So far ……
Meiosis Egg
(Oogenesis)
Fuse Fertilized egg Development
(Zygote) begins
Meiosis Sperm
(Spermatogenesis)
Animal Development
Fertilized egg divides mitotically
Two-cell stage Four-cell stage
(first mitotic division) (Second mitotic division)
CLEAVAGE OF THE ZYGOTE
• Cleavage consists of repeated mitotic
divisions of the zygote, resulting in a rapid
increase in the number of cells. These
embryonic cells-blastomeres-become
smaller with each successive cleavage
division .
compaction
• After the nine-cell stage, the blastomeres
change their shape and tightly align
themselves against each other to form a
compact ball of cells. When there are 12 to
32 blastomeres, the developing human is
called a morula .
Animal Development
Fertilized egg divides mitotically
Two-cell Four-cell Eight-cell Sixteen-cell
Morula
Animal Development
Morula (“mulberry”)
fluid filled cavity - blastocoel- begins to form
Blastula - hollow ball of cells
- has between 25 and 27 cells
blastocoel
The Blastula - Polarity
A frog egg
Two cell
Four cell
Eight cell - in animal
hemisphere
Morula
• After compaction
Internal cells of the
morula (inner cell
mass) are surrounded
by a layer of cells that
form the outer cell
layer.
FORMATION OF THE
BLASTOCYST
• Shortly after the
morula enters the
uterus (approximately
4 days after
fertilization), a fluid-
filled space called the
blastocystic cavity
appears inside the
morula .
• As fluid increases in the blastocystic
cavity, it separates the blastomeres into
two parts:
• A thin, outer cell layer, the trophoblast
(Greek, trophe, nutrition), which gives rise
to the embryonic part of the placenta
Implantation
• The embryo implants in the wall of the uterus on about the 7th day
of development
• A group of centrally located blastomeres,
the inner cell mass, which gives rise to
the embryo; because it is the primordium
of the embryo, the inner cell mass is called
the embryoblast
COMPLETION OF IMPLANTATION AND CONTINUATION
OF EMBRYONIC DEVELOPMENT
• As the blastocyst implants more
trophoblast contacts the endometrium and
differentiates into
• The cytotrophoblast, a layer of cells that is
mitotically active and forms new cells .
• The syncytiotrophoblast, a rapidly
expanding, multinucleated mass in which
no cell boundaries are discernible
• The inner cell mass or embryoblast,
meanwhile, differentiates into (a) the
• epiblast and (b) the hypoblast, together
forming a bilaminar disc .
• Epiblast cells give rise to amnioblasts that
line the amniotic cavity superior
to the epiblast layer.
Trilaminar disc
• The most characteristic event occurring
during the third week of gestation is
Gastrulation, the process that establishes
all three germ layers (ectoderm,
mesoderm, and endoderm) in the embryo
• Gastrulation begins
with formation of the
Primitive streak on
the surface of the
epiblast . The
primitive groove and
pit result from the
invagination (inward
movement) of
epiblastic cells.
• it is clearly visible as
a narrow groove with
slightly bulging
regions on either
side .
Importance of primitive streak
• As soon as the primitive streak appears, it
is possible to identify
• its cranial and caudal ends,
• its dorsal and ventral surfaces,
• its right and left sides.
• The cephalic end of
the streak, the
Primitive node,
consists of a slightly
elevated area
surrounding the small
Primitive pit
Upon arrival in the
region of the streak,
epiblast cells become
flask-shaped, detach
from the epiblast, and
slip beneath it . This
inward movement is
known as
invagination.
• Once the cells have
invaginated, some
displace the
hypoblast, creating
the embryonic
Endoderm, and
others come to lie
between the epiblast
and newly created
endoderm to form
Mesoderm.
• Cells remaining in the
epiblast then form
Ectoderm. Thus, the
epiblast, through the
process of gastrulation, is
the source of all of the
germ layers , and cells in
these layers will give rise
to all of the tissues and
organs in the embryo.
Differentiation of Primary Germ
Layers (from the gastrula)
Ectoderm Mesoderm Endoderm
Nervous Skeleton Digestive
system tract
Epidermis of Muscles Respiratory
skin system
Circulatory Liver,
system pancreas
Gonads Bladder
What happens to the germ
layers?
Ectoderm Mesoderm Endoderm
Skin Skeletal system Lining of digestive,
circulatory, excretory,
respiratory and
Nervous system Muscles
reproductive systems
Liver
Tooth enamel Kidneys
Pancreas
Circulatory system
Reproductive
system
.
TERATOGENESIS OF
GASTRULATION
• The beginning of the third week of
development, when gastrulation is
initiated, is a highly sensitive stage for
teratogenic insult. At this time, fate maps
can be made for various organ systems,
• these cell populations may be damaged by
teratogens.
• Doses of alcohol at this stage kill cells in
the anterior midline of the germ disc,
producing a deficiency of the midline in
craniofacial structures and resulting in
holoprosencephaly. In such a child, the
forebrain is small, the two lateral ventricles
often merge into a single ventricle, and the
eyes are close together (hypotelorism).
• Situs inversus is a condition in which
transposition of the viscera in the
thorax and abdomen occurs.
Tumors Associated With
Gastrulation
• Sometimes, remnants of the primitive
streak persist in the sacrococcygeal
region. These clusters of pluripotent cells
proliferate and form tumors, known as
sacrococcygeal teratomas, that commonly
contain tissues derived from all
• three germ layers
