Bilaminar &
Trilaminar
Embryonic Disc
Dr. Zeenat Zaidi
Bilaminar Embryonic
Disc
The Second Week
Formation of Amniotic Cavity
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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
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The inner cell mass
becomes flattened forming
a circular bilaminar plate,
the embryonic disc,
consisting of two layers:
 Epiblast (ectoderm),
thicker layer, consists of
high columnar cells, and
is related to the
amniotic cavity
 Hypoblast (endoderm),
consists of small
cuboidal cells, and lies
adjacent to the
blastocyst cavity
Formation of Primitive Yolk Sac
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The blastocyst cavity
becomes lined with
exocelomic membrane
and is called
exocelomic cavity
The hypoblastic cells
soon replace the
exocelomic membrane
and the cavity is then
named as the
primitive (primary)
yolk sac
At this stage the embryonic disc is:
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A circular bilaminar
disc, that lies between
the amniotic cavity
and the primitive yolk
sac
The epiblast forms the
floor of the amniotic
cavity & the hypoblast
lies in the roof of the
primitive yolk sac
Formation of Extraembryonic
Mesoderm
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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
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Isolated spaces
appear in the EEM
These spaces rapidly
fuse to form a large
fluid filled, C-shaped
cavity, the
extraembyonic
celome surrounding
the amniotic cavity
and the yolk sac
Formation of Connecting Stalk
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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
The site of the
connecting stalk
determines the caudal
pole of the embryonic
disc
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With the formation
of extraembryonic
celome:
The extraembryonic
mesoderm splits into
two layers:
• an outer
extraembryonic
parietal (somatic)
mesoderm
• an inner
extraembryonic
visceral (splanchnic)
mesoderm
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The primary yolk sac
decreases in size and
becomes the
secondary (definitive)
yolk sac
Wall of the yolk sac,
amnion & chorion are
formed
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Amnion: Two layers:
• Amnioblasts
• Extraembronic
splanchnic mesoderm
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Wall of the yolk sac:
Two layers:
• Endoderm
• Extraembronic
splanchnic mesoderm
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Chorion: Three layers:
• Extraembryonic somatic
mesoderm
• Cytotrophoblast
• Syncytiotrophoblast
chorion
amnion
bilaminar
disc
wall of the
yolk sac
Connecting
stalk
EEC
Extraembryonic celome is now called the
CHORIONIC CAVITY
Trilaminar
Embryonic Disc
The Third Week
The significant event of third
week is Gastrulation
Gastrulation
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The process by which the bilaminar
disc is converted into a trilaminar
disc
It is the beginning of morphogenesis
(formation of body form)
Consists of formation of the primitive
streak, the three germ layers & the
notochord
Embryo is referred to as a Gastrula
Primitive Streak
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The primitive streak
results from proliferation
of the epiblastic cells in
the median plane, in the
caudal half of the epiblast,
and lies along the craniocaudal axis.
Its cranial end forms
primitive node
A groove, primitive
groove, appears in the
primitive streak, which
continues with a small
depression, primitive pit,
in the primitive node.
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A circular thickening
appears in the
hypoblast near the
cranial end, in the
midline, to form the
prechordal plate, that
marks the future site
of mouth
A circular thickening
appears in the
hypoblast caudal to
primitive streak in
the midline to form
the cloacal
membrane, the future
site of the anus
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By this stage of
development, it is
possible to identify
the embryo’s:
craniocaudal axis
cranial and caudal
ends
dorsal and ventral
surfaces
right and left
sides.
Connecting
stalk
Formation of Intraembryonic
Mesoderm
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The epiblastic cells from
the primitive streak
(groove) proliferate to
form mesenchymal
tissue
The newly formed cells
invaginate, migrate
ventrally, laterally &
cranially between the
epiblast and hypoblast
& organize to form the
intraembryonic
mesoderm
Formation of Intraembryonic
Mesoderm cont’d
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Intraembryonic
mesoderm merges with
the extra-embryonic
mesoderm at the
periphery of the
embryonic disc
By the end of 3rd week,
mesoderm lies between
embryonic ectoderm and
endoderm everywhere
except in the region of
prechordal plate and
cloacal membrane, as the
embryonic ectoderm &
endoderm are fused at
these regions
Formation of Intraembryonic
Mesoderm cont’d
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Some mesenchymal
cells displace the
hypoblasts forming
the embryonic
endoderm
Cells remaining in
the epiblast form
the embryonic
ectoderm
Thus the EPIBLAST gives rise to all
three germ layers, Ectoderm,
Mesoderm, Endoderm in the embryo
Each of the three germ layers gives
rise to specific tissues and organs
Fate of Primitive Streak
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Actively forms mesoderm until the
early part of 4th week
Then it starts regressing and becomes
an insignificant structure in the
sacrocooccygeal regions
Normally it degenerates and disappears
by the end of 4th week
Remnants may persist and give rise to
a large tumor called Sacrococcygeal
Teratomas
Notochord
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A rod of
mesenchymal cells
located cranially, in
the midline,
extending between
the primitive node
and the prechordal
plate
Formation of Notochord
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Mesenchymal cells
migrate cranially from
primitive pit toward
the prechordal plate,
and form a rod like
notochordal process
The notochordal
process becomes
canalized forming a
hollow tube, the
notochordal canal,
communicating with
the primitive pit.
Formation of Notochord cont’d
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The floor of the
tube and the
underlying
endoderm break
down, forming a
notochordal plate
The notochordal
plate becomes
continuous with the
endodermal layer.
Formation of Notochord cont’d
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A temporary
communication
is established
between the
amniotic cavity
and the yolk sac,
termed the
neurenteric
canal.
Notochordal plate folds to form the notochord.
Functions of Notochord
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Defines primordial axis of the embryo
Provides rigidity to the embryo
Serves as a basis for the development
of the axial skeleton
Indicates the future site of the vertebral
bodies/column
Regulates differentiation of surrounding
structures including the overlying
ectoderm (neural plate) and mesoderm
(somites).
Fate of Notochord
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Degenerates and disappears as the
bodies of the vertebrae develop, but
it persists as the nucleus pulposus of
each intervertebral disc
Remnants of notochordal tissue give
rise to tumors called Chordomas
Differentiation of the
Intraembryonic Mesoderm
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Induced by the
notochord
Differentiates (in the
region of notochord)
into:
• Paraxial mesoderm
• Intermediate cell
mass
• Lateral plate
mesoderm