Embryology is the study of prenatal development from gametes through embryos and fetuses. The key stages of development include fertilization, cleavage, formation of the morula and blastocyst, and implantation. The blastocyst undergoes further differentiation, with the outer trophectoderm forming the placenta and fetal membranes, and the inner cell mass forming the embryo proper. Implantation of the blastocyst in the uterine wall marks the beginning of the fetal stage of pregnancy. The placenta and fetal membranes develop from the trophoblast and allow for nutrient/waste exchange between the developing fetus and mother.

1. Development of placenta and fetus
Definition:- Embryology is the branch of
biology that studies the prenatal development of
gametes, fertilization, and development of
embryos and fetuses. Additionally, embryology
encompasses the study of congenital disorders
that occur before birth, known as teratology
Embryo:- The embryo is defined as the
developing pregnancy from the time of
fertilization until the end of the eighth week of
gestation.
Embryonic period - first 8 weeks Development
of the three primary germ layers give rise to all
structures and Basic body plan takes shape
Fetal period - remaining 30 weeks. Structures
and organs continue to grow and develop
1st
week of development :-

2. • Fertilization
• Cleavages
• Implantation
Fertilization :- Fertilization is the process of
fusion of the spermatozoon with the
mature ovum. It begins with sperm egg
collision and ends with production of a
mononucleated single cell called the zygote.
Its objectives are:
(1) to initiate the embryonic development
of the egg and (2) to restore the
chromosome number of the species.
Almost always, fertilization occurs in the
ampullary part of the uterine tube.

3. APPROXIMATION OF THE GAMETES: The
ovum, immediately following ovulation is
picked up by the tubal fimbriae which partly
envelope the ovary, especially at the time of
ovulation. The pick up action might be muscular
or by a kind of suction or by ciliary action or by
a positive chemotaxis exerted by the tubal
secretion.
The ovum is rapidly transported to the
ampullary part. Fertilizable life span of oocyte
ranges from 12 to 24 hours whereas that of
sperm is 48 to 72 hours.
Out of hundreds of millions of sperms
deposited in the vagina at single ejaculation,
only thousands capacitated spermatozoa enter
the uterine tube while only 300-500 reach the
ovum. The tubal transport is facilitated by

4. muscular contraction and aspiration action of
the uterine tube. It takes only few minutes for
the sperm to reach the fallopian tube.
CONTACT AND FUSION OF THE GAMETES :
Complete dissolution of the cells of the corona
radiata occurs by the chemical action of the
hyaluronidase liberated from the acrosomal
cap of the hundreds of sperm present at the
site
- Penetration of the zona pellucida is facilitated
by the release of hyaluronidase from the
acrosomal cap.
More than one sperm may penetrate the zona
pellucida.

5. - Out of the many sperms, one touches the
oolemma. Soon after the sperm fusion,
penetration of other sperm is prevented by
zona reaction (hardening) and oolemma block.
This is due to release of cortical granules by
exocytosis from the oocyte.
- Completion of the second meiotic division of
the oocyte immediately follows, each
containing haploid number of chromosomes
(23, X). The bigger one is called the female
pronucleus and the smaller one is called second
polar body which is pushed to the perivitelline
space.
- In the human, both the head and tail of the
spermatozoon enter the cytoplasm of the
oocyte but the plasma membrane is left behind
on the oocyte surface. Head and the neck of

6. the spermatozoon become male pronucleus
containing haploid number of chromosomes
(23, X) or (23, Y).
The male and the female pronuclei unite at the
centre with restoration of the diploid number
of chromosomes (46) which is constant for the
species. The zygote, thus formed, contains both
the paternal and maternal genetic materials. In
some instances, an antigen called fertilizin
present on the cortex and its coat of the ovum,
reacts with the antibody called antifertilizin
liberated at the plasma membrane of the
sperm head Thus the union between the two
gametes may be an immunological reaction
(chemotaxis)
Sex of the child is determined by the pattern of
the sex chromosome supplied by the
spermatozoon

7. If the spermatozoon contains 'X' chromosome,
a female embryo (46, XX) is formed; if it
contains a Y chromosome, a male embryo (46,
XY) is formed.
Cleavage :- After Fertilization there is
repeated mitotic division of zygote take place
to form a solid ball of cell ,this mitotic division
termed as cleavage.
MORULA :- After the zygote formation, typical
mitotic division of the nucleus occurs producing
two blastomeres
The two cell stage is reached approximately 30
hours after fertilization. Each contains equal
cytoplasmic volume and chromosome
numbers. The blastomeres continue to divide
by binary division through 4,8,16 cell stage until

8. a cluster of cells is formed and is called morula,
resembling a mulberry. As the total volume of
the cell mass is not increased and the zona
pellucida remains intact, the morula after
spending about 3 days in the uterine tube
enters the uterine cavity through the narrow
uterine ostium (1 mm) on the 4th day in the 16-
64 cell stage. The transport is a slow process
and is controlled by muscular contraction and
movement of the cilia. The central cell of the
morula is known as inner cell mass which forms
the embryo proper and the peripheral cells are
called outer cell mass which will form
protective and nutritive membranes of the
embryo.
BLASTOCYST
While the morula remains free in the uterine
cavity on the 4th
and 5th
day it is covered by a

9. film of mucus The that passes through the
canaliculi of the zona pellucida which separates
the cells of the morula and is now termed
blastocyst. Zona hatching is the next step so
that trophectoderm cells interact with
endometrial cells and implantation occurs
Due to blastocyst enlargement the zona
pellucida becomes stretched, thinned and
gradually disappears. Lysis of zona and escape
of embryo is called zona hatching The cells on
the outer side of the morula (polar) become
trophectoderm and the inner cells (apolar)
become inner cell mass by the mediation of E
cadherin (protein)
Trophectoderm differentiates into chorion
(placenta) and the inner cell mass into the
embryo. Completely undifferentiated cells are
called the pluripotent embryonic stem (ES)
cells. ES cells are able to produce mature

10. somatic cells of any germ layers (ectoderm
mesoderm and endoderm).
Implantation:- Implantation occurs in the
endometrium of the anterior or posterior wall
of the body near the fundus on the 6th day
which corresponds to the 20th day of a regular
menstrual cycle. Implantation occurs through
four stages eg apposition, adhesion,
penetration and invasion.
CHANGES IN THE BLASTOCYST: The polar
trophoblast cells adjacent to the inner cell mass
are primarily involved in adhesion to the
endometrial cells. The factors responsible for
blastocyst attachment are P Selectin, heparin
sulphate, proteoglycan, EGE, integrins,
troponin, tasin and others. The signals for
trophoblast multiplication arise from the inner
cell mass

11. ENDOMETRIUM AT THE IMPLANTATION SITE:
(1) The endometrium is in the secretory phase
corresponding to 20-21 days of cycle (2) The
microvilli on the surface of the trophectoderm
interdigitate with the decidual cells to form the
junctional complexes. Endometrial receptivity
and molecular signalling during implantation is
induced by progesterone, LIF (leukaemia
inhibitory factor), prostaglandins and COX-2
Trophoblast:- the cells of the blastocyst
differentiate into an outer trophectoderm and
an inner cell mass. Just before implantation,
the trophectoderm is further differentiated into
an inner mononuclear cellular layer called
cytotrophoblast or Langhan's layer and an
outer layer of multinucleated syncytium called
syncytiotrophoblast. The cytotrophoblasts that
line the villous stems are the villous
cytotrophoblasts . The cytotrophoblast cells

12. that invade the decidua are known as
'interstitial extravillous cytotrophoblast and
those that invade the lumens of the maternal
spiral arteries are known as 'intravascular
extravillous cytotrophoblast'. Throughout
pregnancy, syncytiotrophoblast is derived from
the cytotrophoblast.
Placenta and the fetal membranes are
developed from the trophoblast. It is involved
in most of the functions ascribed to the
placenta as a whole Thus it serves at least 3
important functions -- invasion, nutrition and
production of hormones for the maintenance
of pregnancy. Local cytokines regulate the
invasion of the cytotrophoblasts in the decidua.
Decidua :- The decidua is the endometrium of
the pregnant uterus It is so named because
much of it is shed following delivery.

13. Decidual reaction: The increased structural and
secretory activity of the endometrium that is
brought about in response to progesterone
following implantation is known as decidual
reaction.
The well developed decidua differentiates into
three layer(1) Superficial compact layer
consists of compact mass of decidual cells gland
duct and dilated capillaries. The greater part of
the surface epithelium is either thinned out or
lost. (2) Intermediate spongy layer (cavernous
layer) contains dilated uterine glands, decidual
cells and blood vessels. It is through this layer
that the cleavage of placental separation occurs
(3) Thin basal layer containing the basal portion
of the glands and is apposed to the uterine
muscle. Regeneration of the mucous coat
occurs from this layer following parturition

14. After the interstitial implantation of the
blastocyst into the compact layer of the
decidua, the different portions of the decidua
are renamed as(1) decidua basalis or serotina-
the portion of decidua in contact with the base
of the blastocyst (2) decidua capsularis or
reflexa- the thin superficial compact layer
covering the blastocyst and (3) decidua vera or
parietalis the rest of the decidua lining the
uterine cavity outside the site of implantation.
Chorion and chorionic villi:- The chorion is
the outermost layer of the two fetal
membranes (chorion and amnion). It consists of
two embryonic layers- outer trophoblast and
inner primitive mesenchyme which appears on
9th day. At the beginning of the 3rd week, the
syncytiotrophoblast produces irregular finger
like projections which are lined internally by
the cytotrophoblast. These finger like buds are
called primary stem villi surrounded by lacunar

15. spaces which will later form into intervillous
spaces
After the appearance of the primitive
mesenchyme and the development of the
chorion, the primary stem villi are named
chorionic villi.With the insinuation of the
primary mesoderm into the central core of the
villi structures, secondary villi are formed on
16th day. Later on mesodermal cells in the villi
begin to differentiate into blood cells and blood
vessels, thus forming villous capillary system.
These vascularised villi are called tertiary villi
which are completed on 21st day. Later on this
extra embryonic circulatory system establishes
connection with the intra-embryonic circulatory
system through the body stalk.
Development of inner cell mass:- Along
with the changes in the trophoblast, on the 8th
day, the embryoblasts differentiate into
bilaminar germ disc.

16. Two cavities are appears one on each side of
the germ disc (1) a fluid filled space between
the ectodermal and the cytotrophoblast which
is called amniotic cavity . It's floor is formed by
the ectoderm and the rest of it's wall by
primitive mesenchyme (2) the yolk sac appear
on the ventral aspect of the bilaminar disc.
Extraembryonic coelom : extraembryonic
mesenchyme, derived from the trophoblast
appears to separate the yolk sac from the
blastocyst wall and also the amniotic cavity
from the trophoblast of the chorion. Small
cystic spaces (lacuna) now appear within the
extraembryonic mesenchyme . These spaces
gradually enlarge and fuse to form
extraembryonic coelom . Progressive
enlargement of the extraembryonic coelom,
separates the amnion from the inner aspect of
the chorion except at the caudal end of the
embryo. There, the mesenchymal attachment

17. persists to form body stalk . Umbilical cord
develops from this body stalk.
After invagination some cells displace hypoblast
creating embryonic endoderm. Others lie
between epiblast and newly created endoderm
form mesoderm. Cells remaining in epiblast
form ectoderm.
ECTODERMAL LAYER :-Central and peripheral
nervous system, epidermis of skin with its
appendages pituitary gland chromaffin organs,
salivary gland mucous lining of the nasal cavity,
paranasal sinus roof of the mouth etc.
MESODERMAL LAYER:- Bones cartilage,
muscles, cardiovascular system, kidney ,gonads
,spleen , most of the genital tract mesothelial
lining of pericardial pleural and peritoneal
cavity etc.
ENDODERMAL LAYER:- Epithelial lining of the
gastrointestinal tract, liver gall bladder,

18. pancreas, epithelial lining of respiratory Tract
and most of the mucous membrane of urinary
bladder and urethra, bulbourethral and greater
vestibular glands etc.
Development of placenta:- The placenta is
developed from two sources. The principal
component is fetal which develops from the
chorion frondosum and the maternal
component consists of decidua basalis.
When the interstitial implantation is completed
on 17th day, the blastocyst is surrounded on all
sides by lacunar spaces around cords of
syncytial cells, called trabeculae. From the
trabeculae develops the stem villi on 13th day
which connect the chorionic plate with the
basal plate. Primary, secondary and tertiary villi
are successively developed from the stem villi.
Arteriole-capillary-venous system in the

19. mesenchymal core of each villus is completed
on 21st day This ultimately makes connection
with the intraembryonic vascular system
through the body stalk
Simultaneously, lacunar spaces become
confluent with one another and by 3rd
- 4th
week, form a multilocular receptacle lined by
syncytium and filled with maternal blood. This
space becomes the future intervillous space
As the growth of the embryo proceeds, decidua
capsularis becomes thinner beginning at 6th
week and both the villi and the lacunar spaces
in the abembryonic area get obliterated,
converting the chorion into chorion laeve. This
is, however, compensated by (a) exuberant
growth and proliferation of the decidua basalis
and (b) enormous and exuberant division and

20. sub-division of the chorionic villi in the
embryonic pole (chorion frondosum). These
two I.e. chorion frondosum and the decidua
basalis form the discrete placenta. It begins at
6th week and is completed by 12th week .
Until the end of the 16th week, the placenta
grows both in thickness and circumference due
to growth of the chorionic villi with
accompanying expansion of the intervillous
space. Subsequently, there is little increase in
thickness but it increases circumferentially till
term