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Dr. Zeenat Zaidi
Results of Fertilization
Stimulates the secondary oocyte to
complete its second meiotic division
Restores normal diploid number (46)
of chromosomes in zygote
Results in variation in human beings
Determines the chromosomal sex of
the embryo
Causes metabolic activation of
zygote & initiates cleavage (cell
division of zygote)
Repeated mitotic division
of zygote
Begins about 30 hours
after fertilization
There is rapid increase in
number of cells. The cells,
blastomeres, become
smaller with each division
Normally occurs as the
zygote passes along the
uterine tube to the uterus
During cleavage, zygote
lies within the zona
Cleavage cont’d
After nine-cell stage,
the cells become
12-16 cell stage is
called morula. It is
formed about 3 days
after fertilization and
enters the uterus
Internal cells of the
morula, inner cell
mass, are surrounded
by a layer of cells that
form the outer cell
Cleavage cont’d
Fluid filled space called the
blastocyst cavity
(blastocele) appears inside
Blastomeres are separated
 Outer cell layer, the
trophoblast, which gives
rise to embryonic part of
 Centrally located, inner
cell mass (embryoblasts)
which gives rise to
Cleavage cont’d
At this stage, the
conceptus is called
Blastocyst. It has two
poles: embryonic &
Zona pellucida
gradually degenerates
and disappears
Blastocyst takes its
nourishment from
uterine secretions and
enlarges in size. It is
ready to get attached
and implanted to the
uterine wall
Embryonic pole
Abembryonic pole
The process by which the
developing mass gets
embedded within the
uterine wall
Begins 6 days
after fertilization:
The blastocyst
attaches to the
epithelium, usually
adjacent to the
embryonic pole
Implantation cont’d
Trophoblast proliferates
rapidly and differentiates
into two layers:
 inner cellular
 outer mass of
protoplasm with no cell
Finger like processes of
syncytiotrophoblast extend
through the endometrium
and invade the endometrial
connective tissue
Implantation cont’d
By the end of 7th
day, the blastocyst
gets implanted in the
superficial compact
layer of endometrium
and derives its
nourishment from
the eroded
Implantation cont’d
The blastocyst
gradually embeds
deeper in the
By 10th day it is
completely buried
within the ‘Functional
layer’ (stratum
compactum + stratum
spongiosum) of the
Implantation cont’d
The defect in the
epithelium is filled by
closing plug (day 10)
The defect gradually
disappear as the
epithelium is repaired
(day 12 & 13) by the
proliferation of the
surrounding cells
Implantation cont’d
Small cavities, the
lacunae appear in
and get filled with
maternal blood,
Normal Implantation Sites
The implantation
site determines the
site of formation of
the placenta
Normally it occurs in
the upper part of the
body of uterus, more
often on the
posterior wall
Abnormal Implantation Sites
Implantation in the
lower segment leads
to placenta praevia
Extrauterine: leading
to ectopic pregnancies
Fallopian tube
Abdominal cavity
Formation of the Bilaminar
Embryonic Disc
The Second Week
Formation of Amniotic Cavity
As implantation of the
blastocyst progresses,
changes appear in the inner
cell mass (embryoblast)
A cavity, amniotic cavity
appears separating
embryoblast from the
trophoblast, which soon
becomes lined by
amnioblasts derived from
inner cell mass
The cavity gradually
increases in size and is
filled with amniotic fluid
Formation of Embryonic Disc
The inner cell mass
becomes flattened forming
a circular bilaminar plate,
the embryonic disc,
consisting of two layers:
 Epiblast, thicker layer,
consisting of high
columnar cells, related
to amniotic cavity
 Hypoblast (primary
endoderm), consisting
of small cuboidal cells
adjacent to the
blastocyst cavity
Formation of Primitive Yolk Sac
The blastocyst cavity
becomes lined with
exocelomic membrane
and is called
Exocelomic cavity
The hypoblast
(endodermal cells)
soon replace the
exocelomic membrane
and the cavity is then
named as the
Primitive (Primary)
Yolk Sac
Embryonic Disc
Lies between the amniotic
cavity and the primitive
yolk sac.
 Epiblast forms floor of
the amniotic cavity
 Hypoblast lies in the
roof of the primitive
yolk sac
gives rise to the germ
layers that form all the
tissues and organs of
Formation of Extraembryonic
Endoderm of the yolk
sac gives rise to a
layer of loosely
arranged connective
tissue, extraembryonic
mesoderm (EEM),
which surrounds the
amniotic cavity and the
yolk sac.
Formation of Extraembryonic Celome
Isolated spaces
appear in the EEM
These spaces
rapidly fuse to form
a large fluid filled,
C-shaped cavity,
celome surrounding
the amniotic cavity
and the yolk sac
Formation of Connecting Stalk
The region where
no cavity has
appeared, forms
the connecting
stalk that connects
the amniotic
cavity, yolk sac
and the embryonic
disc to the outer
wall (later chorion)
The site of the
connecting stalk
determines the
caudal pole of the
embryonic disc
By this stage, a
circular thickening
appears in the
hypoblast near the
cranial end, in the
midline, to form the
prechordal plate
Formation of Amnion, Chorion &
Secondary Yolk Sac
With the formation of
extraembryonic celome:
The EEM is splitted into two
• an outer extraembryonic
parietal (somatic)
• an inner extraembryonic
visceral (splanchnic)
The primary yolk sac
decreases in size and
becomes secondary
(definitive) yolk sac
Wall of the yolk sac,
amnion & chorion are
• Amnion: Two layers;
amnioblasts &
extraembronic splanchnic
• Wall of the yolk sac: Two
layers; endoderm &
extraembronic splanchnic
• Chorion: Three layers:
Extraembryonic somatic
cytotrophoblast &