• Fertilization takes place in the ampulla of the oviduct
• Cleavage takes place as the zygote moves down the oviduct towards
the uterus
•The morula enteres the uterus 3 days after fertilization
•The morula is 12 – 16
cells forming a solid ball
of cells and is still
surrounded by the zona
pellucida
• As cleavage divisions
continue cavities appear in
the interior of the morula
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The cavities become filled with fluid from
the uterine cavity to become the
blastocyst cavity
The embryo is now a blastocyst,
consisting of the trophoblast,
surrounding the blastocyst cavity with
the inner cell mass/embryoblast/
formative cells at the embryonic pole
The inner cell mass is basophilic - stains
bright when stained with basic dyes.
The trophoblast gives rise to the
extra-embryonic membranes, while the
embryo proper develops out of the inner
cell mass.
The zona pellucida (fertilization
membrane) is still intact around the
blastocyst and the blastocyst lies free in
the uterine cavity.
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The zona pellucida starts to
degenerate and eventually
disappears.
This enables the blastocyst to
become attached to the
endometrial epithelium.
The embryo is now referred to as
a conceptus.
The conceptus orientates itself in
such a way that the embryonic
pole lies against the endometrium
of the uterus. The trophoblast
attaches itself to the endometrial
epithelium in the following way:
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The trophoblast cells in contact
with the endometrial epithelium
undergo multiple divisions
It differentiates into two layers:
inner cellular layer =
cytotrophoblast and an outer
= syncytotrophoblast.
Cell membranes of the outer
layer disappear to form a
multi-nucleated protoplasmic
mass = a syncytium
The syncytotrophoblast forms
finger-like processes which grow
into the endometrium.
The embryo is superficially
implanted by the end of the first
week.
• A new layer becomes visible during
attachment and superficial implantation
• A flat layer of cells possibly originating
from the trophoblast arranges, through
a process of migration, into a thin layer
of flat cells lining the inner surface of the
inner cell mass to form the hypoblast.
• The hypoblast therefore forms a
ceiling to the blastocyst cavity,
separating the inner cell mass from the
blastocyst cavity.
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The inner cell mass + hypoblast
arrange into a double layered
disc = the bilaminar or
embryonic disc, consisting of a
top layer of columnar cells = the
epiblast and a second layer of
cuboidal cells = hypoblast.
The epiblast + hypoblast
develops into all three germ
layers of the embryo proper.
The cells of the hypoblast
develop into endoderm or part of
it. It is believed that its
contribution is mostly extra embryonic in that it mainly
contributes to the endoderm of
the extra embryonic
membranes.
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The syncytotrophoblast
invades deeper into the
endometrial stroma. The
cells of the stroma
degenerate in the area of
the penetrating
syncytotrophoblast.
The cytotrophoblast
continue to divide producing
cells that migrate into the
increasing mass of the
syncytotrophoblast where
they soon loose their cell
membranes.
The formation of the syncyto
trophoblast expands around
the cytotrophoblast of the
conceptus as implantation
progresses.
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Cavities form between the
inner cell mass and the
trophoblast. These spaces
join each other to form the
amniotic cavity.
The part of the
cytotrophoblast overlying
the epiblast splits into 2
layers.
The inner layer consists of
amnioblast cells
The amnioblasts becomes
the amnion.
The cells of the epiblast
form the floor of the
amniotic cavity and are
continuous peripherally with
the amnion.
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Delamination of the
cytotrophoblast
surrounding the blastocyst
cavity also takes place
It is a thin membranous
layer called the
exocoelomic membrane
The exocoelomic
membrane around the
blastocyst cavity is
continuous with the
hypoblast
The blastocyst cavity is now
referred to as the
exocoelomic cavity.
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The syncytotrophoblast invades deeper
into the endometrial stroma as the
cytotrophoblast continue to produce cells
that migrate into the syncytotrophoblast
where they soon loose their cell
membranes.
The cells of the endometrial stroma
degenerate in the area of the penetrating
syncytotrophoblast.
The syncytotrophoblast expands around
the cytotrophoblast and the conceptus
penetrates deeper into the stroma until it is
completely embedded.
A closing plug (blood clot and cellular
debris) remains on the surface for a short
while until the regenerating epithelium
covers over the conceptus.
This type of is called interstitial
implantation.
 Lacunae appear in syncytotrophoblast filled
with maternal blood from the ruptured
capillaries + secretions from the eroded
endometrial glands.
 = Nutritive fluid is embryotroph and
reaches the embryonic disc by diffusion.
 A lacunar network is established giving the
syncytotrophoblast a sponge-like appearance.
 The lacunae become connected with
maternal blood vessels & a primitive
uteroplacental circulation is established.
 Oxygenated blood reach the lacunae
through the endometrial arteries, and the
deoxygenated blood is removed from the
lacunae by the endometrial veins.
The lacunar network is initially formed in the
area of the embryonic pole, but increases and
spreads throughout the whole endometrium to
surround the conceptus completely.
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The delamination of cytotrophoblast continues and
the extra embryonic mesoderm is laid down between
the cytotrophoblast and the exocoelomic
membrane or primary yolk sac, and between the
cytotrophoblast and the amnion.
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The extra-embryonic mesoderm
continues to increase
Isolated cavities appear in the
mesoderm forming the extra
embryonic coelom
The hypoblast spread out along the
inner surface of the exocoelomic
membrane, partially surrounding
the exocoelomic cavity to form
the extra embryonic endoderm.
The part surrounded by only the
exocoelomic membrane is now
termed the primary yolk sac
The exocoelomic membrane + the The formation of a yolk sac together
extra-embryonic endoderm
with the fact that the embryo develops
resemble the yolk sac of the bird out of a blastodisc indicate that the
embryo although it is filled with
ancestors of the mammals had large
fluid- this part is the secondary
yolky eggs with meroblastic discoidal
yolk sac forming
cleavage.
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As the extra-embryonic coelom
becomes continuous and the extra
embryonic mesoderm splits into:
- extra-embryonic somatic
mesoderm, lining the trophoblast
and
- extra-embryonic splanchnic
mesoderm around the yolk sac and
the amnion.
The extra embryonic somatic
mesoderm and the cytotrophoblast
together form the chorion.
The chorion envelops the entire
embryo with its amnion and yolk sac
and is called the chorionic sac. The
embryo with amnion and yolk sac is
suspended in the chorionic sac.
The extra embryonic coelom now
becomes the chorionic cavity.
The amnion remains attached to
the chorionic sac in one area to
become the connecting stalk
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The primary embryonic yolk
sac decreases in size.
The extra-embryonic
endoderm lines only a
section of the exocoelomic
membrane.
Together the
- exocoelomic membrane,
- extra-embryonic
splanchnic mesoderm
- extra-embryonic
endoderm form the wall of
the secondary yolk sac
The remaining remnant of the
primary yolk sac is 'pinched
off, leaving only the
secondary yolk sac
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Towards the end of the 2nd week
the chorionic villi make their
appearance.
The cytotrophoblast produces
finger-like protrusions = primary
chorionic villi that push deep into
the syncytotrophoblast.
Cells of the hypoblast situated
opposite to the connecting stalk
become columnar.
This thickened disc-shaped area is
called the prochordal plate.
This plate indicates the head or
cranial region of the future
embryo as the endoderm of the
mouth and pharynx develop from
it. The opposite side is the caudal
region of the developing foetus.
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Gastrulation is the process by which the bilaminar embryonic
disc is converted into a TRILAMINAR embryonic disc.
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Some of the cells of the epiblast detach from the rest of the epiblast on
caudal side and move toward the midline.
These cells converge either side of the midline to form two ridges,
The new structure called the primitive streak, consisting of the
primitive groove that ends anteriorly in a thickening known as the
primitive pit or primitive knot.
As the cells converge progressively towards the primitive streak, the
migrating cells get forced down and move in between the epiblast and
the hypoblast to form a loose network (a mesenchyme) of cells the
mesoblast.
Some cells of the mesoblast, once part of the epiblast spread out
sideways as well as anteriorly in the direction of the prochordal plate to
form the embryonic mesoderm.
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The primitive streak extends in an
anterior direction as more cells
converge into the two ridges
The section of the mesoblast tissue
that migrates over the anterior part
of the primitive streak and the
primitive knot into the space
between the epi- and hypoblast
converge on the middorsal line to
form the notochordal process.
This develops into the notochord
while the intra embryonic
mesoderm flanking it on either side
develops into the somites.
Other mesoblastic cells join the
hypoblast, displacing the original
hypoblastic cells laterally to form a
new layer with the cells of the
hypoblast. This layer is the
embryonic endoderm.
The remainder of the epiblast that is
left behind on the surface now
become the embryonic ectoderm.
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The notochordal process becomes longer as more mesoblast cells roll
over the primitive pit, and will eventually reach the prochordal plate
More of the migrating cells move into the interior between the epi- and
hypoblast until the convergence of the cells start to decrease. The ridges
of the primitive streak therefore become shorter and the primitive streak
eventually disappears.
The epiblast is therefore the origin of the embryonic ectoderm, the
embryonic mesoderm and probably most of the embryonic endoderm.
The migration of the mesoblast cells into the primitive streak and in
between the epi- and hypoblast and the subsequent formation of the
notochordal process causes the elongation of the embryonic disc. By the
end of the third week it is no longer disc shaped but quite elongated.
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The embryo is now known as the
TRILAMINAR embryonic disc and is
triploblastic, which means that it consists of
three germ layers.
After the formation of the notochordal process
the ectoderm overlying the notochord and the
mesoderm on either side of it, thickens to form
a neural plate and the process of neurulation
i.e. the development of the neural tissue now
commences, exactly as in Branchiostoma.