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Introduction To Embryology
Definition
 a science which study the processes and regulations of development of human fetus.(from 1 cell(zygote) to
(5-7)X1012 cells )
 total 38 weeks:
 preembryonic period: before 2 weeks
 embryonic period: 3-8 weeks
 fetal period: after 9 weeks
 perinatal stage: 26 weeks to birth

Teratology: abnormal development of fetus

1. Fertilization
 definition: the process by which the male and female
gametes(sperm and ovum) unite to give rise to zygote
 place: in the ampullary region of the fallopian tube
 conditions:
1. maturation of oocyte

2. maturation of spermatozoon: ductus epididymus: 2-3 weeks,

forward motility protein
3. capacitation of spermatozoa:
 decapacitation factor: exist in epididymus, to inhibit the release of acrosome enzymes
 Capacitation factor: secreted by epi. cells of oviduct, capacity of releasing acrosome enzymes
4. Quality and quantity of spermatozoa:
 2-6 ml, 100,000,000/ml,
 <1.5 ml; or <10,000,000; abnormal sperm >30%; or capacity for mobile< 70%
5. meeting of sperm and ovum
 sperm: 20-24h
 ovum: 15-18h(1-3ds)

 processes of fertilization:
1. acrosome reaction:
 Hyaluronic acidase: dissolve corona radiate
 Interact with ZP3(receptor glycoprotein, exist in zona pellucide)
molecules
 release acrosin: dissolve zona pellucide
 enter perivitelline space

2. spermatozoa fuse with the membrane of ovum: nucleus enter

cytoplasm of ovum
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 3. Monospermy:
 cortical reaction: cortical granules(located in cytoplasm of ovum) are released into perivitelline space,
and cause
 zona reaction: change of ZP3 molecules in zona pellucide

4. Formation of zygote(fertilized ovum)

 secondary meiosis complete: form second polar body
 male pronucleus fuse with female pronucleus, to restore the 2n chromosome

 significance of fertilization:
1. a new life begin(initiation of cleavage)
2. restoration of the diploid number of chromosomes: cross-over
3. determination of the sex of the new individual:
 ova(22+X) + X-bearing spermatozoon = girl klinefelter’s syndrome: 47XXY, 48XXXY
 ova(22+X) + Y-bearing spermatozoon = boy

2. Blastocyst formation and implantation

1. Cleavage and blastocyst formation
 Cleavage
 blastomere: cells formed by cleavage
 30h: 2 || 40h: 4 || 72h: 12-16- morula

 formation of blastocyst:
 By 5th day, blastomere reach to 107, small space appear between them
 blastocoele: filled with liquid
 trophoblast
 Inner cell mass
 polar trophoblast

2. Implantation
 Definition: the process by which the
blastocyst is embedded in
endometrium
 Time: 5th –6th day begin; 11th –
12th day complete

 processes:
 Zona pellucide disappear → polar
trophoblast touch the
endometrium→secrete proteolytase
→dissolve the endometrium → embedded into
endometrium→coagulation plug seal the space
 trophoblast become into two layers when blastocyst is embedding into
endometrium
 syncytiotrophoblast
 cytotrophoblast
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  place:
 Posterior wall of fundus and body of uterus
 placenta praevia
 ectopic pregnancy
 Decidual response of endometrium
 stroma cell→predecidual cell→decidual cell(cell
become larger and rich in glycogen and lipid
droplet)
 endometrium →decidua:
 Decidua basalis
 Decidua capsularis
 Decidua parietalis
 conditions:
 Endometrium is in secretory phase
 Morula reach the cavity of uterus on time
 Zona pellucide disappears in time
3. Formation and differentiation of trilaminar germ disc
1. Formation of endoderm and ectoderm: early of 2 weeks, inner cell mass
differentiate into two layers of cells
 bilaminar germ disc: epiblast (columnar) +hypoblast (cuboidal)
 epiblast: primary ectoderm
 amniotic membrane: amnioblast
 amniotic cavity
 amniotic fluid
 amnion
 hypoblast: primary endoderm
 extraembryonic endoderm→exocoelomic membrane→primary yolk sac →
exocoelomic vesicle
 extraembryonic mesoderm: → extraembryonic cavity: chorionic cavity
o visceral layer
o parietal layer
 secondary yolk sac: yolk sac
 body stalk: formed by extraembryonic mesoderm

2. Formation of mesoderm: early of 3 weeks

 primitive streak: cells of epiblast proliferate to form a longitudinal arranged
cell cord
 primitive groove
 primitive node
 primitive pit
 mesoderm: intraembryonic mesoderm
 endoderm: hypoblast cells are replaced by epiblast cells
 ectoderm: epiblast changed the name into ectoderm
 Trilaminar germ disc: endoderm + mesoderm + ectoderm
 Determination of head and tail of germ disc
 head process→notochordal tube → notochord
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  buccopharyngeal membrane
 cloacal membrane

3. differentiation of trilaminar germ disc: 4th –8th weeks

 differentiation: same cells which are primordial and inmuture differentiate into different cells which
have specific structure and function
 induction: some tissues effect the differentiation, and determine the differentiating orientation of another
tissue

 differentiation of ectoderm: from 18th –19th days

 Neural plate: neuro-epithelium (neural ectoderm):
pseudostratified columnar epi.
 neural fold
 neural groove
 neural tube: →CNS
 Anterior neuropore: closed by 25th days
 posterior neuropore: closed by 27th days
 neural crest (mesoectoderm): two lines of cell
cords→ganglion

 Differentiation of mesoderm: 17th days

 Paraxial mesoderm
 somite: 20th days, 3 pairs/per day, 42-44 pairs by the end of 5th weeks
o sclerotome: →bone, cartilage
o myotome: →skeletal muscle
o dermatome: dermis and hypodermis
 intermediate mesoderm: →kidney and reproductive
gland
 nephrotome: segmentation
 nephrogenic cord:

 lateral mesoderm:
 intraembryonic coelom: →body cavity
 somatic or parietal mesoderm: →muscle, CT, parietal layer of pleura,
peritoneum and pericardium
 splanchnic or visceral mesoderm: →muscle, CT of digestive tract,
visceral layer of pleura, peritoneum and pericardium
 mesenchyme: →cardiovascular and lymph system

 differentiation of endoderm:
 primitive gut: →digestive, respiratory
and urinary system
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 4. The development of fetal membrane and placenta
1. chorion:
 formed by trophoblast and extraembryonic mesoderm
 primary stem villus: projections of cytotrophoblast and syncytiotrophoblast
 secondary stem villus: extraembryonic mesoderm enter the primary stem villus
 chorionic plate: trophoblast + extraembryonic mesoderm
 chorion: secondary stem villus + chorionic plate
 tertiary stem villus: extraembryonic mesoderm differentiate into CT
and BV
 free villus: branches
 anchoring villus
 cytotrophoblastic cell column: →cytotrophoblastic shell
 chorion leave: 6 weeks later
 chorion frondosum
 hydatidiform mole
 chorion carcinoma

2. Yolk sac:
 blood island: primitive blood cell- derived
from extraembryonic mesoderm on the
wall of yolk sac
 primordial germ cell: derived from
endoderm of yolk sac

3. amnion:
 Amniotic membrane: amniotic epi. + extraembryonic mesoderm
 amniotic fluid:
 Secrete by amniotic epi.
 Slight basic fluid: 500-1000ml
o polyhydramnios: >2000 ml, abnormal digestive system or CNS
o oligohydramnios: <500 ml, abnormal urinary system
 function:
 intraenvironment
 Protecting
 Preventing from adherence
 Wash germ tract

4. allantois
 allantoic A: paired, →umbilical A
 allantoic V: paired
 right: degenerate
 left: umbilical V
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5. umbilical cord
 Cylindrical structure
 Surface: amniotic membrane
 Cord: mucous CT, umbilical A,V, yolk sac and allantois
 40-60 cm long, 1.5-2.0 cm in D
 > 80 cm, or < 35 cm

6. placenta:
 the structure by which exchange of material between
fetus and mother takes place
 size: round, disc-shaped, 15-20 cm in D, 2.5 cm
thickness, 500g in weight
 fetal surface: smooth, covered by amniotic
membrane
 mother surface: rough, 15-30 cotyledons
 structure:
 chorionic plate
 chorion and chorion space
o chorion: 60 chorion stalks→branches
o chorion space: space between chorion, filled with mother blood
 basal plate: cytotrophoblastic shell + deciduas
o Placental septa: separate the chorion into cotyledon
 blood circulation of placenta
 Fetus: umbilical A →cap. of chorion →umbilical V
 Mother: spiral A → chorion space → uterus V
 placental barrier: the structure between fetal and maternal blood
 components:
o Endothelium of chorion capillary and its basal lamina
o CT in the core of the villus
o Trophoblast epithelium and its basal lamina
 function:
a. the exchange of material between the maternal and fetal blood streams
b. defense barrier
c. the production of hormones:
o human chorionic gonadotropin, HCG:
begin: end of 2nd week
highest lever: 9th –11th week
lowest lever: 20th week, until birth
o human placental lactogen, HPL
Similar to HGH
Highest lever: 36th –37th week
o human placental progesterone, HPP and human placental estrogen, HPE, begin: 4th month
o other hormones: human chorionic thyrotropin, HCT; human chorionic adrenocorticotrophic hormne,
HCATH; prostaglandin

‫تم بحمد الله‬

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