis 2). They get spermatids gonads
for the secondary spermatocyte ● Recognizable at an early stage of
and mature oocyte for the development in some of the
secondary oocyte. vertebrates, like in the case of
● In the case of oogenesis, there are anuran amphibians (frogs)
formations of polar bodies such
that at the end of one round of
oogenesis, there is only one
functional sex cell produced.
 ANURAN AMPHIBIANS EXTRAEMBRYONIC MEMBRANES
● Amnion - extraembyonic
membrane most intimate with the
developing embryo
● Yolk sac - color-coded yellow in
the image below
● Allantois - yellow as well
○ In anuran amphibians, in its
● Chorion - pink in the image below
unfertilized egg, it can
easily be identified in the
PGCs MIGRATION TO THE GONADS
circumscribed region closer
to the vegetal pole
○ If you traced it after
fertilization to the cleavage
stage to the blastula stage,
it is still midway between
the animal pole and vegetal
pole
○ In gastrula stage, it ● Recall that a blastocyst has an
becomes unidentifiable in ICM and the trophoectoderm for
the endoderm layer mammals
● ICM forms the embryo proper.
AMNIOTES: BIRDS, Embryo proper will split into
REPTILES, MAMMALS epiblast and hypoblast
● Primordial germ cells (PGCs) ● Take note of the fore gut (farthest
can be identified in the yolk sac away from the allantois), mid gut
endoderm because of the (closes to the yolk sac’s center),
presence of alkaline phosphatase and hind gut (closest to the
● PGCs are large sized, high content allantois) in the picture above
of alkaline phosphatase ● Allantois is an outpocketing of
○ alkaline phosphatase - the hindgut
marker for PCGs ● In humans, by 3rd week, 24-days
● In the case of humans, PGCs can post fertilization, the PGCs wander
be recognized in the yolk sac in an amoeboid motion
encoderm at 24-day post ○ From the primary ectoderm,
fertilization they go down to the yolk
○ Roughly 3 weeks of sac wall, and exit to the
embryonic development allantois

HANNEL & EDDY (1986)

● In the mouse, PGCs originally
reside in the epiblast of gastrula

GINSBURG (1990)
● Found that the PGCs are localized
in the extra embryonic
mesoderm posterior to the
primitive streak in a 7.25th day old
embryo
● Red dots represent the primordial PAANO ANG MOVEMENT
germ cells (PGCs) NG PCGs? (SUMMARY)
● The image above shows that the ● From the yolk sac endoderm,
PGcs are now extra embryonal in passing through the allantois,
the yolk sac wall, they get moves through the hindgut wall,
separated from the rest of the passing through the dorsal
embryo. They are now outside the mesentery, and to the left and
embryonic body right genital ridges
● From here, PCGs will migrate from ● Mema mnemonic: YAHDG (Yah
the the extra embryonic mesoderm Dog)
of the allantois back into the
embryo proper Rodents (as mammalian model)
● Development of PGCs depend on
signals.
● If the image above is going to be
○ These signals based from
cut, by the blue line below. A
the studies is BMP (bone
cross-section passing through the
morphogenetic protein)
notochord , hindgut, mesentery
signal factor.
suspending the hind gut, and the
● Radical expression of pluripotency
left and right genital ridges
markers.
○ Oct-4+, NANOG, and
SOX2 genes
● Studies strongly suggests:
2

○ BMP (bone morphogenetic

I
protein) signal factor

● Then it will look like this:

● Here comes, the signals. It acts as on the

2
Precursor PGCs, located at the
posterior of the epiblast.
● The precursor PGCs increasing
number.
● And then, the big number they
● Notochord is the blue migrate by amoeboid motion into
● Hind gut is the orange the yolk sak, and at the allantois
● Dorsal mesentery suspending the they get specified.
hindgut is the small bridge-like ● What is meant by specification?
structure that connects hindgut to ○ They get specified, kung
the genital ridges baga magkakaroon na sila
● Genital ridges are the future sex ng genetic markers (tattoo),
organs which means sila lang
 pwede maging germ cells; The Life Cycle of Murine Germ Cells
they are marked.
○ They also start to express
pluripotency gene markers,
such as the Oct-4+,
NANOG, and SOX2
genes, which are among
the pluripotency gene
markers of the primordial
germ cells.
● If na sa Allantois na sila that
means specified na sila. The next ● After fertilization: zygote →
step is to determined a certain cleaving forming a 2-cell embryo
developmental pathway, that is to → giving rise to the blastocyst
become sex cells. (middle row).
○ Blastocyst consists of the
Proposed Migration Pathway trophectoderm (green),
and the inner cell mass
(orange).
● After implantation: inner cell mass/
epiblast (orange) → cavitation and
epithelializes
● At gastrulation: a new germ layer
→ the mesoderm (blue).
○ The primordial germ cells
(PGCs) are specified at
E7.25 (black arrow) at the
base of the mesodermal
allantois.
● Thereafter, the PGCs migrate
● So this is the proposed migration thrpugh the forming hindgut →
pathway. From the primary dorsal mesenterium → genital
ectoderm of the developing ridges.
embryo, to the yolk sac endoderm, ● Upon sex determination: female
to the allantois (passing to the (oogonia) and male
hindgut wall), to the dorsal (spermatogonia) germ cells follow
mesentery, and to the left and right different fates.
genital ridges in the embryo
proper.
● So in the yolk sac endoderm, they
become extraembryonal.
● In the allantois, they become
specified at the base, which means
they start to get their markers,
nakakatattoo na sila. They now
have their genetic markers, so they
are now destined to become the
sex cells.
● The primordial germ cells undergo
migration, from the left and to the
right genital ridges, but not all of
them can reach their destinations
(since some of them nawawala),
and those who went astray can’t
reach their destination, in result
they develop a teratoma.
● It is a bizarre form of tumor, and it
looks like a mixture of different
types of differentiating cells. ● The different timing and duration of
● So this is oropharyngeal teratoma, meiosis in human males and
and they are fatal. females. a.) in males, meiosis are
● They undergo mitotic division. initiated continuously from puberty
on, and each meiosis is completed
within a few weeks. b.) in females,
all meiosis begin prior to birth, but
normally only one oocyte
completes meiosis during each
reproductive cycle between
puberty and menopause.
● In males, mitosis is continuous,
during embryonic development,
after birth, all throughout life;
mitosis is continuous.
● In females, mitosis stops early,
● The 2nd phase, proliferation of
mitosis can only proceed up to a
germ cells, by ordinary mitotic
fifth month of embryonic
division.
development, in a female embryo.
● So from their journey from the yolk
By the time it is at its seventh
sac and into the allantois, to the
month embryonic development, all
hindgut, to dorsal mesentery, and
of the oogonial have already enter
all through their journey the
the first meiotic division, so
primordial germ cells are
oocytes na sila.
undergoing continuous mitotic cell
division.
CHANGES IN HUMAN
● If it is a female embryo, the PGCs GERM CELL NUMBER
will divide to form an oogonia, the
PGCs will divide to form a
spermatogonia in males. These
are the mitotical active sex cells.
● Ordinary mitotic division are
basically the same, they only differ
in patterns.
● Mitotic patterns in the gonads differ
widely between males and
females. ● The peak of germ cells (in 7
million) is at 7 months of
 pregnancy before birth. All the ● Third phase is where the
oogonia already became the chromosome number was reduced
primary oocytes at the 1st meiotic by half (meiosis)
arrest at birth
● So at birth, all female mammals
have in her ovary primary oocytes. GAMETOGENESIS
This is why oogonia was not asked ● Random Trivia: All living beings
for us to be located in the cats’ come from an egg (Frontispiece of
ovaries. At time of birth, wala nang William Harvey’s book on The
oogonia sa ovaries ng girls Generation of Animals)
● At birth, 2 million of those oocytes,
only around 400,000 oocytes are GAMETOGENESIS: DEFINITION
retained at puberty. ● Gametogenesis - generation and
● These 400,000 oocytes, only maturation of the sex cells
400-500 oocytes can complete ○ Oogenesis and
maturation during the reproductive folliculogenesis -
age, between the onset of puberty generation of oocytes and
and the start of menopause formation of surrounding
follicle cells
MATHEMATICAL PART -takes place in ovary of
● The onset of puberty is 13 years females
old ○ Spermatogenesis in the
● The onset of menopause is 48 seminiferous tubules -
years generation and maturation
● Then, the reproductive years if 35 of the spermatozoa
years (48-13) -takes place in the testis of
● 35x12 months, which is equal to males
420 months ● Cross sections of cells responsible
● 1 oocyte will mature every month, for gametogenesis:
so 420 oocytes are matured (which
is within the range)
● Summary:
No. of Fertile Oocytes = (Puberty Age -
Menopause Age in Years) * 12

REDUCTION IN CHROMOSOME
NUMBER BY HALF (MEIOSIS)
SERIES OF CHANGES
UNDER GAMETOGENESIS
● Primordial germ cells passing through
events to become specialized sex
cells, spermatozoa or ovum (mature
oocytes)
● Primordial germ cells - degenerative
sex cells that collectively called as
germplasm
○ Germplasm can be
distinguished easily in
 lower animals other than ● Then they (secondary stage sex cells)
mammals embark in another meiotic phase
○ No obvious germplasm in (meiosis II) and beget four spermatids
mammals before and one oocyte.
fertilization ● Why only one oocyte? Because during
○ A cell-dense association of one of the phases of meiosis I, there is
mRNAs and mRNA-binding an unequal division of the primary
proteins such as VASA oocyte which resulted to the
DAZL, and MILI production of a secondary oocyte and
homologues (found in the a polar body
cytoplasm), which can be ● After the formation of spermatids and
identified by biological a mature oocyte, specialization
staining of the oocytes through differentiation into
spermatozoa and ovum.
OVERVIEW ● One spermatogonium underwent
spermatogenesis results to four sex
cells/spermatozoa produced
● One oogonium underwent oogenesis
resulted to one functional sex cell
produced, one oocyte/ovum

PHASES OF GAMETOGENESIS
1. Generation of germ cells and
migration to the gonads
2. Multiplication of germ cells in the
gonads (mitosis) while in
transit/migration to gonads
3. Reduction in chromosome number
by half (meiosis)
4. Maturation and differentiation

1.GENERATION OF GERM CELLS AND

MIGRATION TO THE GONADS
● Germ cells arise outside the
gonads
● In spermatogenesis, it starts with
● Recognizable at early stage of
spermatogonium
development
● In oogenesis, it starts with oogonium
○ For example, in anuran
● First event, they undergo growth and
amphibians, in the
differentiation and they reach the
unfertilized oocytes, germ
primary stage and become primary
cells can be identified as a
spermatocyte and primary oocyte
circumscribed region in the
respectively.
cytoplasm. This is the
● Primary stage sex cells then undergo
cell-dense region
meiosis I, which begets the secondary
containing mRNAs and
stage: secondary spermatocyte and
mRNA-binding proteins
secondary oocyte.
○ Upon the onset of
fertilization, from one cell
 stage to cleavage stage, ORIGIN OF PGCS: HOW THREE
the cell dense region can FINDINGS ARE HARMONIZED
be traced and the
cytoplasm closer to the
animal pole
○ When traced up to the
gastrula, they colonize the
endodermal layer of the ● Blastocyst, it has an inner cell
gastrula mass and the trophectoderm
○ They are generated and ● Inner cell mass - give rise to the
can be recognized at an embryo proper
early stage of development -embryo proper are the blue cells
○ Germplasm, primordial sex surrounded by the amnion in the
cells, kasi ay nakikita bago picture
pa fertilization -at first, it will split into epiblast and
○ Ang germplasm ay hindi hypoblast
ganito kadaling makita sa ○ Epiblast - where the
higher animals embryo will form as the
primitive streak (groove
AMNIOTES: BIRDS, REPTILES, that becomes distinct germ
MAMMALS
layers)
● Primordial germ cells (PGCs) are
-primordial germ cells are
easily identified in the yolk sac
generated first in the
endoderm
epiblast then migrate out
● PGCs are large sized, high content
the embryo to the
of alkaline phosphatase
endodermal layer of the
● Recognized here as early as, for
yolk sac
humans, at 24-day post-fertilization
● Trophectoderm - gives rise to the
in the yolk sac endoderm
extra embryonic membranes like
○ Roughly three weeks of
the placenta
embryonic development
-this and other components of the
● This is the classical notion.
embryo will expand from the
embryonic area to the sides, which
HANNEL & EDDY (1968):
IN THE MOUSE gets to form the other extra
● Claimed that PGCs originally embryonic membranes like the
reside in the epiblast of gastrula amnion, yolk sac, allantois, and
in the mouse. chorion

GINSBURG (1990) ● In humans, by the 3rd week, the

● Localized the region in the extra PGCs wander in an amoeboid
embryonic mesoderm posterior manner
to the primitive streak in a 7.25th ● Main flow of PGCs: primary
day embryo ectoderm to yolk sac wall to near
● Extra embryonic mesoderm is exit of allantois
found outside the embryo.
● More detailed flow of PGCs:
From the epiblast, primary
 ectoderm, going down here in the ● Hind Gut - suspended by
yolk sac endoderm. From the yolk mesentery
sac endoderm, they will exit ● Genital Ridges - forming gonads
passing through the allantois and of the embryo
go back to the embryo proper.
● PGCs become extra embryonic
ADVANCED STAGE OF here, when they leave via the
PGCs MOVEMENT allantois, and then go back into the
embryo when it reaches the
embryonic body after the hindgut

WHY PGCs GO OUT?

● Rodents (as mammalian model)
○ Development of PGCs
depend on signals
○ Radical expression of
pluripotency markers
Oct-4+, NANOG, and
SOX2 genes
○ Studies suggest
■ BMP (bone
morphogenetic
● Developing embryo above, its
protein) signal
primordial germ cells are
factor
colonizing the yolk sac endoderm
near the attachment of the
● When they go out into the
allantois. Allantois is then
endoderm and aggregate to the
connected to the hind gut.
base of the allantois, they become
● Allantois - outpocketing of the
specified
hind gut
● They start to express pluripotency
gene markers and the most known
If you cut it cross-sectionally like the pic
of primordial germ cells are
below…
Oct-4+, NANOG, and SOX2
genes
● Precursor PGCs are activated by
the BMP, increasing in number by
mitotic division
● When they reach the threshold
population, they start to migrate
away from the embryonic area,
becoming extra embryonal going to
● From the yolk-sac endoderm,
the yolk sac. Then they go to
PGCs will travel to the allantois,
allantois and get specified
then to the hindgut. Then they will
● Basically, para “matattoo” sila na
go up to the body and reach the
maging specialized sex cells
right and left genital ridges
 LIFECYCLE OF MURINE GERM CELLS

● After fertilization
1. Zygote
2. Cleaving forming a 2-cell
PROPOSED MIGRATION PATHWAY embryo
1. Primary ectoderm 3. Giving rise to the blastocyst
● Primordial germ cells arise (middle row)
from the epiblast, giving ■ Blastocyst consists
rise to the primary of the
ectoderm trophectoderm
2. Yolk sac endoderm (green) and the
● Where they first become inner cell mass
distinguishable (orange)
● Becomes extraembryonal ● After implantation
3. Allantois 1. Inner cell mass/epiblast
● They colonize at the base (orange)
of the allantois 2. Cavitation ad epithelializes
● Becomes specified at the ● At gastrulation
base 1. A new germ cell layer
● Gene pluripotency markers 2. Mesoderm (blue)
are expressed ■ PCGs are specified
○ Well known at E7.25 (black
pluripotency arrow) at the base
markers include of the mesodermal
OCT4, nano ??? allantois
genes ● PCGs migrate through the forming
○ Only these will hindgut
become PCGs 1. Dorsal mesenterium
4. Dorsal mesentery 2. Genital ridges
● Passes to the hindgut ● Sex determination
before the dorsal 1. Female (oogonia) and male
mesentery (spermatogonia) germ cells
5. Left & right genital ridges follow different fates
● Final destination 2. In the ovary (top row), the
● Left and right developing oocytes mature in their
ovary (females) follicles
● Left and right developing ■ Released during
testis (males) ovulation
● Embryo proper 3. In the testis (bottom row),
6. After specification, they travel back spermatogenesis takes
to the embryo place to generate motile
sperm cells
 mitotic division, increasing their
population
● Final proliferation phase will take place
within the genital ridges
● Mitotically active developing sex cells
in the developing ovaries are the
oogonia (female) and spermatogonia
(male)
*(slide skipped) ● Mitotic patterns in the gonads differ
widely between males and females
TERATOMAS ○ Proliferates by ordinary
● Not all PCGs reach their final mitotic division, increasing
destination, some get strayed their population by batches
○ Those that stray cannot find ● Mitosis is the same in terms of the
their final destination process but the pattern differs
○ They can’t find the genital ○ Differs widely between
ridges males and females
○ They migrate somewhere ○ Human female vs male
else, forming teratomas ■ Mitotic patterns vary
● Teratomas: tumor that is a mixture of
scrambled differentiated structures
which is fatal
○ Bizarre tumor

● Timing of mitosis and meiosis

● Oropharyngeal teratoma (right) ● (left) male (right) female
● Coccygeal teratoma (left) ● Male
○ Mitosis is continuous
How would migrating PCGs know where ■ Embryonic stage
to go? ■ Before birth
● Hypothesis: the genital ridges ■ After birth
(developing gonads) secrete ■ Puberty
chemoattractants, marking the ■ Throughout life
pathway for the migration of PCGs ■ Throughout
adulthood
2. PROLIFERATION OF GERM CELLS ○ The first batch of mitosis
(MITOSIS) stays there until puberty
● PCGs, while migrating and after ■ All are at mitotic
specification, will go back to the stage until puberty
embryo and are already undergoing ● Female
○ Mitosis stops early
 ○ Mitosis stops at around the producing mature
5th month of embryonic oocytes
development ○ The 400,000 germ cells
■ Rapid proliferation enters maturity 1 per month
takes place ?
between the second ■ Only 400-500 will
and the fifth month attain final
of embryonic maturation
development ○ 420 oocytes maturated
■ By the 7th month, once per month
mitosis has already ● Mitotic pattern stops early in
stopped, the oogenesis
oogonia now enters ○ Stops before birth
the primary oocyte
stage 3. REDUCTION IN CHROMOSOME
NUMBER BY HALF (MEIOSIS)
● Meiosis in male and female vary
● Male
○ Meiosis starts at puberty
○ The first batch of
spermatogonia will embark
in meiosis 1 and 2 to
develop into mature sperm
cell
○ Spermatozoa are produced
● Oogonia number is at its peak at the ○ Spermatogonia will enter
5th month of embryonic development meiosis 1 and 2 in waves
and drops down at the 7th month to ■ Would take 64 days
around 2 million left to complete for each
○ The 2 million have entered batch
the primary oocyte stage ○ Will go on throughout life
● At puberty adolescent stage, this is the ■ Will continue as
start of menstruation long as the flesh is
○ Only 400,000 germ cells strong
left because all the rest ● Female
have undergone natural ○ Meiosis starts early before
degeneration birth
○ Puberty is at around 13,14 ■ Around 5th month of
years old embryonic
● Menopause development
○ Occurs at around 48,49 ○ By the 7th month, they
years old become primary oocytes
○ 35 years between puberty ■ Enters meiosis 1
and menopause ○ At birth, all a female
■ Reproductive years mammal has in her ovaries
■ Years that the are primary oocytes in the
human female is first meiotic stage
 ○ Before birth and puberty, ○ Meiosis starts early but
the first phase of meiosis is gets arrested and will
arrested (it stops) resume at the onset of
○ At the onset of puberty, puberty
meiosis will proceed again
■ For every month, SUMMARY OF MAJOR STAGES OF
one of the primary MEIOSIS IN A GERM CELL
oocytes will
complete the first
meiotic division and
release a secondary
oocyte
■ 1 per month
○ Because mitosis stops
early, primary oocytes is
limited
■ There is a limited
period for the
production of
oocytes unlike that
of males

Which among the spermatogenic and

oogenic cells enter meiosis?

MEIOSIS I

● Primary spermatocyte and oocyte ● The first phase of meiosis

enters meiosis 1 and becomes ○ Primary spermatocyte &
secondary spermatocyte and oocyte primary oocyte
● Meiosis in male is continuous ● Reductional division: chromosome
○ Meiosis 1 and 2 is number is reduced by ½
completed and produces ○ In a female, chromosome
spermatozoa number is 46 (23 pairs)
● Meiosis 1 in females is arrested and ○ At 46, it will be reduced to
will resume at puberty 23-23 in daughter cells
○ Will be ovulated and ● Long prophase
release a secondary oocyte ○ Resumes during the onset
○ Will enter meiosis 2 and will of puberty
get arrested again
 PROPHASE I ○ Diakinesis
■ Nuclear membrane
breaks so that
spindle fibers can
attach to the
homologous
chromosomes
● Very long because it has subphases ■ Gain of new genetic
○ Leptotene stage components
LIPDD
■ chromosome ● 46 Human sex chromosomes (23
materials are very maternal chromosomes in pink, 23
thin (threadlike, fiber paternal chromosomes in gray)
like)
■ converge on one METAPHASE I
side of the nucleus
■ Bouquet stage
○ Zygotene (synaptene
stage)
■ Pairing of
homologous
chromosomes
■ Make 23 pairs
■ They align side by ● Homologous chromosomes align at
side and closely the equatorial plate
oppose ? each ● Arranged in a random manner
other point by point ○ One paternal chromosome
■ Synapsis: Precise on one side, and another
lining up on the other side (random
○ Pachytene distribution
■ Chromosomes
become thicker and ANAPHASE I
shorter
■ Pairing is completed
■ Exchange of genetic
material occurs
(crossing over)
■ No distinct physical
evidence of
crossing over
■ Tetrad formation for ● Homologous chromosomes separate
distinct sister ● Move toward respective poles
chromatids
○ Diplotene
■ Distinct evidence of
crossing over
■ Chiasmata: point of
exchange in
crossing over
 LEPDD
TELOPHASE I PROPHASE I (CONT.)

● Pairing begins at synaptene

● Pairing of chromosomes at pachytene
○ Chromosomes become
● Chromosomes are reduced by ½ thick and stocky
(reductional division) ○ Pairing is completed, they
● Haploid number of chromosomes are pairing side by side
● End of meiosis 1: 23 chromosomes on closely opposed ??? each
one daughter cell and another 23 for other
the other daughter cell ● Chiasma formation
○ Exchange of genetic
MEIOSIS II material by crossing over
○ Physical evidence in
diplotene
● Pulling apart of double-structured
chromosomes
○ Diakinesis
○ Exchanging chromosomes
will separate or pull apart
● The second phase of meiosis from each other
● Proceeds by ordinary mitotic division ○ Each with its own gained
● Equational division (other term for genetic material (genetic
ordinary mitotic division): distributing exchange)
the chromosome number
○ Enter meiosis II ANAPHASE I (CONT.)
○ Begets two daughter cells
● Sister chromatids separate, each
acting as a chromosome
● The final progeny ?? is the daughter
cells are totally genetically not similar
with each other

SPERMATOGENESIS
● In spermatogenesis, meiosis II and ● (E) Homologous chromosomes
meiosis II proceed uninterrupted separate
● (F) First meiotic division (right)
OOGENESIS ○ Cells contains 23 double
● In the case of Oogenesis, meiosis has structured chromosomes
two arrested phase (left)
○ diplotene stage
○ Metaphase II
 TELOPHASE I (CONT.) ○ Nondisjuntion of
chromosome 18 (trisomy
18): Neurological disorders
■ prominent occiput,
cleft lip,
● (G) Formation of daughter cells micrognathia,
○ Left: cell contains 23 single low-set ears, and
chromosomes one or more flexed
○ Right: cells resulting from fingers
second meiotic division
● Haploid number of chromosomes at
the end of meiosis I

COMPARISON OF NORMAL AND

ABNORMAL MEIOTIC DIVISIONS

● A: Normal meiotic division

○ In normal condition
○ Chromosomes separate (in
meiosis I) and embark in
the second meiotic division
○ Beget the normal number
● Nondisjunction: the failure of the of chromosomes (23 single
chromosomes to separate chromosomes)
○ can happen in the first and ● B: Nondisjunction first meiotic
second meiotic division division
○ Can result to abnormalities ○ Condition where some
● Chromosome numbers prone to pairs of chromosomes fail
nondisjunction are chromosome 13, to separate
chromosome 18, and chromosome 21 ★ For example: the
(most familiar) chromosome on the left
○ Nondisjunction of side, fail to separate, one of
Chromosome 21 results to the cells got 24
Down syndrome chromosomes while the
other just got 22. Their
progenies here are 24 and
the other is 22.
● C: Nondisjunction second meiotic
division
● Sometimes, chromosomes fail to
separate
 NORMAL GAMETOGENESIS ○ If female sperm fertilizes an
egg cell, the sex of the
embryo will be female (XX)

ABNORMAL GAMETOGENESIS

Left: spermatogenesis; Right: Oogenesis

● Spermatogonium with diploid number
of chromosomes (46, XY; 44
autosomes, 2 sex chromosomes) will
embark into a period of growth and ● There is nondisjunction
beget primary spermatocyte ○ The first spermatocyte has
● Primary spermatocyte (diploid) will 24 chromosomes and gets
enter first meiotic division and begets both of the sex
secondary spermatocytes (23, X & 23, chromosomes (X and Y)
Y → haploid) ○ The other spermatocyte
● First meiotic division is reductional has 22 chromosomes with
division no sex chromosomes
○ The secondary ● Klinefelter Syndrome
spermatocyte will have ○ Sex-linked syndrome
haploid number of ○ Can result to neurological
chromosomes (23, X & 23, disorders
Y) ○ If the abnormal sperm
■ 22 autosomes, 1 fetilizes an abnormal
sex chromosomes oocyte (23, X), 24XY + 23X
● Secondary spermatocytes (haploid) = 47XXY, total number of
will enter the second meiotic division chromosomes would be 47,
(equational division) and the sex chromosome is
○ Progeny: four (4) XXY
spermatids (23, X; 23, X; ○ There is an extra X
23, Y; and 23, Y) chromosome
● The sex of the sperms can be male or ○ Affects only males
female (sex classification)
○ Male sperm with Y
chromosomes
○ Female sperm with X
chromosomes
○ If a sperm carrying an Y
chromosome fertilizes an
egg cell with X
chromosome, the sex of
embryo will be male (XY)
TIMING AND DURATION OF MEIOSIS ● There is only one functional oocyte
produced per cycle with the
addition of polar bodies that are
produced.

MAMMALIAN OOGENESIS AS A
REFERENCE
● PGCs: they become oogonia
(mitotically active sex cells) These
undergo a period of growth. They
● In spermatogenesis, mitosis is become primary oocytes.
continuous from the embryonic ● At the primary (1°) oocytes stage,
stage throughout life. they enter meiosis I, the long
● In oogenesis, mitosis stops early. prophase:
● Meiosis starts at puberty. ○ Leptotene
● Meiosis is continuous in ○ Zygotene
spermatogenesis. ○ Pachytene
● Meiosis is arrested in females. ○ Diplotene
● At diplotene, meiosis I is arrested
PHENOMENA/SERIES OF CHANGES… (1st meiotic arrest). There are
cellular and molecular changes
going on.
● What are examples of molecular
and cellular changes going on?
○ The chromosomes become
relaxed., forming an oiling.
○ Portions of the
chromosomes are exposed
and they may be prone to
damage.
● The arrested stage in females from ● The longer it stays in the
primordial cells, a series of arrested stage, the longer it may
changes, and then they become be exposed to chromosomal
mature oocytes. damage.
● From the primordial germ cell to ● Imagine a chromosome that is
the mature oocytes stage, there compact; there is no extended one.
are phenomena taking place at But when a chromosome is
the: extended, the portions are
○ Cellular level exposed.
○ Molecular level ● Imagine the test tube brush. There
○ Physiological level are bristles on the sides. That is
what chromosomes look like when
DISTINCT FEATURES OF OOGENESIS they are relaxed or extended while
● Oocytes undergo arrested stages in the diplotene stage.
of meiosis. ○ While they are in the
● There is an equal scientific extended stage, they can
cytoplasmic division. be prone to damage.
● What can lift off the 1st meiotic ○ One secondary (2°)
arrest? oocyte will be released per
○ The first meiotic arrest is month.
lifted off with the onset of ● When it is released from the ovary,
puberty. It's like, "Oh, the secondary (2°) oocyte will
dalaga na si Sabel!" enter meiosis II → prophase II →
● At puberty, physiological metaphase II.
phenomena taking place is that ○ At metaphase II, it gets
there are no surges in the sex arrested again (2nd
hormones. meiotic arrest).
○ Release of FSH, LH, ● What can lift off the 2nd meiotic
progesterone, and arrest?
estrogen. ○ The 2nd meiotic arrest at
■ The hormonal metaphase II is lifted off
interplay activates with the onset of
the maturation of fertilization.
the oocyte, thus ● With the onset of fertilization
lifting off the meiotic metaphase is lifted off. The
arrest at the secondary oocyte will complete
diplotene stage. So, anaphase, and telophase, resulting
diplotene arrest is to:
lifted off the first ○ Mature oocyte
meiotic arrest. ○ 2nd polar body
● The oocyte proceeds to finish ● Functional oocyte is the
meiosis I → metaphase I → fertilized oocyte then it will
Anaphase I → Telophase I. After become known as the zygote.
this, meiosis I is completed. ● What happened to the 1st polar
○ The result of meiosis I is body? It can opt to divide again. In
the production of a some cases, it just degenerates.
secondary (2°) oocyte and ● If it were to embark on another
a 1st polar body. division, there will be three polar
■ The chromosome bodies produced.
number in the ○ In most cases in mammals,
secondary (2°) just the first polar body and
oocyte is already the second polar body are
reduced by produced.
one-half. This now ● Features of oogenesis:
has a haploid ○ There is a presence of
number of meiotic arrest at:
chromosomes. ■ Diplotene of meiosis
■ There is an equal I
cytoplasmic ■ Metaphase II of
division. meiosis II.
● This secondary (2°) oocyte will ■ There is an equal
then be released from the ovary by cytoplasmic division
a process called ovulation. ■ There are polar
bodies produced.
 DIPLOTENE ● Fetal Period
● It is the reason why older women ○ Follicular histology: no
are said to have a greater follicle cells around
probability than younger women ○ Meiotic event in ovum:
to give rise to children with birth Oogonium
defects. ■ Oogonium just
● For instance, when a woman gets undergo mitotic
pregnant at 48 has a greater division and
chance of giving birth to a baby therefore, increase
with birth defects than the in number.
probability of a woman who got ● Before or at birth
pregnant at the age of 25. ○ Follicular histology:
○ The reason for that is that primordial follicle
the oocyte fertilized at the ○ Meiotic event in ovum:
age of 25 was arrested at primary oocyte
a shorter stage, so lesser ■ At the primary
exposure of the oocyte stage, an
chromosomes to incomplete layer of
damage. follicle cells are
○ Whereas the oocytes invested around.
fertilized at the age of 48 ● There are just
were arrested for 48 two follicle cells.
years. It indicates longer ● After birth
exposure to ○ Follicular histology:
chromosomal damage. primary follicle
○ Meiotic event in ovum:
SUMMARY OF MAJOR EVENTS IN primary oocyte
HUMAN OR GENESIS AND ■ Primary oocytes
FOLLICULOGENESIS
increased in size
■ The number of
follicles around
increased.
● From 2 layers of
follicle cells
(before or at
birth), there are
now complete
layers of follicle
cells.
■ This is the primary
oocyte arrested in
the diplotene stage
of meiosis I.
● After puberty
○ Follicular histology:
secondary follicle
○ Meiotic event in ovum:
primary oocyte
 ■ The primary oocyte cavity is called an
has rapidly antrum.
increased the follicle ● It is filled with
cells around. antrum liquid
■ REMEMBER: or antral liquid
follicle cells are the ■ Granulosa cells
same as the (black dots): this
granulosa cells. will differentiate into
■ The secondary two layers:
cells are composed ● Theca externa
of two to three ● Theca interna
layers of the follicle ■ These two layers
or granulosa cells. are now the
■ The oocyte Graafian follicle.
increased in size. ■ The tertiary follicle
■ The distinct feature is otherwise known
of the secondary as the Graafian
follicle is the initial follicle with:
appearance of a ● the large antrum
small cavity within with the theca
the granulosa or externa and
follicle layer. theca interna
■ Zona pellucida ● with the
(blue shadow): cumulus
translucent oopherus
■ Antrum (white surrounding the
space): the initial oocytes and still
appearance of the with the zona
antrum is evident. pellucida
immediately
surrounding the
oocytes (blue
colored layer)

○ Follicular histology:
tertiary follicle
■ It is still within the
oocyte
○ Meiotic event in ovum:
secondary oocyte and polar ● REMEMBER: The secondary
body I oocyte is the one that is
○ What is the difference released from the ovary or the
between secondary and oocyte that is within the ovary
tertiary? shortly just a few hours before
■ In the tertiary, there the ovulation period.
is already a large ○ While inside the ovary, it is
cavity formed. This a primary oocyte or a
 tertiary follicle. So if it is a ● What happens to the granulosa
tertiary follicle or a primary cells (theca externa and theca
oocyte, the meiotic division interna) and the liquid in the
is not yet complete and antrum?
thus it is not yet reduced ○ With the release of the
in chromosome number. oocyte, the liquid is
○ REMEMBER: once released as well. It will
released from the ovaries, it become shrunken and
is a secondary oocyte. crumpled. It forms the
corpus luteum.
● In the lab, the Graafian follicle is ■ The Corpus luteum
classified as still in the primary eventually and
oocyte stage. gradually
○ For the primary oocyte degenerates and
stage, it has: will go back as the
■ Primary follicle component of the
■ Secondary follicle ovary.
■ Tertiary follicle
GRAAFIAN FOLLICLE

● Ovulated oocyte
○ It is in the secondary
oocyte stage.
*In the illustration: Graafian follicle (A) is
younger; Graafian follicle (B) could be
shortly after ovulation. Probably this had
already completed the first meiotic
division.

○ Chromosome number is
** Let us agree that the Graafian follicle
reduced by one-half (23).
is still a primary oocyte.
○ Released from the ovary.
ACTUAL PHOTOGRAPH OF FOLLICLE ● Secondary follicle- with the
formation of the initial antrum
● Tertiary follicle- with
well-developed antrum and thick
layer of the granulosa cells
● The gradual investment thickening
of the granulosa cells
accompanied by the gradual
maturation of the oocytes is
governed by hormones
● Gonadotropic hormones- start to
be produced with the onset of
* In the photograph: The secondary puberty
follicle with the initial appearance of an ● FSH- follicle-stimulating
antrum can be seen. hormone (comes first before LH/
Luteinizing hormone)
HORMONAL CONTROL OF OOCYTE ○ The target cells of FSH are
MATURATION granulosa cells and act on
● Controlling the increase in the it; as a response, the
number of follicle cells or granulosa cells increased
granulosa cells around the growing further in number so they
oocyte are stimulated to increase
● The growth of the oocyte which is their population
basically the oogenesis is always ○ The higher the FSH
coupled with folliculogenesis, aka stimulation, the greater the
the formation of granulosa cells increase in the population
around it of the granulosa cells
● Hormonal control sets in with the ● The increase in granulosa cells are
onset of puberty accompanied by synthesis
○ This is the time when the ● The increase in estrogen levels
maturing female is starting which will further activate the other
to produce the hormones granulosa cells to increase in
● At puberty, the primary oocyte from number; leading to a peak in
the primary follicle gradually get estrogen level which will send a
transformed into the secondary feedback regulation on the pituitary
follicle gland
○ Still at the primary oocyte ● So FSH acting on the granulosa
stage cells, the granulosa cells increase
in number, as well as they are
activated to synthesize and secrete
estrogen, a female sex hormone
● On the other hand, LH- luteinizing
hormones which are released at
lower levels than FSH; its release
is activated by the levels of
estrogen released by the
granulosa cells so the pituitary
gland is activated to release LH;
 LH released will then act on the
theca (interna and externa) cells
● In response to LH, the theca cells
synthesize and secrete the
hormones testosterone
● From the theca cells (outside)
surrounding or lining the wall of the
antrum, so the testosterone
produced by the theca cells will
cross the membrane of granulosa
cells inside and as it crosses, this
is converted by an aromatase ● To summarize, the FSH acts on the
enzyme and a cyclic AMP follicle cells or the granulosa cells
generating system such that ● In response, the granulosa cells
these testosterone produced by secrete and synthesize estrogen
the theca cells are converted into and aromatase
estrogen, so estrogen levels ● The estrogen, the female hormone,
increased even more will now exert its function on the
● The levels of estrogen synthsized female reproductive activity
and secreted by the granulosa ● It will send a feedback regulation to
cells plus the testosterone the pituitary gland to stimulate LH
converted into estrogen, this is the release
level that will send a feedback ● LH acting on the theca cells
regulation to the pituitary gland to ● The theca cells, in response to LH,
activate more LH release will synthesize and secrete
● As a result, there are now testosterone
increased receptors in anticipation ● Testosterone will cross the
of higher release of LH and FSH membrana granulosa
○ This is the cycle every ● Testosterone is converted by
month aromatase and cyclic AMP
○ The release in LH will degenrating system into estradiol
activate more estrogen ○ Estradiol- the potent
level formation and this estrogen
increased level of estrogen ● High level of estrogen at puberty,
will activate more LH sex hormones are surging up, they
receptors act on follicle cells
■ A cyclical hormonal ● Formation of more LH receptors
activity ● The secondary follicle is further
● The maturation and growth of the stimulated to grow and become the
oocyte, together with the formation Graafian follicle
of the follicle cells, are governed by ● With the peak in the LH level, the
an interplay of hormones: FSH, Graafian follicle will then be
LH, and the hormone estrogen released from the ovary by the
process of ovulation
● This whole cycle will happen every
month, meaning every month,
there would be one Graafian
follicle that completes meiosis I
 and gets released from the ovary the primordial follicles will then go
as a secondary oocyte through a stage of maturation to
● So maturation into the Graafian become Graafian follicle and the
follicle involves the interplay of the cycle is repeated. This cycle is
FSH, LH, and estrogen level governed by the interplay of the
sex hormones.
SEQUENCE OF FOLLICLE
MATURATION IN THE OVARY

Figure: Actual photograph

● Difference between atretic follicle and
corpus luteum
○ Corpus Luteum: Bigger
Size; Distinct membrana
granulosa; thicker theca
● At the time of birth, all of female externa and interna
mammal has in her ovaries are the ○ Atretic Follicle:
primary oocyte stages Undifferentiated wall
● Primary oocyte at various levels:
primordial follicle, primary follicle, OOGENESIS IN AVIAN
secondary follicle, Graafian follicle ● Mature eggs go nearer the wall of the
● With so many primordial follicles, ovarian epithelium
only 1 will complete maturation up ● The bulging yellow large cells in the
to Graafian follicle per month to figure below are called the ovarian
become the secondary oocyte follicle/ovum
released at ovulation ● Everything aside from the egg cell
○ This will enter meiosis II, (egg shell, egg white, thicker albumin)
prophase II, metaphase II are accessory layers for protection
■ At metaphase II, it ● Mature ovum would contain the yolk,
gets arrested again region containing the nucleus, and
● The process of releasing the region containing the active cytoplasm
secondary oocyte from the ovary is ● The whitish, sticky jelly-like liquid
ovulation, which happens at the structure outside the yellow yolk is
middle of the month called the white yolk
● Antrum- the cavity that is filled ● The substance attached to the yolk is
with liquid, the antral liquid the albumin
● With the release of the liquid, this ● The egg is difficult to beat because of
will crumple into a shrunken the presence of chalaza
structure called the corpus luteum ○ Chalaza’s function is to
which undergoes further crenation make sure that the
to crumple, the cavity is now blastodisc is always
closed, degenerating corpus oriented on top (this is for
luteum is termed corpus albicans thermoregulation - mother
which goes back and gets hen sits on top of the egg
incorporated as connective tissue where the blastodisc is
of the ovary. Next month, one of oriented)
● A cleaving blastodisc is called a cytoplasm/blastodisc. The circle area on
blastoderm the line drawing (upper figure) is the yolk.
● Blastodisc is unfertilized
● The blastoderm will give rise to the
chick embryo, not the whole yolk
● Aves, in all vertebrates, are known to
have the longest oviduct relatively in
proportion to the body
○ Oviduct is divided into the
infundibular region, magnus
region, shell region, and
shell gland region
● If the egg is to be fertilized, it must be
fertilized before the accessory
coverings are added
● VitaLink PH - Hen layers injected with
hormones to hasten ovulation so that
Figure: Reproductive Organs of Aves.
at any one time, there can be 2
oocytes that are released which will
OOGENESIS IN AMPHIBIAN:
simultaneously go down the oviduct ILLUSTRATION
and will get enclosed by the accessory
coverings together

● Cross section of the ovary, in

which oocytes can be seen at any
stage (younger ones, growing
Figure: Left - Line drawing of the ovary,
ones, matured ones)
Right - Actual specimen
● Characteristic of amphibian oocyte:
○ Mitosis is continuous
○ Egg maturation requires 3
years – starts after
metamorphosis
○ 3-year cycle of oogenesis

Figure: Developing chicken/avian embryo

with labeled components. Take note:
Yellow yolk/sex cell contains the nucleus
which contains the active
Phases of oocyte maturation ○ Also results to intense RNA
(oogenesis) in amphibians synthesis (germinal vesicle)
■ Formation of
lampbrush
chromosomes
nucleoli in
preparation for the
production of
proteins
● Vitellogenic phase
○ Yolk accumulates
○ Cortical granules
● Previtellogenic phase - first year to ○ Pigment granules
around middle of second year ● In most animals, growing oocytes
○ Previtellogenic - before the are actively transcribing genes
accumulation or deposition ● Gene products
of yolk ○ (cell metabolism,
● Vitellogenic phase - approximately oocyte-specific processes,
after mid of the second year early development prior to
towards the third year nuclear function)
○ Vitellogenic (vitellogenin =
precursor protein for the
yolk, coming from the liver)
- period or phase of actual
accumulation or deposition
of yolk

● At any one time, when a frog’s

ovary is histologically prepared for
observation under a microscope,
● Previtellogenic phase - increase in there can be different populations
mitochondrial number for the of oocytes observed in a cross
eventual need of cellular section
respiration once the oocyte gets ○ Oogonia, growing oocytes,
activated and fertilized maturing and matured
○ The accumulation of oocytes
mitochondria results in the
formation of dense body
called Balbiani body
 ● Figure showing stages of arrested
arrest in amphibians (similar to
● Line drawings showing the series
mammalian) – diplotene block (at
of changes
prophase I) and metaphase block
1) Oogonia surrounded by
○ At sexual maturity (puberty
follicle cells, theca interna
in mammals), progesterone
and theca externa
levels surge – hormone
● Theca externa -
activator of amphibians;
layer surrounding
gets activated by
the wall of the ovary
phosphorylation
● Theca interna -
■ c-Mos protein
extension of the
activates p34
theca externa
component of the
surrounding the
Active MPF – gets
individual growing
phosphorylated and
sex cells
becomes activated
2) Oocyte is growing in size,
■ Completion of the
surrounded by the theca
first meiotic division
layer; the nucleus starting
■ Second meiotic
to grow as well
metaphase is
3) The nucleus is enlarged,
blocked by a
termed now as germinal
cytostatic factor –
vesicle
metaphase block
4) Matured oocyte -
■ Fertilization saves
accumulation of pigment
the day!
granules at one pole –
● In oogenesis, there are two
animal pole; vegetal pole
arrested phases: diplotene block at
filled with yolk granules;
prophase I and metaphase block at
theca externa and follicle
metaphase II
cells could still be seen
5) Frog’s oocyte at the time of
SPERMATOGENESIS
laying; it has a plasma ● Starting point: primordial germ
membrane surrounded by a cells (PGCs) → become
vitelline membrane outer to specialized sex cells, specialized in
the plasma membrane; structure and function
outside the vitelline, the (spermatozoa)
jelly coat (to prevent ● The same: mitotic multiplication,
desiccation) meiosis (I and II)
● Differentiation stage: spermatogonia,(which lining the
spermeiogenesis walls of the seminiferous tubules),
● Mitosis in male occurs throughout primary spermatocytes (has the
life biggest cells of the tubules), sperm
● PGCs at the genital ridge (final head (color red), sperm tails (color
destination: left and right) → green).
incorporated in the sex cords →
Seminiferous tubules (at around
the time of puberty) → Tubular
epithelium (lining the seminiferous
tubules) where the Sertoli cells
would form

● Closer look of the drawing.

● Spermeiogenesis is an important
step in spermatogenesis. It is also
the maturation of the spermatids to
become structurally and
functionally specialized.

● Type A spermatogonia, derived

from the spermatogonial stem cell
population, represent the first cells
in the process of spermatogenesis.
● Line drawing, is the testis, which Clones of cells are established and
shows the different seminiferous cytoplasmic bridges join cells in
tubules. each succeeding division until
● The cross-section of this portion individual sperm are separated
(the one with the box), it shows the from residual bodies. In fact, the
seminiferous tubules, the number of individual
 interconnected cells is ● In the spermatogenesis, has
considerably greater then depicted residual bodies. And they are
in this picture. basically speramtids strip for
● Shows the transformation of the actions. With a compact head and
spermatogonia into the formation a long motile tail.
of spermatozoa, the mature sperm
cell. CATEGORIES OF CHANGES DURING
● Once within the genital ridges, the SPERMATOGENESIS
primordial germ cells undergo
series of divisions to generate
waves of different subpopulation of
spermatogonia.
● All of the stem cell (synchronized
cell divisions) are capable of self
renewal.
● In one of the subpopulations, the
type A4 (last batch of type A pale
spermatogonia), has the option to
undergo to have another round of ● Reorganization of the cytoplasm:
cell division, or undergo program ○ Golgi Apparatus
cell death/ apoptosis, or they have - They fused,
the option to be committed to flattened over the
follow the developmental option of nucleus, and
becoming a primary eventually form a
spermatocytes. cap over the
● This committed cells will undergo nucleus, which form
another self-renewal to become an acrosome cap.
the type B spertamgonia. ○ Centrioles
● The type B spermatogonia, is the - They aggregates on
last one to undergo mitosis. This one side.
are now committed to become the ○ Mitochondria (locate in
primary spermatocytes. neck)
● The primary spermatocytes will
begin entering meiosis 1, and will
begin the secondary
spermatocytes.
● Secondary spermatocytes will
complete meiosis 2 and will get the
spermatids.
● The spermatids now will undergo
spermeiogenesis.
● From oogonial stage down to the
spermatids stage, they are
interconnected by cytoplasmic
bridges, that is to make sure that
per batch will mature
synchronously.
 ● Progressive reduction in nuclear
size/compaction of the nucleus
○ Nuclear elongation
○ Loss of water
○ Centrioles
○ Elimination of RNA; leaving
● These will take approximately 24 only DNA
days, for the spermatids to the 1. DNA is initially packaged by
spermatozoa. histones.
● 64 days, complete maturation from 2. At the secondary spermatocyte
the days that it starts meiosis. histones are replaced first by
transition proteins and then by
protamines.
3. Solenoid structures is replaced
by torroids (doughnut shapes),
which are in turn supercoiled into
torroidal loops.
4. Highly compacted structure shuts
down transcription during
spermiogenesis.
● Not all histones are replaced
● In humans, 15-20% remain in
● Summary of the major stages in nucleosomal configuration.
spermeiogenesis starting with
spermatid (A) and ending with
mature spermatozoon (I).
● Short arrow, starting point of the
cycle.
● Long arrow, ending point of the
cycle.

● Parts of the sperm.

● Acrosome cap, covers the oviduct and thus the
nucleus. swimming movements must
● Midpiece, surrounded by the be agile, and the head with
mitochondria that shape can greatly help
● Tail, provides swimming with motility.
movement of the sperm.
● Some sperms are normal and LECTURE 3: FERTILIZATION
some are abnormal. ● The union of the sex cells which is
● Types of spermatozoa fertilization
abnormalities: ● This begins the formation of a new
○ Normal organism
○ Head defects
○ Midpiece defects- not TWO TYPES OF FERTILIZATION:
energize and have a AMONG VERTEBRATES
1. External fertilization (ex vivo)
difficulty in swimming.
● Characteristic of aquatic
○ Acrosomeless- no enzymes
vertebrates such as the fish and
and can’t penetrate the
the amphibians
oocyte.
● The challenge for aquatic
○ Tail defects
vertebrates is to release
thousands of eggs in a single
spawning to ensure the survival of
a species
● The challenge for external
fertilization is to ensure
species-specific attraction
between sex cells, the solution for
this is that there is a presence of
● The thin, elongated heads are chemoattractant released by the
spermatids. jelly coat of the oocyte
● The tail ones are the spermatozoa. ○ Chemoattractant -
responsible for
species-specific sperm
attraction and activation

● Spawning - the release of eggs

into the aquatic environment

● Why does the species specific

differences are in the head?
- It is because, the coats are
different from each species.
● In the image above, here is a male
For instance, for birds and
fish waiting to spread his sperm
reptiles the oviduct of the
over the spawned oocytes (eggs)
hen have a very long
2. Internal fertilization (in vivo) FERTILIZATION PROCESS: SEA
● Characteristic of avians and URCHIN FERTILIZATION
mammalians ● Most of what we know about
● It requires the insemination, the fertilization process was based
deposition of sperm cells into the on sea urchin fertilization
female reproductive tract ● These are the results of the studies
● Fertilization takes place in the of the Hertwig brothers (Oscar and
female reproductive tract, Richard Hertwig)
specifically in the oviduct also ● Hertwig brothers, the first guys
known as the uterine tube or who observed fertilization that
fallopian tube used sea urchins as model
organisms

MAJOR EVENTS IN FERTILIZATION

● The details of fertilization vary from
species to species, but generally
these are the major events:
1. Contact and recognition
between the sperm and the
oocyte
2. Regulation of sperm entry ● Image above shows a schematic
into the oocyte. drawing of an aquatic animal’s
a. AKA the prevention oocyte, a sea urchin or an
of polyspermy, amphibian or fish.
prevention of the ● Outer to the plasma membrane of
entry of more than the egg cell is the vitelline
one sperm into the envelope and the presence of the
oocyte jelly coat
3. Fusion of the genetic ● Jelly coat is believed to be the one
materials of the sperm and secreting the chemoattractant for
the oocyte species-species specific
a. AKA pronuclear recognition of sex cells in the
fusion or aquatic environment
amphimixis
4. Activation of the oocyte
metabolism to start
development
a. Comes into two
phases: early
responses and late
responses
 1. CONTACT AND RECOGNITION ● Next is the movement of the
BETWEEN THE SPERM AND OOCYTE sperm into the cytoplasm of the
oocyte, and in the process
causing cortical reaction
○ No. 4 and 5 in the pic
above
○ Cortical reaction - simply
the bursting of the cortical
granules, releasing their
chemical contents in the
perivitelline space
● Image above shows the sea urchin ○ Perivitelline space - space
oocyte and the sperm between the plasma
● There is the egg plasma membrane of the oocyte
membrane, vitelline layer, and the and the vitelline layer (yung
jelly coat white area kung nasaan
may blue dots)
CONTACT AND ● Vitelline layer is lifted off from
ACROSOMAL REACTION the plasma membrane of the
● Here comes the sperm (to the jelly
oocyte, which forms the
coat), the sperm head starts the
fertilization envelope
acrosomal reaction which starts
S
with the breakdown of the
CORTICAL REACTION
acrosoma cap, releasing the
hydrolytic enzymes
○ No. 1 in the pic above
● Hydrolytic enzymes start to
degrade or digest the jelly coat
○ No. 1 also in the pic above
● Further as part of the acrosomal
reaction is the formation of the
acrosomal process, a fibular
protein to further penetrate the ● Image above illustrates the
vitelline layer cortical reaction, the breakdown
○ No.2 in the pic above of cortical granules resulting to
● The vitelline layer is penetrated. the release of chemical
Here we can see the formation of a substances forming the
cone referred to as the fertilization membrane
fertilization cone, which is the ● In here, formation of the
actual fusion of the sex cells’ fertilization cone (No. 4 in the pic
plasma membranes above), the fusion of plasma
○ No. 3 in the pic above membranes of the sex cells
○ Vitelline layer - contains ● Then cortical reaction, releasing
the species-specific the contents. The contents are
receptor for the sperm proteases,
○ Fertilization cone - fusion mucopolysaccharides, and
of the plasma membrane of peroxidases
the sperm and oocyte
 ○ Proteases - enzymes that FIRST BLOCK TO POLYSPERMY
breaks down molecular ● Fast block to polyspermy
bonds between the vitelline ● Electrical in nature
envelope and the plasma ○ Change in membrane
membrane potential of the cell
○ Mucopolysaccharides - becoming more positive
carbohydrate types that inside relative to the
produce osmotic gradients, outside
causing water to rush ○ Very fast and transient
between the vitelline ○ May not be sufficient,
envelope and the plasma hence the need of the
membrane to separate second block
them ● Plasma membrane contact (gamete
■ Imagine niyo class fusion)
ang sago na ○ The sperm acting under
naibabad sa tubig, receptors present on the
tapos magblobloat. plasma membrane of the
Kapag nagbloat ang oocyte
mucopolysacchari ○ The previously inactive
des, iseseparate oocyte gets activated
niya na itong ○ Membrane depolarization
vitelline membrane ○ Cell at rest (oocyte), gets
from the plasma activated by the sperm,
membrane once activated, the sodium
○ Peroxidases - enzymes channels in the membrane
that harden the fertilization open causing sodium influx
membrane by cross-linking into the cell
tyrosine residues of ● Cell that is at rest has a membrane
adjacent proteins to prevent potential ranging from –50 to –70
other sperms to attach to millivolts
the oocyte membrane ○ More negative inside than
outside the cell
2. REGULATION OF SPERM ENTRY ● There is a fast reversion of the
INTO THE OOCYTE electrical potential – negative to
positive
○ From -70 to around +10
millivolts (fertilization
potential) – very fast, thus
needs a backup → slow
block to polyspermy

SECOND BLOCK OF POLYSPERMY

Upper box: First block to polyspermy; Lower box: ● Chemical in nature
Second block to polyspermy ○ Involves calcium release
● The first and second blocks to that triggers cortical
polyspermy are early metabolic reaction
responses or activation of oocytes ○ Lasts longer
● Resting MP: approximately -70 mv
● Slow 3. FUSION OF GENETIC MATERIALS
● Fusion of plasma membranes OF MALE PRONUCLEUS AND FEMALE
activates the phospholipase C and the PRONUCLEUS
egg cell plasma membrane
○ Results in the production of
inositol triphosphate and
diacylglycerol
○ Inositol triphosphate (IP3):
activates the release of
calcium ions (Ca2+ from
calcium reserves
(Endoplasmic Reticulum)
○ Increase of cytoplasmic
calcium triggers cortical
reaction (breakdown of
cortical granules that will
release proteases,
peroxidases,
mucopolysaccharides
resulting in the formation of ● Union of gametes
fertilization cone ● The illustration shows a mammalian
oocyte
EXPERIMENT SHOWING HOW FAST T ● The mammalian oocyte with the zona
IONS SPREAD WITH THE ONSET OF pellucida and corona radiata
FERTILIZATION ● The sperm penetrating causing the
depolarization of the plasma
membrane
○ First block to polyspermy –
change in electrical
potential
● Then, the onset of cortical reaction,
the mechanism of the second block to
polyspermy, that results to the
hardening of the zona pellucida and
the vitelline membrane forming the
fertilization envelope in the sea urchin
● The movement is traced by a ● The male nucleus that is very
fluorescent dye compact, sperm head that is very
● Very fast spreading of calcium ions compact, while it gets into the oocyte
cytoplasm, the head undergoes
decondensation, nuclear breakdown,
and starts to form the new male
pronucleus
○ At this point the metaphase
II arrest is lifted off and the
second polar body is
released
 PHOSPHOLIPASE C ACTIVATION
● This pathway (boxed in red in the
image) is basically similar pathway
in mammals except the formation
of hyaline layer. Hyaline layer
(characteristic of aquatic animals)
● Activation of the phospholipase C,
● Photo above shows pronuclear fusion tyrosine kinase C can also be
(pronuclei fusion or amphimixis) stimulated possibly in the oocyte
○ The male and female plasma membrane
pronuclei meet forming the ● What is certain is that
zygote nucleus phospholipase C is activated
○ You can see polar bodies resulting to the production of IP3
○ Fertilization is completed and DAG (diacylglycerol)
● Protamines are replaced by histones ● IP3 causes the release of calcium
● Zygote nucleus (functional oocyte) ions
formed when metaphase II arrest is ● Calcium ions activate
lifted off, go on to complete meiosis II calcium-dependent kinases like
NAD+ kinase
4. METABOLIC ACTIVATION ● NAD is phosphorylated to NADP+
OF THE OOCYTE (IN SEA URCHIN) ● NADP can then serve as a
coenzyme involved in the
synthesis of lipids

WHY SYNTHESIS OF LIPIDS

IMPORTANT?
● This is preparation of the fertilized
oocyte for the generation of
plasma membranes of the newly
formed cells within the onset of
cleavage division and the
● These are based on studies using succeeding stage of
sea urchins embryogenesis
● As I said before, fast block to ● Also, calcium ions causes the
polyspermy is actually an early cortical reaction that sets up the
response of the cell slow block to polyspermy
● Cortical reaction (cortical granule ● Calcium ions also activate other
exocytosis) is an early response of calcium-dependent kinases that
the cell stimulate protein synthesis
● Here, fusion of sex cells and ● DAG stimulates protein kinase C
plasma membranes which can phosphorylate other
○ Sinabi lang ito ni miss target proteins that can result in
habang binibilog gamit ng the activation of DNA replication
cursor niya yung “Sperm and cytoplasmic movements of
binding and/or fusion to egg morphogenetic material.
cell membrane sa taas. ○ These are now the later
Walang further explanation metabolic responses of the
whatsoever. oocyte
 1.TRANSPORT OF GAMETES AND
FERTILIZATION IN MAMMALS

● DAG (diacylglycerol) activates

protein kinase C, phosphorylates
target proteins, that results in FEMALE REPRODUCTIVE TRACT
cellular responses such as proteins ● The female reproductive tract and
that involve in DNA replication, cell its association with the ovary.
cycle regulation, associated with ● Ostium or the opening or mouth of
chromosomes the fallopian tube surrounded by
○ Cyclins and finger-like structures called the
Cyclin-Dependent Kinases fimbriae
- proteins that regulate cell ● Fimbriae are the finger-like
cycle structures, ostium is the opening
○ Histones - proteins ● During ovulation, the oocyte is
associated with released and captured by the
chromosomes fimbriae. Then it gets to travel
down to the uterus
● Fertilization is actually a wake-up ● It takes 4 to 5 days for the oocyte
call for the oocyte into a fast-paced to travel from the oviduct down to
action the uterus
● Very rapid lahat. Rapid DNA ● By around 7 to 8 days, it is now
replication, synthesis of DNA already in the vicinity of the uterus.
blocks, synthesis of chromosomal
proteins (like histones), synthesis
of proteins for cell cycle regulation
(like cyclins and cyclin-dependent
kinases)
● Fertilization is complete. The
fertilized oocyte has been fully
activated and ready for a
fast-paced action
OOCYTE TRANSPORT IN THE FEMALE ● From the upper vaginal canal, the
REPRODUCTIVE TRACT sperms had to pass through
○ The cervix then traverse
the muscular uterus
(number 2 in the image
above),
○ Then the utero-tubal
junction (number 5 in the
image above)
○ Then the ampulla of the
fallopian tube (number 3 in
● Here, the finger-like structures are the image above)
the fimbriae ● Here is the ampulla where there is
● Opening is ostium hyperactivated motility of sperm in
● The little red circle highlighted by the fallopian tube to penetrate the
the blue arrow is the oocyte oocyte
captured by the fimbriae
● It goes along the fallopian tube and SPERM BARRIERS
● During the transport of the sperms
it becomes fertilized in ampulla
in the female reproductive tract
(fertilization site)
they pass through these barriers:
○ Ampul is the site of
○ Natural vaginal acidity (of
fertilization
pH 3.5)
● Fertilized or not it gets to travel
■ From 3.5 pH, very
down to the nucleus
acidic. But during
insemination, there
SPERM TRANSPORT IN THE FEMALE
REPRODUCTIVE TRACT is a buffering
capacity of the
semen so it reverts
back fast to pH of
7.2
○ Thick cervical mucus
■ If insemination
takes place during
ovulation, because
of hormonal
changes usually, the
mucus here is more
● The process of deposition of sperm watery in its
into the female reproductive tract is consistency which
insemination. helps the swimming
● Site of insemination in common movement, that
mammals is in the upper vaginal lessens the barrier
canal (number 1 in the image difficulty of the
above) sperm swimming
● But in the case of rodents like through the cervical
mice, the site of deposition is in the mucus
uterus
 ○ Wide uterus TRANSPORT OF THE
■ Sperms travel the FERTILIZED OOCYTE
muscular uterus
■ Considering their
size, they are like
crossing the Pacific
Ocean
○ Utero-tubal junction
■ In common
mammals, this is
not much of a ● If and when the oocyte got
problem; not very fertilized forms the zygote on the
constricted unlike first and second day, then it
that of rodents undergoes cleavage
● Final destination of the sperm for ● At around Day 3-4, 4-cell stage
fertilization is the ampulla of the ● At around Day 8, 8-cell stage
fallopian tube. But then again, ● A week after, Day 6-7, it is already
once here, it gets to pass through in the uterus and gets implanted
the blocks to polyspermy around this time (Day 8-9) as a
blastocyst, at the blastula stage
SPERM UNDERGO CAPACITATION
● Sperm undergo capacitation along UNION OF GAMETES IN MAMMALS
its journey in the male reproductive ● In mammals, the acrosome cap
tract releases hyaluronidase and it
● Capacitation is a period of digests the corona radiate
conditioning for the sperms as they
pass through the female
reproductive tract such as the ff.
○ Removal of the
glycoprotein protein coat
and other semenar proteins
that cover the acrosome
cap
● The length of capacitation may
vary from species to species.
○ In the case of mouse, it is
typically one hour.
○ For rabbits, 6 hours.
○ For humans, about 5-8 ● In the image above, corona radiata
hous. gets digested.
● For in vitro fertilization, where ● Male sperm had penetrated the
sperms do not travel through the zona pellucida
female reproductive tract, ○ Zona pellucida contains
collected sperms are incubated the sperm specific
in a capacitate (or capactiating receptor
medium or capacitation medium) ● Finally sperm penetrating into the
to take off the glycoprotein coat ooctye’s cytoplasm
covering the acrosome cap
 ○ The egg had penetrated ACCOMPLISHMENTS
with this part of the OF FERTILIZATION
sperm surrounded by the ● Completion of the second meiotic
mitochondria (refer to the block, metaphase II arrest (of the
image above). oocyte) is lifted off
○ The tail is left outside ● Restores normal diploid number of
● Penetrating the zona pellucida chromosomes (23 + 23 = 46
involves other hydrolytic enzymes chromosomes)
like acrosin. ● Sex of the future embryo is
○ Acrosin - hydrolytic determined
enzyme that helps in ● Genetic variation
digesting the zona pellucida ● Metabolic activation of the egg
○ Ready to embark the
X AND Y SPERMS: fast-paced action of
SEX DETERMINATION cleavage division

FERTILIZATION IN AMPHIBIANS

● In the last meeting, we mentioned

● Image above shows sperm with an
progesterone is the hormone
X-chromosome and another with a
that lifts off diplotene block once
Y-chromosome, the “female” and
the frog has reached the sexually
“male” sperm respectively
mature stage
● 23 X plus 23 X, where female
● But the oocytes are arrested at
sperm fertilizes oocyte results to
metaphase II
female embryo
● At fertilization, metaphase II
○ 46 chromosomes (23 paits)
arrest is lifted off. Calcium ions
○ 44 autosomes (XX)
released, calcium binds with
○ 2 sex chromosomes (XX)
calmodulin and the complex
○ Sex of embryo: Female
activates Calmodulin-Dependent
● 23 Y plus 23 X, where male sperm
Protein Kinase II (Cam-PKII)
fertlized oocyte results to male
● Cam-PKII is activated and
embryo
degrades cytostatic factors
○ 46 chromosomes (23 paits)
(CSF).
○ 44 autosomes (XX)
● CSF is degraded, meiosis II
○ 2 sex chromosomes (XY)
proceeds to complete
○ Sex of embryo: Male
● Fertilization is completed, you can
see the polar bodies released
 LECTURE 4: CLEAVAGE TO of the house and have it
BLASTULATION positioned in a specific
corner of the house; the
CLEAVAGE same phenomenon in the
embryonic development
● 2. Generates many copies of the
zygotic genome

● One must show the greatest

respect towards anything that
increases exponentially no matter
how small… - Garrett Hardin
(1968) ○ The cleaving embryo
● Fertilization is the wake up call for becomes a progeny of
the fertilized oocyte to embark on cells, each having their own
the next phase of development – copy of the zygotic genome
fast paced action: cleavage ○ Because cell division is
division ordinary mitotic division, all
of the resulting cells,
FUNCTIONS OF CLEAVAGE DIVISION blastomeres or cleavage
cells, are genetically equal;
they have their own copy of
the zygotic genome
● 1. Generates large number of ○ This is very essential
cells because having their own
copy of the zygotic genome
will allow each of them or
give the individual
○ From a newly fertilized blastomeres the freedom to
zygote, a single-celled express the subset of the
zygote, it becomes genome, and them having
two-celled, four cells, eight that would guide them onto
cells, and so on, until it their developmental
becomes a multicellular pathway to follow
embryonic phase ● 3. Segregates cytoplasmic
○ These are embryonic cells components into different
that are capable of moving blastomeres (maternal
relative to its other position cytoplasmic factors)
○ By analogy, if you’d like to ○ These cytoplasmic
build a house using one big components are that of the
boulder of stone, like the preformed guidelines –the
newly fertilized zygote, it maternal cytoplasmic
must be cut into smaller factors
○ bits so that it is easier to ○ In an oocyte, after
build the specific structures oogenesis (completion of
 an oogenesis), when it is ● It can influence
not yet activated or which subsets
fertilized, the cytoplasmic of the gene can
components are in the form be activated,
of maternal mRNAs can be
(silenced maternal mRNAs) deactivated,
ready to be translated into and so, by
proteins once activated by doing this, it
fertilization can lead the
blastomere or
the cleavage
cell onto its
developmental
pathway, or on
which to
embark into
○ A cleaving embryo with ● 4. Increases the ratio of nuclear
different varieties of volume: cytoplasmic volume
cytoplasmic components, ○ This is important because
represented on the image this is the universal aspect
as colored circles, turn into of cell division
a progeny of cells (different ○ Cell division increases the
blastomeres which ratio of nuclear volume to
acquired different cytoplasmic volume
combinations of the
cytoplasmic components
■ An unequal
distribution of
○ A zygote has a different
cytoplasmic
nuclear volume to
components
cytoplasmic volume as that
■ The different
of the next cells in the
combinations of
series of cell division
cytoplasmic
○ The cytoplasmic volume is
components in the
gradually distributed into
progeny of cells
smaller cells, so increasing
provide new
the nuclear volume to
cytoplasmic
cytoplasmic volume, or the
environment to the
other way around,
cells’ nucleus
decreasing the cytoplasmic
■ A cytoplasmic
volume to nuclear volume
component has an
○ The cytoplasm is
influence on the
distributed into smaller and
activity of the cell’s
smaller cells until a balance
nucleus; it is called
is achieved
cytoplasmic
○ Universal aspect of
influence on
cleavage
nuclear activity
 ○ At a certain level, pace of HOW DOES MPF WORK?
cell division slows ● Subunits must be bound together
■ At some point, a as a complex
balanced volume, a ○ The subunits, the large and
“set point” is already small must be bound
attained; by then, together as a complex
the pace of cell ○ Then the mpf structure or
division slows down substance is active
■ This set value or set ○ MPF is periodically active
level of nuclear - It is used by mitosis
volume to and gets degraded.
cytoplasmic volume Once degraded, it
ratio is also has to be
species-specific synthesized to fuel
mitosis, it is used
FERTILIZATION (ZYGOTE) → up, it is degraded, it
CLEAVAGE hs to be
● The transition from a fertilized synthesized again
stage (the zygote) into a cleavage to fuel mitosis.
stage requires a fuel molecule, ○ Cyclin B is broken down
MPF (mitosis promoting factor) during mitosis
● MPF is a factor or protein - Re-synthesized
translated from maternal mRNAs during S phase
○ Since the oocyte is already - When cyclin B
activated or fertilized, so broken down, the
the maternal mRNAs that cyclin B
used to be silenced were deactivated.
activated and translated - When cyclin B is
into specific proteins synthesized, it
● MPF has 2 subunits: Cyclin B and forms a complex
cdc2/cdk1 = cyclin dependent with cdc2, thus the
kinase kinase is now
○ Cyclin B is a large subunit activated.
○ Cyclin dependent kinase 1 - There is a series of
(cdk1), which is degradation
homologous to cell division synthesis, activation
control 2 (cdc2), is a small and deactivation
subunit
● The MPF, with cyclin B (large
subunit) and cdc2 (small subunit),
is the fuel for the transition from
the zygote phase to the cleavage
phase
 CYCLING OF CELLS ➔ Spindle fibers
● Synthesis and degradation of promotes the
Cyclin B organizations of
● Activation / deactivation of cd mitotic spindle fibers
kinases ➔ Spindle fibers are
necessary for
CYCLIN B-CDC2 COMPLEX (ACTIVE mitosis to occur
MPF) ➔ Can now access
chromosomes to the
centromere
● Results to: onset of mitosis that
leads to the series of cleavage
divisions

SOURCE OF MPF IN THE EARLY

EMBRYO
● Cyclin B is translated from
maternal RNAs
● In an active form, then it ○ All the other necessary
phosphorylates target proteins components for cell cycling
inside the blastomere or cleavage are also maternally derived
cells - Like the cdc 2 or the
● Kinase is an enzyme that cdc k 1 or the
phosphorylase a target proteins cycline dependent
● What are the target protein that kinases
gets phosphorylated? ○ The cell is independent of
- Histone proteins the nuclear genome for
➔ Are phosphorylated, several cell divisions
they make the ● Regulators of Cyclin B reside in the
chromatin cytoplasm of the egg
condensation
➔ Chromosomes that DEPLETION OF THE
are condense make MATERNALLY-LOADED CYCLIN B AND
a distinct OTHER CELL CYCLE FACTORS
chromosomes
- Proteins in the nuclear
membrane
➔ It undergoes
nuclear membrane
depolarization
➔ Results to the
breakdown of the
nuclear membrane
➔ Degradation of the
nuclear membrane ● Zygotic transcription must begin
- Regulatory cytoplasmic ○ Transition from maternal to
proteins zygotic genome
transcription
● Beginning of new phenomena: - Loss of synchronous
1. Addition of gap phases division
2. Loss of synchronous - Transcription of new
division mRNAs
3. Transcription of new - Synthesis of different
mRNAs regulators in the new cells
4. Synthesis of different
regulators in the new cells WHEN DOES THE EMBRYO STOPS
● When they are all depleted, they DIVIDING?
need to be replenished and this is ● A new balance in the nuclear:
the time when there is a switch cytoplasmic volume ratio
from maternal genome ○ If the embryo stops
transcription into zygotic dividing, it embarks to the
transcription. zygotic transcription.
○ Zygotic transcription ● Depletion in the maternal reserve
- Where the maternal of mRNAs and proteins
contribution ● Result to:
participates ○ Zygotic genome activation
- Zygote nucleus: a (ZGA); new gene
result of the fusion transcription in the zygote
of the male and the nucleus
female ○ Synchronous cell division is
● With the transition into the zygotic lost (because
genome transcription new asynchronous)
phenomena are now taking place:
- Addition of gap phases THE TIMING OF CLEAVAGE DIVISION
➔ The usual s phase,
m phase, so the
cells will now start
to grow
➔ Because this is a
new genome
transcription, new
mRNA are
transcribed, new ● Left pic: this is late embryonic
mRNA is coding for stage going on to a mature cell
new proteins, which cycle (asynchronous and slow)
means new and ● Right pic: this early embryonic cell
different regulators cycle (synchronous and fast)
in the cell ● You can easily spot the differences
➔ Thus this will have in the late embryonic cell, it has the
an allover effect on characteristic cell cycle of a mature
affecting the or a typical somatic cell: there is
synchronous cell the m phase and the s phase, and
division. From the inclusion of the g phases or
synchronous it will growth phases.
now become
asynchronous
 ● In the early embryonic cell cycle, it
is by phasic of m phase and s
phase. So this is very fast, there is
still no growth. Gradually, it starts
to have the G2 phase, but it occurs
in a short while

CELL CYCLE OF A TYPICAL MATURE

SOMATIC CELL:
● Includes M (mitosis) phase, S
phase (DNA synthesis) two gap
phases, G1 and G2 b) Cell cycle in a typical somatic cell
○ (when the cells grow, or late embryonic cell stage
produce mRNAs and ● We can see here new players and
proteins necessary for cell regulators: cyclins (cyclin E, D, A)
cycle) and the addition of gap phases.
● There is no growth.
CELL CYCLE OF EARLY CLEAVAGE
DIVISIONS: In normal cell cycles:
● No gap phases ● Presence of the cyclin-cdc2
● Cells cycle between M phase and complex promotes the transition
S phase (biphasic) from G2 to M phase
○ If G2 would occur, it is short ● Slow, asynchronous
○ There is no growth
In early cleavages:
WHAT CONTROLS THE CELL CYCLE? ● The cyclin-cdc2 complex promotes
● MPF - Mitosis Promoting Factor
the transition from S to M phase
● M-phase Promoting Factor
● Fast
○ They must be in complex to
be active
FACTORS INFLUENCING CLEAVAGE
PATTERN
1. MATERNAL CYTOPLASMIC
FACTORS
● Angle of mitotic spindle is believed
to be the site of the 1st cleavage
furrow

a) Cell cycle in a cleavage stage

embryo

● Invariably, cytokinesis occurs in a

plane perpendicular to the axis of
mitotic spindle
 ● Maternal gene products may orient ○ Proceeds from the animal
mitotic spindles hemisphere (animal pole)
going down to the vegetal
hemisphere (vegetal pole)
○ Can be:
■ Radial
■ Rotational
■ Spiral
■ Bilateral
○ Holoblastic radial and
rotational are
characteristics of
2. AMOUNT AND DISTRIBUTION OF vertebrates.
YOLK ○ Holoblastic spiral and
● Types of egg as to yolk content:
bilateral are characteristics
○ Isolecithal/ oligolecithal
of invertebrates
■ Contain small
amount of yolk but
evenly distributed
○ Mesolecithal
■ Contain moderate ● Meroblastic- telolecithal oocytes
amount of yolk ○ Can be:
slightly ■ Discoidal-
concentrated in one characteristic of
pole vertebrates
○ Telolecithal ■ Superficial/
■ Dense amount of meroistic-
yolk heavily invertebrates
concentrated on (terrestrial and
one pole aquatic arthropods)
○ Centrolecithal
■ Concentration of
yolk at the center of
the oocyte
● Isolecithal, mesolecithal, and
telolecithal are all characteristics of
vertebrates
● Centrolecithal- characteristic of
aquatic and terrestrial arthropods
like the insects

PATTERNS OF CLEAVAGE &

CLEAVAGE SYMMETRY
Cleavage can be:
● Holoblastic- isolecithal and
mesolecithal oocytes
○ Complete
● The frog is classified as slightly ● Amphibians and amphioxus are
telolecithal or mesolecithal. closely similar
● Birds, reptiles, and fishes have ● For amphibians, the first plane of
telolecithal type of oocytes cleavage is holoblastic complete
● Mesolecithal= holoblastic cleavage cleavage, proceeds from the
○ Total cleavage from top to animal hemisphere going down to
bottom, from animal the vegetal hemisphere. But
hemisphere to the vegetal because amphibian egg is slightly
hemisphere telolecithal, there is thick amount of
○ Cleavage is complete yolk in the vegetal hemisphere so
○ Involves the whole oocyte cleavage division slows down and
● Telolecithal= discoidal cleavage before it gets completed, the
○ Meroblastic means second plane of cleavage is
incomplete already taking place at right angle
○ Cleavage involves only the to the first one. The second plane
active cytoplasm or the gives rise into a 2-cell stage. The
blastodisc, it does not third plane of cleavage is
involve the other horizontal, cuts the oocyte
components of the oocyte horizontally, more displaced
○ The yolk doesn’t cleave towards the animal pole than that
○ Cleavage happens only in of the vegetal pole because of the
the blastodisc thick yolk content.
○ Characteristic of birds, ● The next cleavage division is
reptiles, and fish another horizontal and it cleaves
the animal hemisphere faster than
HOLOBLASTIC RADIAL this, resulting into tiers of cells and
smaller group of cells: 1st, 2nd, 3rd
tier.
● Micromeres- smaller blastomeres
at the top portion
● Macromeres- big blastomeres at
the bottom portion

CLEAVAGE TO BLASTULA
AMPHIBIAN CLEAVAGE TO BLASTULA
DIFFERENCE BETWEEN FERTILIZED
OOCYTE AND UNFERTILIZED OOCYTE

● One can easily spot the difference

between fertilized and
unfertilized oocytes because of
the presence of a gray crescent
in a fertilized oocyte.

● At around 128 cells, the

developing/cleaving embryo starts
to form an internal cavity called
blastocoel.
○ By this time, the embryo
has now embarked into the
blastula stage.
● In amphibians, there is a very
distinct transition from late
cleavage to blastula.
○ The transition is the morula
○ Formation of the gray
stage. It is the solid ball of
crescent opposite to the
a cell.
entry point of the sperm.
○ Gradually that solid ball of
○ Formation of the gray
cells inside, they start to
crescent during fertilization
pump secreted fluid and the
is due to cortical reaction
pressure created inside will
causing the mass
push the blastomeres to the
movement of pigment
side, creating the cavity.
granules towards the sperm
The cavity is the blastocoel.
entry leaving the gray
When the blastocoel is
crescent area somehow
formed, it is now at a
devoid of the pigment
blastula stage.
granules so it appears
● At the end of the blastula stage,
lighter in color.
there are three distinct regions
○ The gray crescent region is
recognized:
formed in intermediate
○ Gray crescent region
shade between the
○ Vegetal region with yoke
vegetal pole and the
○ Pigmented with the
animal pole.
pigment granules
 the other hand, cell
CD is still starting to
divide (can be
horizontally or
vertically).
■ As a result of the
completion of
division of cell CD is
the formation of a
3-cell stage.
■ In a 4-cell stage, it
MAMMALIAN CLEAVAGE
can divide
horizontally first.
■ Transient window
time for formation of
an odd numbered
cell
○ The cells do not always
proceed regularly from 2
● In mammalian cleavage, the type cell stage > 4 cell stage > 8
of cleavage is holoblastic cell stage.
rotational. It is in rotational mode. ■ There is a quick
○ "Ikaw muna and then ako transition between
then ikaw ulit, ako naman, 2 to 4. Transient
'yung isa naman, siya time before it
naman…" becomes 4, short
● In mammalian cleavage: while it becomes
○ Cleavage is characterized 3-cell stage.
as extremely slow; (during ○ Why is cleavage slow in
its journey down the mammals?
oviduct. ■ Maybe because it is
■ But it is still the an evolutionary
usual biphasic that adaptation.
has mitosis and ■ The embryo is
synthesis phase but secured in the
in this case, it is uterus. Unlike when
slow. However, fertilization is
there is no growth, external, where the
the same the same cleaving embryo is
as the early early exposed in the
embryonic cell of embryonic
amphibians. environment making
○ Unusually asynchronous it prone to
because it is rotational. predation.
■ Cell AB divides first ● It may be a
horizontally before it predation at a
gets completed. On stage that an
amphibian is still
 helpless. Thus (3) Cavitation: Formation of the
cleavage blastocoele.
should be ○ The polarized inside cells
faster in will form the inner cell
amphibians mass (ICM).
because the ○ The cells polarized
developing outside will form the
embryo is trophoblast or
exposed in the trophectoderm.
aquatic
environment.
● In contrast to
the developing
embryo in
mammals is
very secured in ● This is the blastocysts stage in
the uterus. mammals (32-64 cell stage)

MAMMALIAN CLEAVAGE TO
BLASTULA
IMPORTANT EVENTS DURING THE
8-CELL STAGE

Figure: Blastocyst is still within the zona

pellucida (upper image), Later stage of
blastocyst/hatched (lower image) - zona
pellucida is removed, ready for infantation
(1) Compaction: At the 8-cell stage, it
undergoes compaction.
● ICM has pluripotent stem cells
○ Here, the points of
because of pluripotency gene markers
contact are not yet
(Oct4, Nanog, Stat 3)
maximized.

(2) Polarization: Compaction is

followed by polarization at around
the 16-cell stage.
○ Around 14-19 outside
cells are polarized.
○ Around 2-7 inside cells are Figure: Holoblastic Spiral, characteristic of
polarized vertebrates
 AVIAN CLEAVAGE TO BLASTULA

Figure: Aftermath of the creation of the

subgerminal cavity. The cells left on top
start to fall off and these cells that fell will
form another layer (hypoblast). Those left
on top would form the epiblast.

● Hypoblast splits the “laminies” (can’t

Figure: Top-view of cleavage. make out the word) from the epiblast,
● 1st plane of cleavage - involves the blastocoel is formed at the
only the blastodisc expense of the germinal cavity.
● 2nd plane of cleavage - cuts ● When the epiblast and hypoblast starts
through the 1st plane of cleavage forming, that is the signal for the
at a right angle transition of blastula to gastrula
● 3rd plane of cleavage - it may not ● The cavity between the epiblast and
be asynchronous (the almost hypoblast area will appear
c-shaped ones) light/translucent under the microscope
● 4th plane of cleavage - and this will become known as the
circumferential and as a result it area pellucida
forms a centrally located and ● The cells on the sides will still remain
peripherally located cells. in close contact with the underlying
○ Circumferential cleavage is yolk, and when viewed under the
followed by another microscope it will appear dark and is
circumferential cleavage, known as the area opaca
and goes on, and as a ● The embryo will form on the area
result there is radial pellucida
expansion of the
blastomere AT THE END OF CLEAVAGE:
BLASTULA

Figure: Side-view cut sagittally.

● Subgerminal cavity forms because
the blastoderm will start to absorb
the fluid that was once in the Figure: Amphibian
subgerminal cavity area.
 DEVELOPMENTAL PROPERTIES OF
CLEAVING EMBRYOS
● Regulative development
○ Development of embryonic
cells based on cell-cell
interaction
● Totipotent
○ The cells have “Total
PotentiThe end of cleavage
Figure: Avian
for Amphibians, Aves, and
Mammals are represented
byal”

Figure: Mammalian

Figure: Summary

● The end of cleavage for Amphibians,

Aves, and Mammals are represented
by
○ Amphibians: Ball of cells
○ Aves: Stack of cell layers
○ Mammals: 2 distinct cell
populations
DEVBIOL: Developmental Biology
Lecture 5-7: Gastrulation, Neurulation, and Body Plan Formation
Dr. Gliceria Ramos
Term 2 AY 2022-2023
Transcribed: Berana, Capistrano, Dalapo, Delos Angeles, Juachon, Narbonita, Paclibar, Rabang, Sartorio, Silao, Soliman

● When the blastodisc has started

cleaving, it forms the blastoderm.
Outline
● The formation of the epiblast and
1. Gastrulation hypoblast is already the start of the
2. Neurulation early phase of gastrula. So there is
3. Body Axis Formation a swift transition from the last part
of blastulation to early gastrulation
RECAP ON BLASTULATION
AMPHIBIAN BLASTULATION AVIAN BLASTULATION:
BLASTODERM, SUBGERMINAL
CAVITY, EPIBLAST AND HYPOCLAST,
& BLASTOCOEL

● For amphibian development, at the

end of the blastulation process, the
resulting embryonic phase is a
hollow ball of cells with a cavity ● In blastulation, there is the
● The cavity is called as the formation of the blastoderm (it
blastocoel looks like the black spider in the
● At the end of blastulation, there is image above)
already the establishment of the ● Imagine that you are looking at the
future dorsal side of the embryo. top view of the oocyte, this is the
And it resides into the previous yellow yolk (yellow part) and the
position occupied by the gray blastoderm (black part)
crescent area ● Blastoderm cells underneath
● Here in the image above, start to fall off. When they fall off,
approximately at the marginal zone they coalesce and form this layer
(MZ) is the future dorsal side of the called as the hypoblast
embryo. ● When the hypoblast and epiblast
split, they form the blastocoel
AVIAN BLASTULATION ● Blastocoel is the cavity between
epiblast and hypoclast. But before
the blastocoel was there, there is
already a formation of the
subgerminal cavity.
● Subgerminal cavity is the cavity
underneath the blastoderm. It is
formed when the blastoderm are
● At the end of blastulation in avians,
increasing in number, they then
there is a resulting stack of cell
start to absorb the fluid in the yolk
layers: this is the epiblast and
underneath. This area then
hypoblast. But before this, there is
becomes devoid of fluid and
a formation of a massive cells that
becomes the subgerminal cavity.
get formed where blastodisc used
This is a characteristic of
to be.
blastulation
● But subgerminal cavity is not ● Towards late gastrula, the dorsal
the actual blastocoel. The lip of the blastopore now becomes
blastocoel is yet to form with the very distinct and well-formed
splitting of the blastoderm into surrunding the completely formed
epiblast and hypoblast opening, the blastopore.
● Similar with amphibians, avians ● Because of the movement of cells,
has established polarity early. laying down the three embryonic
How? The site where the first germ layers which are called as
falling off cells to form the the primary germ layers:
hypoblast appears, that will ○ Ectoderm - outermost layer
designate the future posterior ○ Endoderm - innermost layer
end of the embryo. ○ Mesoderm - layer
sandwiched in-between
MAMMALIAN BLASTULATION
GASTRULATION
● From “gaster” which is the Latin
word for stomach
● In the case of mammalian ● The main goal of gastrulation is to
blastulation, the end of the process establish the precursor of the
is 2 distinct populations of cells digestive gut called as the
generated: ICM or inner cell primitive gut or archenteron
mass and the tropoblast or (arche + enteron = primitive + gut)
trophectoderm + the blastocoel ● Laying down the primitive gut
● The embryo proper has yet to form starts with establishing the dorsal
in the ICM. At the end of lip of blastopore and initiating the
blastulation, polarity is not yet formation of the blastopore or the
established in mammalian opening
development
● It is the most labile in terms of MAIN GOALS OF GASTRULATION
establishing body polarity
● Then gastrulation follows

LECTURE 5: GASTRULATION

1. Laying down the primitive gut -

the precursor of the digestive gut
● This is gastrulation. marked by the formation of a
● The most distinguishing feature, blastopore
the most remarkable ● In the image above, you
developmental landmark of early see arrows indicating cell
gastrula, is the formation of the movements
dorsal lip of the blastopore 2. Cell movements and
● Coming with that is the initial rearrangements (morphogenetic
formation of laying down the movements) - Prelude to
blastopore, laying down the morphogenesis (gradually
opening. generating the over-all body plan)
 ● With these movements, ● Mesoderm, this is the
embryonic cells are mesenchymal type
gradually acquiring (loosely-arranged; plenty of ECM)
molecular cues, collectively ○ This will remind you of
called positional neural crest cells and head
information mesenchymal cells in the
3. Starting to acquire positional 4mm and 10mm frog
formation - cells acquire embryo, surrounding the
molecular cues that will tell an neural tube
embryonic cell where it is relative ● Their behavior, because they are
to the body axis loosely-arranged they can…
4. Forming the three germ layers - ○ Migrate - migrate
Ectoderm, endoderm, mesoderm individually one after the
other with a mass
THE THREE GERM LAYERS movement
● The 3 primary germ layers vary in ○ Intercalate - interdigitation
characteristics -It’s like… open your
fingers and let your palms
ECTODERM AND ENDODERM move at the midline closer
and closer. (So parang
maghoholding hands
kamay mo pero di mo
isasara)
○ Ingress - movement of
cells from a layer of flat
sheet of cells moving to a
● Ectoderm and endoderm, these cavity
are the epithelial type (flat sheet;
closely-packed cells; little amount
of ECM)
● Their behavior, because they are
closely-packed, is that they can…
○ Spread - when they divide
○ Roll - easily because they
are flat and thin ● Now, the epithelial type and
○ Fold mesenchyma type of the ectoderm,
○ Buckle endoderm, and mesoderm… they
○ Bend have the ability during embryonic
stage to undergo transition (with
MESODERM each other), called as the
epithelial-mesenchymal
transition
○ As a result of this transition,
these behaviors and
cellular activities occur in
various combinations
 DIFFERENT MORPHOGENETIC ■ CAMs and SAMs
MOVEMENTS are
● These cell movements and cell morphoregulatory
activities occur in various molecules of not
combinations only gastrulatio but
● Regulated by gene activity also of embryonic
● Genes are activated so there are development
gene products, protein products ■ CAMs = Cell
that can facilitate the onset of Adhesion Molecules
morphogenetic movements and ■ SAMs = Substrate
cellular activities Adhesion Molecules
★ Example: ● Combinations:
○ Expression of genes coding ○ When there is a change in
for cyclins and cyclin the rate of cell division, it
dependent (cd) kinases will eventually cause the
■ In cleavage division, cell to spread out called
cyclins (cyclin B and epiboly (spreading out of
CDK1) are players cells caused by rapid cell
in the cell cycle – division)
they get to regulate ○ Involution would require the
cell division. migration of cells – mass
Change in the rate movement of cells
of cell division
requires the activity EPITHELIAL TYPE
of the gene coding ● Ectoderm and Endoderm
for cyclins & cd ● Can undertake major
kinases. morphogenetic movements
○ Expression of regulatory ○ Invagination
genes for activation of other ○ Epiboly
genes ○ Involution
■ Regulatory genes: ○ Convergent Extension
genes that code for ○ Demlamination
transcription factors ○ Passive Movement
■ Transcription
factors: factors that MESENCHYMAL TYPE
can regulate the ● Can undertake changes in cell
activity of other behaviors or activities
genes such as to ○ Migration
upregulate or ○ Intercalation
downregulate, ○ Change in cell shape
activate or ○ Change in the degree of
deactivate the cell adhesiveness
activity of a certain ○ Change in the rate of cell
gene division
○ Selector genes (example of ○ Ingression
regulatory gene) control the ○ Apoptosis (programmed
expression of CAMs and cell death)
SAMs genes
 TYPES OF MORPHOGENETIC ● Driven by mitosis – when cells
MOVEMENTS divide, they become thinner, they
1. INVAGINATION spread out
● Can involve a monolayer (i.e. a
sheet of cells one cell layer thick)
in which case the individual cells
must undergo change in shape
○ If the cell is double layered,
● Inpocketing of cells it becomes monolayered
● Epithelial sheet bends inward to
form an inpocketing
● This takes place in the formation of
the dorsal lip of blastopore
(initiating blastopore formation)
● Only few cells are involved –
localized movement of cells (don’t
actually move inside)
● The classical example in
● To illustrate, imagine that the figure
textbooks: when you pinch your
above is an amphibian oocyte, the
thumb in a bloated balloon, it
upper half is the animal
creates a localized depression and
hemisphere, and lower half is the
invagination
vegetal pole
○ This is how invagination
○ In the animal hemisphere,
operates during embryonic
there are smaller
development
blastomeres called
micromeres
○ In the vegetal pole, there
are macromeres (bigger
cells) because of impeded
cell division due to the thick
yolk
○ Faster rate of cell division
in the animal pole (smaller
● In the picture, a depression can be quantity of yolk) – cells
seen, initiating the formation of the divide smaller and smaller
blastopore, and the shaping of the and they spread
dorsal lip.
3. INVOLUTION
2. EPIBOLY

● Spreading of cells ● In contrast with invagination

● A sheet of cells spreads by ● Mass movement of cells rolling
thinning, that is, the sheet thins, inward to form an underlying layer
while its overall surface area via bulk movement of cells
increases in the other two ● The photo above represents the
directions dorsal lip of blastopore
 ● Cells converge by intercalating
perpendicular to the axis of
extension
○ Intercalation can be top to
bottom
○ Result would be the overall
extension of the tissue in a
preferred direction
● The cells converge and change the
dimensions of a sheet of cells (i.e.
● Illustrated during gastrulation – dimension of width or dimension of
cells are moving fast, epiboly is length)
taking place (animal pole), there ○ Dimension of width: it
are nowhere to go because of the elongates and becomes
invagination in the red portion. narrower
Hence, the cells have the tendency ● Decrease in one dimension,
to move inward increase in the other dimension
● Classic example is the eating of ○ I.e. Narrower but longer,
halo-halo in chowking with the narrower but thicker
ingredients on top. When you push ● Coupled with convergent
the spoon on one side of the tall thickening
glass, all ingredients on top will
move downward to the bottom of
the tall glass.

CONVERGENT EXTENSION

● Two or more rows of cells

intercalate, but the intercalation is
highly directional
○ This is a result of
intercalation of cells ● The process by which tissue
○ Intercalation is axis of elongates along the
extension – from top and anterior-posterior (AP) axis, and
from bottom, will meet to becomes narrower along the
the midline medio-lateral (ML) axis, is called
convergent extension
● Frog or chick development
○ In the photo above is a
blastopore, cells are
involuting moving inward
(anteriorly-posteriorly; very
thick)
○ As the cells move,
eventually becoming longer
but narrower
 INTERCALATION CAN BE ● What is shown here is the ICM
LATERAL OR RADIAL (inner cell mass), forming an
epiblast and a hypoblast
● Delamination is splitting of a layer
of cells, but not a literal split
● Layers split by means of cells
falling off from the thick layer

5. PASSIVE MOVEMENT OF CELLS

● Intercalation can result to AND MIGRATION OF CELLS
convergent extension and
convergent thickening

CONVERGENT THICKENING

MIGRATION STRATEGIES
● Involves tissue becoming thicker in ● Ameboid migration
the direction at right angles to the ○ Just like what happens in
convergent extension the primordial germ cells
● Sketch showing how the ○ At first nandoon sila sa
convergence force (y-axis) epiblast embryonal area
increases through gastrulation, and then they migrate by
then plateaus (during early amoeboid motion.
neurulation) before increasing ○ The primordial germ cells
again (during late neurulation). go down to the yolk sac
○ There is an increase in endoderm, and then up to
movement in late the allantois passing
neurulation through the dorsal
mesentery, amoeboid
4. DELAMINATION motion pa rin, until they get
to the genital ridges

Mesenchymal migration
● Happens involving neural crest
cells and the head mesenchyme
● Neural crest cells are powerful
● Formation of the epiblast and migratory cells that form the
hypoblast different ganglia of the body
Collective migration
● Happens with gastrulating cells,
like those involuting from outside
moving inward

GENES AND MORPHOGENETIC

MOVEMENTS
● These morphogenetic movements,
these cellular activities, are
regulated by gene activity
● Genes activated resulted to the ● In the above above, it shows a
formation of gene products that are sagittal side of a gastrulating chick
involved in these movements embryo. This is the epiblast on top
● For example: actin cytoskeleton, of the hypoblast. Blastocoel is
integrins, and adherins junctions between epiblast and hypoblast
are all components of the CAMs ● These ingressing cells colonizing
(cell adhesion molecules) and the blastocoel or the cavity, this is
SAMs (substrate adhesion ingression
molecules)
● Their expression or presence at a CHANGE IN
specific age of the embryo at CELL SHAPE AND POSITION
specific locations is via regulation
of gene activity coding for these
proteins

INTERCALATION

● Cells can undergo change in cell

shape and position
● “Naulit ko lang ata ito” sabi ni miss. ● Observed in the blastocyst of the
● Intercalation results to convergent mammalian embryo
extension and convergent ● During the 16-cell stage, cells
thickening undergo polarization
● There are cells that are polarized
INGRESSION inside, some are outside
● Those polarized inside, they get to
form the ICM (or inner cell mass)
● Those polarized outside, they form
the trophoblast or the
● Cells leave an epithelial sheet by
trophoectoderm
transforming into freely migrating
● So they change position,
mesenchyme cells into a cavity
rearrangement. Some of them
● Epithelial sheet of cells going into a
goes changes in cell shape as well
cavity
 CHANGE IN CELL ADHESIVENESS

● In the image above, this is a

clearer illustration of chage in cell
shape: formation of bottle-shaped
cells in the formation of amphibian ● Closely-packed, very adhesive with
blastopore each other and eventually they
● These (bottle cells) are epithelial acquire different degrees of
type of cells, and epithelial tissue adhesiveness from the cellular
always have plasma membranes level to the tissue level to the
or basal membranes. Also known embryonic level
as basement membranes.
● Apical surface of the bottle cells
are free or unattached
● When these cells change their cell
shape, their apical surfaces
elongate and pull along with them
the basement membrane. When
they do so, they form a concave
structure, creating this cavity called
the blastopore

APOPTOSIS

Refer to the image above

● Cellular Level: Interface-specific
localization of cadherin and the
actomyosin cortex determine the
shape and the strength of the
adhesion contact between two
● Programmed-cell death
cells
● In the image above, it shows digits
○ Here at cell level, it involves
(fingers) with web between them
CAMs, cell-to-cell contact.
became gone bcause of
Example of a CAM here are
apoptsosis
cadherins with the help of
actomyosin cortex that
facilitate this degree of
adhesiveness
 ● Embryonic Level: The interaction
between tissues, forming at
different phases of development
and characterized by different
cortical and adhesive properties,
controls correct germ layers
formation during gastrulation
○ Some can have a greater
degree of adhesiveness,
some lower
○ Nagvavary na ang
adhesiveness
● Cadherins are cell adhesion
molecules (CAMs)

Refer to the image above CAMs and SAMs

● Tissue Level: Cortex tension and ● CAMs (cell adhesion molecules)
cell-cell adhesion determine the facilitate cell to cell contact
shape of multicellular aggregates ○ Example: compaction of
and the sorting order in heterotypic blastomers at 8-cell stage
aggregates in mammalian blastocyst
○ Cells are closely-packed ○ At 8-cell stage there is
and this is facilitated by maximized point of cell-cell
cortex tension and cell-cell point contact in mammalian
contact here blastocyst
○ Cortex tension refers to ● CAMs also undergo dynamic
the plasma membrane expression patterns correlated
○ If cell is turgid or bloated, with cell fates.
like in plant cell wall, then ○ Examples:
the cells are more packed ■ Cells fated to be
together (dikit-dikit) epidermis they get
to express
E-cadherin
(epidermal cell
adhesion proteins)
■ Cells fated to be the
neural plate they get
to express a high
degree of
N-cadherin (neural
cell adhesion
proteins) genes
■ E-cadherin - high
expression in
prospective
epidermis
Refer to the image above
■ N-cadherin - high
expression in
 prospective neural ● So that gene products are there at
plate the right time and right place when
● SAMs (substrate adhesion needed by the developing
molecules) facilitate cell to ECM embryonic cells of the embryo
contact
○ Establishes structural MORPHOREGULATORY MOLECULES
integrity of cells with the
surrounding matrix
○ These cells attached to the
ECM, they provide contact
guidance to the migration of
cells. ECM ang parang
tulay na ginagapangan ng
mga cells
● CAMS and SAMs are both
● The process of morphoregulation
morphoregulatory molecules
(according to the concept
● CAMS and SAMs are both capable
proposed by Edelman) that
of reversible adhesion, quick
regulates morphogenesis during
attachment/detachment
development, adaptation and
● They attach then detach then
regeneration involves cellular
attach, because they are
programmes such as cell division,
morphoregalutory molecules. Kung
movement, adhesion and death
nasaan sila kailangan agad, kung
and is controlled by molecules.
nagawa na nila ang attachment,
● With CAMs, SAMs and JAMs,
they detach at attach naman to the
eventually they will have an effect
other ones
on the activation of intracellular
signaling cascades resulting to
SAMs
development morphogenesis.
● Establish structural relationship
● The coordinated expression and
between cells and the ECM
function of these morphoregulatory
● Provide contact guidance to
molecules:
migrating cells and ingressing cells
○ CAMS: cell adhesion
● Examples: laminin, fibronectin,
molecules
integrins (present in ECM)
○ SAMS: substrate adhesion
characteristic of connective tissues
molecules
○ JAMS: cell junctional
SPATIO-TEMPORAL
GENE EXPRESSION molecules provide an
● Embryonic cells synthesize essential link between
stage-dependent and genetic and epigenetic
region-spefific ECM components in mechanisms.
accord with cell activities ■ These molecules
● CAMs and SAMs are expressed at exert critical
specific times at a specific interactions at both
embryonic stage at a specific the cell surface and
location. This is spatio-temporal the cytoskeleton
gene expression and mediate their
effects through
 activation of ■ These cell
intracellular movements and
signalling changes in cell
cascades such as shape, are drivers
the for
p21ras/MAP-kinas morphogenesis.
e pathway. ● So the cells move, change shape,
and interact with other cells or new
ROLE OF CAMS AND SAMS IN neighboring cells. That means new
MORPHOREGULATORY CYCLES cell-to-cell interaction and new
intercellular signaling.
○ This can have feedback on
the selector genes on
whether to activate more
CAMs/SAMs genes.
○ Changes in cell shape can
also have feedback on
intracellular signaling.
● Selector genes: they code for ● Intracellular and intercellular
transcription factors. signaling by different mechanisms
○ These transcription factors can activate selector genes and
control the expression of the cycle distributed.
the CAMs and the SAMs. ● CAMs and SAMs are needed for
It depends on the stage of embryonic cell positioning.
development or embryonic
stage. AMPHIBIAN GASTRULATION
● At what specific region of the
embryo these CAMs and SAMs
are needed at a specific time?
○ CAMs and SAMs are
synthesized and deployed
into different parts and
regions of the embryo.
● After CAMs and SAMs got
synthesized, they undergo
cell-to-cell adhesion with other
cells.
● Next, interaction with the CAMs
and SAMs of other cells and other
cells extracellular matrix.
● Cell-to-cell adhesion interactions,
they now cause various cell
movements and may involve
changes in: *Please refer to this illustration
○ Cell shape ● The presence of the gray
○ Movements crescent region will determine the
○ Ingression site of the dorsal lip of the
○ Involution blastopore (DLB).
 INVAGINATION because there is a depression
here. So the tendency is for the
spreading cells to move inward.
This refers to involution.
● In amphibian gastrulation, at the
start, there is an orchestration of
the three morphogenetic
movements:
○ Invagination
○ Epiboly
○ Involution: the cells
● The dorsal lip of the blastopore moving inward can
formed because of the formation of intercalate with the
the bottle shape cells. ectoderm and others can
● There is invagination here also intercalate with endoderm
because of battle shape cells. and eventually cause the
elongation of the whole
EPIBOLY embryo in late gastrula.

● While there is invagination going

on here, the cells in the animal
hemisphere are undergoing mitotic
division. The cells are spreading
and thinning. This refers to
epiboly.

INVOLUTION

*Please refer to this illustration

● While cells here are spreading,
there is nowhere else to go
● Dorsal lip of blastopore (red blastopore with the
structure):this becomes the site residing chordamesoderm
of cell turnover. is called the chief cell
○ The involute cells keep organizer of the embryo.
coming in here. This is because of its
● The cells migrate from the powerful influence on
dorsal lip of the blastopore area neurulation
to the other side. The migration is
guided by a SAM called AVIAN GASTRULATION
fibronectin.
○ Fibronectin provides
contact guidance to the
migrating cells anteriorly.
○ These migrating cells will
designate the posterior
cell of the embryo
(blastopore). Conversely,
this will now form the
anterior axis. Thus, it is
gradually establishing the
body axis.

*Please refer to this illustration

● This is a fertilized egg
● This is the yellow yolk (oocyte or
egg cell).
● Lying on top of the yellow yolk
(oocyte or egg cell) is the
blastoderm. This has undergone
cleavage division.

● The last group of cells in the TOP VIEW OF THE YELLOW YOLK
vicinity of the dorsal lip of the (OOCYTE OR EGG CELL)
blastopore, they could no longer
enter there because it was already
full, they get to form the
chordamesoderm.
● The chordamesoderm has a
powerful influence on the
formation of the neural tube. ● The blastoderm is the black
○ Chordamesoderm is the structure.
pick-up point for ● Because there is a subgerminal
neurulation. cavity underneath when viewed
○ This is the reason why the under the microscope, the light
dorsal lip of the area is the area pellucida.
● On the sides are darker because IN AVIAN GASTRULATION…
this area is still in close contact ● Blastodisk cleaves after
with the underlying yolk, this is fertilization to form blastoderm.
the area opaca. ○ Once fertilization has taken
○ Opaca means opaque or place, the blastodisk starts
dark. to cleave to form the
● The light area occupied by the blastoderm.
blastoderm is called area ● Subgerminal cavity forms
pellucida. underneath the blastoderm.
○ Pellucida from the word ○ The blastoderm will start to
"lucid" which means absorb the fluid underneath
translucent. creating the subgerminal
cavity
TRANSVERSE VIEW OF THE YOLK ● The blastoderm eventually
(OOCYTE OR EGG CELL) delaminates forming the epiblast
and the primary hypoblast.
● Blastocoel forms at the expense
of the subgerminal cavity.
○ In between the formation of
the epiblast and the primary
● Splitting the epiblast to form the hypoblast is the formation
hypoblast. of blastocoel, the real
● Blastocoel is formed at the blastocoel.
expense of the subgerminal cavity. ● Hence, this is the blastula stage
● (Look at the side portion of the but already starting gastrulation.
yolk) This is the side where the first ● Primitive streak forms at the
falling off of cells occurs which epiblast.
forms a thickening. This will also
be the start where the primitive SAGITTAL VIEW OF THE
streak will form. BLASTODERM

SURFACE VIEW OF YOLK

● The epiblast underlying the

subgerminal space.
● Area pellucida: translucent region
occupied by the blastoderm and
● Primitive streak (blue-colored the subgerminal space.
structure): this will designate the ○ The area occupied by
future posterior end of the embryo. epiblast and subgerminal
Conversely, the future anterior end space when viewed under
of the embryo. the microscope, this is the
light in color structure also
known as area pellucida.
● Area opaca: opaque region
because of the close contact with
the underlying yolk; its cells
involved in the processing of the
yolk.
○ (Look at the side region of
the yolk) This region is still
● The primary hypoblast with
in close contact with the
secondary hypoblast interdigitating
yolk.
results in elongation of the
○ This area will appear dark
hypoblast.
(opaque) and this is called
● 1st: Epiblast and hypoblast
area opaca.
converged at the margins of the
area opaca.
○ What happens to the
anterior portion or margins
of the area opaca (red
arrow)?
■ There is a
thickening. This is
where the epiblast
and the hypoblast
● 1st: Cells delaminate and ingress converged.
from the epiblast into subgerminal ● 2nd: Blastocoel is formed
cavity. between epiblast and hypoblast.
○ Delamination does not ○ Epiblast will form embryo
mean it is simply torn and proper
split. It involves the falling ○ Hypoblast will contribute to
off of cells. the formation of
○ Falling down of cells is extraembryonic
the ingression. They enter membranes.
the cavity. ● The thickening (green structure) is
● 2nd: Ingressing cells form the called the Koller's sickle.
primary hypoblast. ○ It refers to the cell
○ Cells will stick together thickening at posterior
● 3rd: Cells from the posterior margin of area pellucida;
margin (secondary hypoblast) induces formation of the
migrate anteriorly and join the primitive streak in avian.
primary hypoblast.
○ The cells that have formed FORMATION OF THE PRIMITIVE
into thickening (green STREAK
structure), they will fuse ● Cells from the lateral region of the
with the primary hypoblast. posterior epiblast migrate towards
These formed the the midline.
secondary hypoblast. ○ Koller’s sickle induces the
formation of the primitive
streak.
 ● The HN surrounds a pit, the
primitive pit continuous to the
primitive groove.
○ Hensen’s node-
homologous to the dorsal
lip of the blastopore.
○ Primitive groove-
homologous to the
blastopore.
○ HN- becomes the site of
cell turnover. Dito
dumadaan ang mga cells
that are undergoing
ingression.
○ Cells woving into the
blastocoel between the
epiblast and hypoblast.
○ The primitive pit continuous
to the primitive groove is
homologoud to the
blastopore.
● As more cells converge (see
arrows), cells forming the primitive
streak migrate anteriorly
● Primitive streak lengthens and
narrows

● The blastoderm cells migrate over

the lips of teh primitive streak and
into the blastocoel.
○ Colonizing underneath the
blastocoel

● As more cells converge towards

the midline, a depression forms
within the streak, the primitive
groove surrounded by primitive
folds
● A thickening of cells forms into a ● More cells are moving towards the
knot at the anterior end of the midline ingressing passing through
primitive groove, the knot is called the primitive groove.
the Hensen’s node (primitive ● The blastoderm cells migrate over
knot) the lips of the primitive streak and
into the blastocoel.
● If cut along the blue line, the mesoderm, dorsal mesoderm, and
following is observed: intermediate mesoderm) and the
axial mesoderm (remaining at the
middle) (chordamesoderm)

● You can see underneath (medyo

sagittal) ● Avian gastrulation is an
● The arrows represent the ingression of epiblastic cells
ingressing cells with the blastocoel which form mesenchyme cells.
or cavity. ○ Epiblastic cells moving into
○ Some cells are moving the cavity.
directly downward and get
mixed with the hypoblast.
○ The hypoblast (green) are
pushed to the sides.
○ Newly-ingressing cells
(yellow) invading the
position of the hypoblast
■ This will form the
endoderm.
● Cells at the anterior end are
○ The other are moving
already starting to form organs,
sideways (to the left and
starts to undergo neurulation
right) which give rise to the
● The primitive streak regresses
mesoderm mesenchymal
○ Primitive streak pushed
type.
downwards (posteriorly)
● Fibronectin guides migratory
○ Neural structures pushed
behavior of cells from the epiblast.
anteriorly
● Hyaluronic acid coats the
● Posterior portions of the embryo
ingressing cells; change adhesive
(from HN) are still undergoing
behavior; adhere to ECM
(late) gastrulation
molecules in the bastocoel.
● First migrating cells move ventrally
to form the foregut endoderm,
displacing the hypoblastic cells.
● The other cells move anteriorly and
form the head process, overlying
on the Hensen’s node.
● Other cells move sideways in the
blastocoel to form the head
mesoderm and lateral ● Hensen’s Node moves posterior;
mesoderm (lateral plate leaves the head process and the
notochord in its wake.
● The primitive streak with the HN
will form the posterior end of the
digestive gut, the anus.
● All of the structures found
anteriorly will give rise to organs
starting out with the neural
structure.

MAMMALIAN GASTRULATION
● Marked with the formation of a
primitive steak similar with that of
avian embryo.
● It is the inner cell mass that will
undergo delamination, so it will
● Neural structures- anterior to the form the epiblast and the
Hensen’s node hypoblast.
● Proamnion- ‘pro’ on top of the ● The epiblast will eventually give
merging point of ectoderm (left at rise to the amniotic ectoderm
the top) and the migrating cells enclosing the amniotic cavity, and
underneath endoderm to the remainder of the embryonic
○ Devoid of mesenchyme so epiblast, where the primitive
it appears lighter streak will form.
○ Within the area pellucida ● Formation of epiblast and
● Notochord- forms the scaffold hypoblast is called bilaminar germ
○ Its role is to provide an disc formation (Epiblast &
axial skeleton for the Hypoblast)
formation of the CNS
○ But eventually, it will
undergo degeneration and
it will be incorporated into
the vertebral column as the
nucleus pulposus
● The area occupied by the embryo
is the area pellucida; outside is
the area opaca.
 ● Primitive streak forming in the
● Embryonic epiblast- yung epiblast, involving ingression
kalahati niya is amniotic ectoderm ● Endoderm forming
○ Where the primitive streak ● Mesodermal cells forming
will form. ● Left at the midline will form the
○ There will be ingresion of notochordal process moving
cells. Those that are anteriorly.
ingresisng will be forming ○ Notochordal process is the
embryonic endoderm and same as that of the axial
embryonic mesoderm. mesoderm
● Left on top (outermost) will be ○ Notochordal process:
forming the embryonic ectoderm. pick up point for neurulation
● So ectoderm, endodern,
mesoderm, the three germ layers GASTRULATION– MAMMAL
are formed.
● This is trilaminar disc formation.
○ Unlike the first one
(bilaminar) wherein only the
epiblast and hypoblast will
form.
● What happened to the amniotic
ectoderm and the rest of the
hypoblast and trophoblast?
○ These will contribute to the
LECTURE 6: NEURULATION
large part of the placenta.

[To be discussed in Placentation (next

topic)]

● Laying down the rudiments of the

CNS (brain & spinal cord)
● Along with the formation of the
precursors of the brain and the
spinal cord, neuralation involves
the generation of the neural
crest cells (NCCs)
 ● Neural crest cells are of RECAP OF GASTRULATION
mesenchymal type and are cells
that have the power to migrate to
the different parts of the body, and
give rise to different structures, in
which the most classic form of
them would be the ganglia ● The formation of the
● Along with the formation of these chordamesoderm in amphibians
migratory cells (the neural crest and the axial mesoderm for the
cells), gradually and eventually, chick embryo is the same for the
with neurulation process, the notochordal process in
various brain divisions become mammalian embryo
distinctly defined ● The axial mesoderm/notochordal
● Because neurulation has a central process have an overlying
importance, it generates the ectoderm derived from the epiblast
central nervous system, – epiblastic ectoderm or embryonic
neurulation, therefore, marks the ectoderm
start of organogenesis (the ○ With this, overlying the
formation of organs) chordamesoderm is the
● The embryonic axis, such as the ectoderm
dorsal-ventral axis, ● The generation of the
anterior-posterior axis, and Chordamesoderm/ the axial
left-to-right axis, starts its mesoderm/ notochordal process
establishment during the gastrula have very powerful inductive effect,
stage, however are continued to hence, they facilitate neural
be well-established in the induction
neurulation stage
NEURAL INDUCTION
SUMMARY OF THE IMPORTANT ● Neural induction is a process
DISCUSSIONS UNDER NEURULATION where these cells, the
● Neurulation refers to laying down mesodermal, axial mesodermal
the rudiments of the CNS (brain & cells, and notochordal cells, act on
spinal cord) the overlying ectoderm, and
● It is involved in eventually will transform the dorsal
○ Generating the neural crest ectoderm to the first precursor
cells (NCCs) stage of neurula (the formation of
○ Defining the various brain neural plate)
divisions ○ Dorsal ectoderm →→
○ Starting organogenesis Neural plate
○ Continuing the
establishment of the FORMATION OF THE NEURAL PLATE
embryonic axis ● This employs the different
epithelial cell behaviors and the
other cellular activities in
gastrulation
○ Example of epithelial
behavior: bending, folding
 ○ Cellular activities: change
in cell shape
○ Eventually resulting into a
neural plate that bends with
edges that will eventually ● During neural plate formation, the
and gradually gets elevated presumptive neural plate cells
● Morphogenic changes: are expressing the genes coding
○ Shaping, bending, and for N-cadherin, which is a neural
folding cell adhesion protein, on the other
■ Neural plate bends hand, presumptive epidermis is
and edges elevate also expressing the genes coding
for E-cadherin, epithelial cell
adhesion protein
● There are CAMs (cell adhesion
molecules) that are expressed or
secreted by the neural plate cells
and the presumptive epidermal
cells – that is a normal
development
● Eventually, these E- and
● There are three different N-cadherins (cell adhesion
populations of cells in the dorsal molecules) will be responsible for
ectoderm – (1) the presumptive making the cells of the
neural plate cells, (2) the presumptive epidermis clump
presumptive neural crest cells, and together, as well as making the
(3) the presumptive epidermal cells neural plate cells clump together,
○ Presumptive neural plate forming a close cell-to-cell contact,
cells: low columnar cells eventually the neural plate cells will
→ tall columnar cells be detached from the epidermal
○ Presumptive epidermal cells – shows how powerful the
cells: flat squamous cells CAMs are (refer to the image)
● The neural plate cells undergo
columnarization, which is the SUMMARY OF THE NORMAL
transformation from the low DEVELOPMENT
columnar cells into tall columnar
cells
○ Basically, the neural plate
in the image is a plate of
tall columnar cells that will ● Expression of N-cadherin and
make it very distinct from E-cadherin adhesion proteins
the surrounding or during neurulation in Xenopus.
bordering neural crest cells ● In normal development of the
and presumptive epidermal neural plate stage:
cells ○ N-cadherin is seen in the
● First phase of neurulation: neural plate
compressed and increased ○ E-cadherin is seen on the
epithelial cells; thickening presumptive epidermis.
● Eventually, the N-cadherin-bearing FORMATION OF THE NEURAL TUBE
neural cells separate from the BY CLOSURE OF THE NEUROPORE
E-cadherin-containing epidermal ● In the formation of neural tube,
cells. there is simply a closure of the
neuropore, meeting of the neural
FORMATION OF THE NEURAL folds completely at the midline, and
GROOVE AND NEURAL FOLDS when they do, the neural crest
cells detach underneath the
epidermal cells, then as a result,
the neural tube pinches off from
the epidermal layer, and the neural
crest cells will start to move away
from the crest, ready to undergo
mesenchymal migration via
● It will involve again one of the amoeboid movement
morphogenetic events taking place
during gastrulation – convergence SUMMARY: FORMATION OF NEURAL
TUBE BY CLOSURE OF NEUROPORE
which is coupled with convergent
elongation and convergent
thickening
● You can see from the image that
the folds are moving towards the
midline, then the formation of an
empty shaped cavity, which is the
● The neural folds then merge, cut
neural groove
off the neural groove, and form the
○ The green portion
neural tube.
represents the neural crest
● The closure of the neural tube
cells while the blue portion
disconnects the neural crest cells
represents the epidermal
from the epidermis
cells
● Migration of the Neural crest cells
○ The neural groove is a
is the final step in the neural tube
result of the neural plate
closure
deepening and with the
○ Again, neural crest cells are
neural folds converging
of mesenchymal type, so
towards the midline,
unlike those of the
thereby creating a
epidermal cells and those
V-shaped form, with the
of the neural plate cells that
cavity at the middle, the
secrete cell adhesion
neural groove
proteins, the neural crest
○ Its margins are increased
cells do not have cell
by
adhesion proteins
■ Convergent
○ The neural crest cells have
extension (cell
neither cadherin, and they
rows shift into one
disperse (kasi hindi sila
another and form
dikit-dikit, therefore, can
the neural folds)
easily migrate and
disperse)
 ● Other mesoderm cells differentiate SECONDARY SIGNALING CENTERS
into the dorsal mesoderm → ESTABLISHED WITHIN THE NEURAL
somites TUBE
1. Roof of the cells express/ secrete
○ The other mesoderm cells
BMP4
will differentiate into the
2. Floor plate cells express/ secrete
dorsal mesoderm or will
Sonic hedgehog
move sideways and form
● Two concentration gradients are
the dorsal mesoderm that
established; Gradients of TGF- ᵚ
will eventually give rise to
and Shh
the somites
● Due to exposure to sonic
○ The dorsal mesoderm
hedgehog, the floor plate of the
actually becomes known as
neural tube becomes the
the paraaxial mesoderm
secondary signalling centers.
○ The other mesoderm will
These signalling centers will
thicken to form the
determine the formation of the
notochordal process or the
dorsal side and the ventral side,
axial mesoderm
depending on the concentration
gradients.
● The concentration gradients is
illustrated by the gradiations in
colors (from darkest to lightest
color; highly concentrated, lesser
concentration)
● Different sets of genes are
activated in response to the
DORSAL-VENTRAL SPECIFICATION OF different concentrations of the
THE NEURAL TUBE signalling factors
● Different concentrations gradients
activate the expression of different
sets of genes
○ Different exposure level of
TWO PRIMARY SIGNALING CENTERS cells results to different
1. Ectodermal cells of the epidermis identities
produce BMP4 and BMP7 (TGF- ● Cells then
ᵚ) - roof of the nerual tube is differentiate to
exposed become
2. Notochordal cells produce Sonic interneurons and
hedgehog protein - floor of the motor neurons
neural tube is exposed ● Motor neurons will
● The first primary signaling center be expressed on the
secrets BMP4 and BMP7 (TGF- more ventral side.
ᵚ), and these are components of ● Sensory neurons on
the transformation growth factor thre more dorsal
beta superfamily side
● Interneurons, in
between the
gradients
 ● Notochord eventually degenerates NEURAL TUBE DEFECTS (NTDs)
- Persists as nucleus
pulposus of the vertebral
discs.
● This is illustrating of the
establishment of embryonic access
● After sending the signals, the ● Anencephaly
notochord degenerates and will be - Failure of the anterior
incorporated as components of the neuropore to close;
vertebral discs in the spinal column portions of the forebrain is
/ vertebral column and we called missing; usually fatal
that as nucleus pulposus ● Spina Bifida
- Failure of the posterior
THE NEURAL TUBE IS TEMPORARILY neuropore to close;
OPEN AT BOTH ENDS different degrees of severity
● Spina Bifida Occulta (“Obscure”
/ “concealed”)
- Mild form, no pain, no
neurological disorder
- An occurrence of a dimple
like at the back, dorsally
● Spina Bifida Cystica
- More severe than occulta;
spinal cord bulges out
● Anterior neuropore dorsally; with neurological
- Opening at the head end; disorder
brain will develop in the ● Anterior neuropore and posterior
anterior region neuropore are in medical interest,
- Closes around 24th to 26th because when there is a failure or
day closure of the anterior neuropore
● Posterior neuropore or in the posterior neuropore, it will
- Opening at the caudal end; result in an embryonic condition.
spinal cord will develop
towards the posterior PRIMARY VS SECONDARY
region NEURULATION
- Closes around 28th day
❖ The cavity of the neural tube later
forms the ventricles of the central
nervous system.
❖ This will allow the passage of
anionic cavity or anionic fluid
❖ Neurocoel
- Cavity of the neural tube
- Will form the ventricles of
the central nervous system
● Primary neurulation gradually forms that will
● Development of the neural lead to the formation of a
tube as induced by the neural tube
notochord and the ● Starts out as a solid rod of
mesoderm cells, the medullary cord.
● Inductive effect of the ● Within the medullary cord,
mesoderm or the is where the neural tube will
notochordal process, acting form
in the overlying dorsal ● In avians, secondary
ectoderm to form the neurulation takes place in
primordium of the CNS the caudal region at about
● It is towards the cephalic the level of the 25th somite
region ○ In human embryo: ends in
○ In human embryo: ends in 6th week; around level of
the 4th week of embryonic 35th somite
development with the
closure of the posterior THE EXTENT OF USE DEPENDS ON
neuropore. THE CLASS OF VERTEBRATES
● Fishes: Exclusively secondary
● Secondary neurulation neurulation
● Birds:
○ Anterior Region of n.t -
primary neurulation
○ Caudal end to the 25-27th
somite - secondary
neurulation
● Amphibians: Mostly primary
neurulation
○ Tail end - secondary
● The caudal end of the neurulation
neural tube then develops ● Humans: Secondary neurulation
into the neural notochord around level of 35th somite
then canalized
● Starts out as medullary
cord then cavitates
● The medullary cord
will undergo
epithelialization
● Epithelial cells
- Cells with
basement
membrane
and apical
surfaces Figure: Primary neurulation of amphibians
● It is towards the lumbar - around the anterior region of the embryo
region (formation of a keyhole), secondary
● Within the juxtapose neurulation - around the caudal/posterior
epithelial cells, cavity end
 ○ B2 - The neural folds are
elevated as presumptive
epidermis continues to
move toward the dorsal
midline
○ C3 - Convergence of the
neural folds occurs as the
dorsolateral hinge point
(DLHP) cells become
Figure: Avian neurulation wedge-shaped and
epidermal cells push
● Involves hinge point cells which toward the center
serves as anchors ○ D4 - The neural folds are
● The cells anchored to the hinge brought into contact with
point cells undergo change in cell one another, and the neural
shape, wedging which creates a crest cells link the neural
v-shape conformation of the plate tube with the epidermis.
● When the cells undergo wedding, The neural crest cells then
they pull along with them to the disperse, leaving the neural
midline the epidermal and neural tube separate from the
crest cells epidermis
● This is augmented by the formation
of dorsolateral hinge point cells NEURAL TUBE FORMATION DOES
NOT OCCUR SIMULTANEOUSLY
(also anchor sites), they eventually
THROUGHOUT THE ECTODERM
pull the epidermal cells to the ● Starts out as elongated primitive
midline, which then the neural tube streak
gets detached as a whole thing ● Neurulation on the cephalic end -
completely formed from the well advanced
overlying epidermal cells (neural ● Neurulation in the caudal end - still
crest cells migrating away from the undergoing late gastrulation
roof) ○ Due to the regionalization
of the neural tube
Add’l on the Avian Primary Neurulation
● Primary Neurulation: Neural tube
formation in the chick embryo
○ A1 - Cells of the neural
plate can be distinguished
as elongated cells in the
dorsal region of the
ectoderm. Folding begins
as the medial neural hinge
point (MHP) cells anchor to
notochord and change their
shape, while the
presumptive epidermal
cells move towards the Figure: Neural tube formation
center
 DIFFERENTIATION OF THE ○ Rhombomeres - Periodic
NEURAL TUBE swellings on the
● Occurs simultaneously in 3 ways rhombencephalon; with the
1. Gross Anatomical Level neural crest cells and will
a. NT and Neurocoel - determine where cranial
chambers of the brain and nerves will arise
spinal cord ■ R2 - cn5
2. Tissue Level ■ R4 - cn7&8
a. Cells in the wall of NT - ■ R6 - cn9
functional regions of the ■ R3&5 - apoptosis
brain and spinal cord ○ The neural crest cells, over
3. Cellular Level the roof of the
a. Neuroepithelial cells - Rhombencephalon and
numerous types of nerve underneath the epidermal
cells (neurons) and cells, will determine the
supportive cells (neuroglial position and location of the
cells) cranial nerves
○ Will form the ganglia thru
thickening of the neural
crest cells
● Ganglia vs Nerves
○ Ganglion - Aggregate of the
nerve cell bodies
○ Nerve - Collection of the
axonal processes of the
nerve cell

Figure: 3 primary vesicles with their adult

derivatives Figure: Cephalons

● Early human brain development

○ The three primary vesicles
are subdivided as
development continues
● Rhombencephalon/Hindbrain
○ Forms distinct
subdivisions/swellings or
rhombomeres
 DEVELOPMENTAL EVENTS OF THE C. 19D embryo (approximately); amnion is
HUMAN EMBRYO cut to expose dorsal view
16D EMBRYO ● Neural plate visible
○ It is thickening going
epithelium columnarization
Recall:
● The gastrula in gastrula, the
epiblast splits into two; amniotic
ectoderm and epiblastic embryonic
ectoderm.
● The amniotic ectoderm forms a
helmet around the epiblastic
ectoderm and encloses a cavity
amniotic cavity that has the fluid.
Figure: Visible primitive streak with the
● Amniotic ectoderm form a helmet
primitive node
sa ibabaw ng developing embryo.
● Image A (19 days) is the top view
18D EMBRYO
after cutting the helmet

20D EMBRYO
C. 20D embryo (approximately)
● Neural groove and fold formed
(there is an elevation)
● Somites forming lateral to the
neural structures
● Image C (20 days) has 3 somites
● Fusion of the neural folds begins in
the cervical region (level of 5th
Figure: Pear-shaped; cephalic end somite)
broader than caudal end ● There is a neural fold closure in the
cervical region.
19D EMBRYO
 22D EMBRYO 28D EMBRYO
A. 22D embryo (approximately)
● 7 distinct so it's on the sides if the
closed neural tube
● The somite number 5 is that start
of the closure of neural fold

23D EMBRYO
C. 23D embryo (approximately)
● Pericardial bulge in each side of
the midline in the cephalic part
● The developing heart is starting to
bulge B. Approximately 28D old
● Anterior and posterior neuropore ● It has 25 somites
is still open ● 3 pharyngeal arches formed
○ The two allows the passage ● Sensory placodes formed
of amniotic fluid ○ lens and otic
● Closure of the posterior neuropore
25D EMBRYO
NEURAL INDUCTION AS A
MULTISTEP PROCESS

● There is a cytoplasmic movement

in the fertilized egg
● With fertilization, there is a cortical
movement
A. approximately 25 days ○ Granules will move to the
● There are 18-20 somites sperm entry dragging the
● 14-somite stage;bulging pericardial pigment granules and
structure making a gray crescent
● 1st and 2nd pharyngeal arches region
forming ○ The gray crescent region
● Closure of cranial (anterior) will determine the dorsal
neuropore side of the embryo
● In later cleavage, only dorsal
blastomeres are able to induce
animal cells to form dorsal modern
○ There is a bias between the
ventral cells, macromeres
and animal cells.
○ The ventral cells express
more on molecular
marker for epidermal
cells (epi 1)
 ○ The animal cells also ● The Goosecoid gene codes for the
express epi 1 but at a lower transcription factor that regulates
level and eventually from the activity of chordin and noggin.
low expression to inhibited ● When chordin and noggin activate
expression the inactivated BMP4 the dorsal
○ In an inductive process, development is allowed and lead to
there must be a synthesis the establishment of the dorsal
of competence factors to mesoderm
respond to the incoming
signal.
● The formation process
○ Dorsal mesoderm - DLB
(organizers) - sends planar
induction signals at gastrula
stage; after evolution -
formation of
● Dorsal development allowed
Chordamesoderm
○ Neurulation
● Chordamesoderm
■ Neural ectoderm
○ Sends vertical signals to
■ Dorsal
the overlying prospective
ectoderm/epidermis
neuroectoderm/dorsal
○ Dorsal Mesoderm
ectoderm
■ Notochordal
● Neuroectoderm is primed to
process/Axial
respond to signal stimuli
mesoderm
● Inductive signal may act by
■ Paraxial
distinction
mesoderm/somites
○ Interfere with inhibitory
● Ventral development
signal that normally block
○ Ventral epidermis
a default program
■ Lateral plate
mesoderm
AXIS INDUCTION
● Vertebrate embryonic cells, after ■ Intermediate
midblastula transition, develop mesoderm
dorsal organ rudiments unless they
are told differently
● Bone morphogenetic protein 4
(BMP 4) as the natural inhibitor of
dorsal axis formation

● Three types of mesoderm

○ Paraxial mesoderm (to the
side of neural tube)
● In late gastrula, the dorsal lip of the ○ Intermediate mesoderm
blastopore expresses the ○ Lateral plate mesoderm (it
goosecoid gene. is in the stomach and
intestinal area)
 BODY AXES FORMATION Gastrulation → Neurulation
● After the process of neurulation is ● From gastrulation onwards to
the body axes formation neurolation. There is gradual
formation of the body axis.
RECALL
Gastrulation
● Establishment of
chorda-mesodermal cells at the
dorsal lip blastopore
○ During gastrulation, there is
the establishment of the
dorsal lip of blastopore and
the chorda-mesodermal ● If you may further recall, this is
cells at the dorsal lip of best illustrated by the
blastophore. dorsal-ventral patterning of the
○ This is for amphibian neural tube where neurolation or
gastrulation. axis induction involves the
● Establishment of primitive streak signalling factor BMP4.
and Hensen’s node; nodal cells in ● When BMP4 activity is inhibited, it
the Hensen’s node favors dorsal development but at
○ During gastrulation, there is times when BMP4 activity is
the establishment of allowed or activated, it favors
primitive streak with the ventral development.
Hensen’s node and ● Allowing dorsal development to
establishing the presence take place and allowing the ventral
of nodal cells around the development to take place, which
Hensen’s node. means the timing of the activation
○ This is for the avian and and inactivation of BMP4 that
mammalian development governs establishment of axis
formation occurs at specific time
● As early as gastrula stage, and at specific location in the
somehow the dorsal side of the embryonic body we call this spatial
embryo is already established. It is temporal expression of genes.
starting to get established.
● Also with the cranio caudal axis. In LECTURE 7: BODY PLAN FORMATION
the avian gastrulation which is ESTABLISHMENT OF BODY AXES
● Pattern formation: the
similar with that of mammalian.
establishment/ development of an
From the Hensen’s node upward,
embryo’s spatial information
the embryonic disc wll form upward
(positional information)
will eventuallye stablished the
○ Body plan formation is
precursors of central nervous
simply establishing spatial
system (the head and the spina
information within the
cord). It is gradually laying down
embryo. It means
the cranio-caudal axis of the
establishing positional
embryo.
information.
○ Positional information tells
an embryonic cells or group
 of embryonic cells where ● With what goes along with cytoplamsic
they are supposed to be rearrangement or particle movement,
located relative to the there are more molecules that are
embryo’s body axis dragged along with it.
■ This positional information is ○ For example, we have
supplied by molecular cues or molecules that specify
molecular signals eventually dorsal fate moving towards
then establishing the body site of gray crescent region,
axes: dorso-ventral axis, establishing eventually
anterior-posterior axis or marking the cells in the
cranio-caudal axis, and grey crescent area as the
left-right axis future dorsal side of the
embryo.
DORSAL-VENTRAL AXIS ● Conversely, there will be moleules that
ESTABLISHMENT are dragged towards the posterior
vegetal pole and will get concentrated
in there which will also established the
vegetal cells that i'll form the central
site of the embryo.
○ Examples of these
moelcules are VegT and
Vg1. These are
● Recall that the left image is an transcription factors coded
amphibian oocyte. This is at by genes that can control
fertilization. the expression or the
● Opposite of the site of sperm entry, activity of other genes.
with the onset of fertilization, there is ■ They regulate the
cortical reaction. This is called activity of other
cytoplasmic rearrangements where genes. These are
cytoplasm at the edge of cells, the members of
cytoplasm immediately underneath the transformation
plasma membrane, it goes a growth factor beta
cytoplasmic rearrangement superfamily.
○ In this activity, there is a ● On the future dorsal side of the
mass movement of embryo at the region of grey cresecnet
cytoplasm towards the area, there is the continuous
point of sperm entry. accumulation of beta catenin.
○ As a result, the opposite ○ These are molecules that
the site of sperm entry can serve as anchors for
becomes devoid of particle cell adhesion molecules
granules and cytoplasm. that gradually eventually
○ It becomes lighter in color will participate in
compared with the site of establishing the cordo
the animal pole and darker mesodermal cells. This
compared with that of the marks the future dorsal side
vegetal pole. It is in of the embryo.
between so it is called gray ● Dorsal ventral axes here is gradually
crescent area. geting established at early
 cleavage/fertilization. The participation ● Goosecoid gene codes for a
of transcription factors, the markers for transcription factor
dorsal side and markers for ventral ○ This is a regulator gene in
side of the embryo the chordamesoderm cells
● The chordamesoderm cells is the cells
MOLECULAR MECHANISM that colonized the dorsal lip of
Key component blastopore
● The reason why the core of the
mesoderm and the dorsal lip of the
blastopore having the goosecoid gene
regulator is termed chip cell or the
organizer of the embryo?
● When goosecoid gene is activated it
● Beta-catenin will undergo gradual
can regulate the chordin and noggin
accumulation in the gray crescent
○ Chordin and noggin are
region
known to be the inhibitor of
● Beta-catenin with spatio-temporal
BMF4 activity
expression (at the right time at the
● When BMF4 is inactivated or inhibited
right location) expressed in blastomere
the dorsal development can proceed
on the prospective dorsal side
● Recall: in neural induction or axis
● The prospective dorsal side is marked
induction, neurulation occurs by
by the gray crescent region which
disinhibition of BMF4
determined the location of the dorsal
● When dorsal development is allowed
lip of the blastopore
the neurulation can proceed
● Beta-captenin-Trf-3 complex gets
○ Neurulation
complexes with transcription factor
■ Notochord (midline)
● This is translocated into the nuclei of
■ Paraxial mesoderm
cells (Cell in the gray crescent region)
(lateral to the
● As they get translocated in the nuclei
position of the
of cells, they regulate the expression
notochord)
of other genes
■ Dorsal
● These genes can regulate another
ectoderm/epidermis
gene which is involved in establishing
(overlayed paraxial
the ventral and dorsal side of the
mesoderm)
embryo
● If goosecoid gene is inactivated it
● The immediate genes are activated by
cannot code for transcription factor
B-Catenin-Tcf-3 complex, which are
that cannot code for chordin and
the siamois and twin genes
noggin
● Siamois and twin genes are expressed
○ Thus, the activity of BMP4
in the dorsal cells
is not inhibited
○ the cells that will form the
● If BMF4 is active it activates the
chordamesoderm at the
development of ventral structure
organizer side
○ Ventral development
○ Siamois, twin and TGF
■ Lateral plate
(Transformation Growth
mesoderm
Factor)-Beta activates the
■ Ventral mesoderm/
goosecoid gene
epidermis
 ANTERIOR-POSTERIOR (A-P) AXIS ● Anterior Visceral Endoderm (AVE)
/CRANIO-CAUDAL AXIS ○ Organizing center
● Mammalian as model organism ○ This region expresses
○ Once gastrulation begins, genes coding for
A-P polarity becomes transcription factors:
specified by Hox genes ■ OTX2
■ LIM1
■ HESX1
○ Also secreted factor
■ Cerberus
■ Lefty
● All the genes mentioned ( )
● The image A is at the top view interplay to establish the cranial
○ The notch or hensen’s end of the embryo
notch or nock or primitive ○ At the AVE
node is anterior to the
primitive streak CRANIAL TO CAUDAL PATTERNING
● Tilted image
○ Sagittal section through the
node and primitive streak
shows the expression
pattern of genes regulating
the craniocaudal and
dorsoventral axes
● Anterior Visceral Endoderm (AVE) is a ● Established through the somites
group of endodermal cells and vertebrae
○ Organizing center for the ○ Repeated segments which
embryo is governed by the HOX
○ Induced at the distal tip of genes
the embryo ■ HOX genes:
○ Migrate to the prospected members of the
anterior homeotic
○ These cells starts out as transcription factor
extra embryonic cells family that play a
○ These cells starts to specify key role in
the neural pattern by controlling the body
inhibiting primitive streak plan along the
formation anteriorly cranio-caudal axis
(also referred to as
ANTERIOR VISCERAL ENDODERM anterior–posterior),
(AVE) ● specify segment
identity of
tissues within
the embryo
● Age of the embryo is based on the ● (Paraaxial
number of somites mesoderm
○ When counting somites eventually giving
become hard, it is then birth to) Somites
determined through CRL or and
Crown-rump length somitomeres (in
○ CRL is measured from the the cranial
vertex of the skull to the region)
midpoint between the
apices of buttocks
■ Expressed in
millimeters

● Primitive Node
○ Expression of the nodal
gene:
■ Initiates formation of
● Top view of the blastoderm primitive streak;
● In addition to the expression of maintains PS
goosecoid, chordin, noggin, ■ Regulates the
follistatin, nodal at the Hensen’s formation of:
node ● Dorsal and
○ Hensen’s node factor 3 ventral
beta (HNF3β) is also mesoderm
expressed ● Head and tail
■ A gene that structures
maintains the node ○ From the Hensen’s node
and will eventually upward, the embryonic disc
induces regional will grow to gradually
specificity in the establish the primordium of
(prosencephalon) the CNS
forebrain and ■ From the primitive
midbrain areas node upward, it will
● Goosecoid, chordin, noggin, develop into the
follistatin, nodal antagonize and growth of the
inhibit the activity of BMP4 (bone embryonic disc
morphogenetic protein 4) ■ Primitive streak will
○ When BMP4 is inhibited: eventually regress
■ Favors dorsalization downward
■ Cranial mesoderm ● Embryonic Disc
dorsalized into: ○ BMP4 (Bone
● Notochord morphogenetic protein 4)
 and FGF (Fibroblast growth ○ Hinds limbs are
factor) are secreted along compromised
the embryonic disc ■ Composed of
■ Evolve and mesodermal core
ventralize the with an ectoderm
mesoderm to form: ○ Components of the
● Intermediate urogential system are also
mesoderm compromised
● Lateral plate
mesoderm LEFT-RIGHT BODY AXIS
● Dorsal-ventral axis/specification is
established by the interplay of the
genes
● The node is the organizer
○ Homologous to the
chordamesoderm in the
dorsal lip of blastopore
● Nodal gene is homologous to the
goosecoid gene
○ Code for the transcription ● Dorsal views of the germ disc showing
factors that regulate the gene expression patterns responsible
activity of the genes at the for establishing the left-right body axis
dorsal lip of blastopore and ● FGF8: secreted by the node;
at the node establishes expression of Nodal
● The BMP4 establishing the ○ Nodal gene is activated by
dorsal-cranial end of the embryo the activation or secretion
of FGF8
○ FGF8: fibroblast growth
factor 8
● With the activation of secretion of
FGF8, neurulation proceeds
○ Lefty2 is also activated and
● Brachyury (T) gene responsible is expressed in the lateral
for establishing mesoderm–middle plate mesoderm
to caudal region ■ Lateral plate
○ Expressed at around the mesoderm
node of the notochordal represents the
cells notochord
○ Essential for cell migration ○ FGF8 induces expression
in the primitive streak of Nodal and Lefty2 (LPM)
○ Dorsal mesoderm formation ○ Immediately lateral to the
(middle to caudal region) notochord are the paraxial
● Caudal dysgensis: absence of mesoderm
Brachyury gene results to the ■ posterolateral to the
shortening of the embryonic axis paraxial mesoderm
○ Result of the misregulation is the intermediate
of the Brachyury gene mesoderm
 ■ Posteroventral to ○ Snail gene is only
the intermediate expressed on the right side
mesoderm is the (right lateral plate
lateral plate mesoderm)
mesoderm ○ NKX 3.2 established
● Nodal and Lefty2 activate the activity expression of snail on the
of PITX2 gene right lateral plate
○ PITX2 gene: codes for a mesoderm
transcription factor that ○ Snails are not expressed
controls left sidedness on the left side
● Lefty 1: expressed on left side; ventral ■ PITX2: left side
aspect of the neural tube ■ Snail: right side
○ Goal is to prevent left sided
signals from crossing-over
to the other lateral plate
mesoderm
○ Remains on the left side
● SHH: sonic hedgehog
○ Repressor of left-sided
gene (PITX2) expression
on the right
● 5HT (serotonin): neurotransmitter
○ Known to be broken down HOMEOBOX / HOX GENES
by monoamine oxidase on
the left side
○ Serotonin products on the
left side favors the
activation of FGF8
○ Activates secretion of FGF8
○ Breakdown of these
neurotransmitters is
favored towards the left
(monoamine oxidase) ● There are 4 classes in vertebrates
○ FGF8 is favored on the left ○ Hox cluster A-D
side ● Hox genes: the family of genes
○ Regulates the cascade of responsible for determining the
chain activation, eventually general body plan such as:
leading to the activation of ○ The number of body
the gene coding for the segments of an animal
transcription factor that ■ The number of
establishes left-sidedness somites
● The gene that is involved in ○ The number and placement
establishing the right side of the of appendages
embryo is not very well known ■ Forelimbs,
○ NKX 3.2 & snail gene: hindlimbs
candidate that has been ○ The animal head-tail
previously identified directionality