Download Third week o Development

Third week of
Development
view of the germ disc at the end of the second
week of development
Dorsal aspect of an 18-day embryo
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The embryo has a
pear-shaped
appearance and
shows the primitive
streak and node at its
caudal end on the
surface o the epiblast
The hypoblast and epiblast are in contact with each
other, and the primitive streak forms a shallow groove
in the caudal region of the embryo
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Primitive streak : a narrow groove with slightly bulging
regions on either sides on the surface of the epiblast in the
caudal region of the embryo .
Primitive node : slightly elevated area on the cephalic end
of the primitive streak surrounding the small primitive pit .
Dorsal side of the germ disc from a 16-day embryo indicating
the movement of surface epiblast cells
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Cells of the epiblast migrate towards the primitive streak
and node and move inwards ( invaginate ) through the
streak and node . Some of the invaginated cells displace
the hypoblast creating the embryonic endoderm . Others
come to lie between the epiblast and newly created
endoderm to form mesoderm . Cells remaining in the
epiblast form ectoderm .
Gastrulation
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Is the process that establishes all three
germ layers (ectoderm, mesoderm, and
endoderm) in the embryo. This process
OCCURS during the third week of
development with the appearance of a
primitive streak and node on the surface of
the epiblast . The epiblast, through the
process of gastrulation, is the source of all
of the germ layers in the embryo .
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As more and more cells move
between the epiblast and
hypoblast layers , they begin
to spread laterally and
cranially gradually, they
migrate beyond the margin of
the disc and establish contact
with the extraembryonic
mesoderm covering the yolk
sac and amnion.
In the cephalic direction, they
pass on each side of the
Prechordal plate
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The prechordal plate: thickened localized region
in the cephalic edge of the endoderm . It forms
between the tip of the notochord and the
Buccopharyngeal membrane .The prechordal
plate will be important for induction of the
forebrain and it contributes to the general head
mesenchyme .
The buccopharyngeal membrane at the cranial
end of the disc consists of a small region of tightly
adherent ectoderm and endoderm cells that
represents the future opening of the oral cavity.
Notochord
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A prolongation undercover the ectoderm . It arises from the
primitive node and extends cephalically as far as the prechordal
plate . This prolongation is known as the notochordal process . The
primitive pit may extend into the notochordal process as the
notochordal canal .The floor of this channel becomes intercalated in
the endoderm and begins to disintegrate allowing communication
between the amniotic cavity and the yolk sac ( Neuroenteric canal ) .
The roof of the notochordal process form the notochordal plate .
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B. cross section
through the region of
the notochordal plate.
Soon the notochordal
plate will detach from
the endoderm to form
the definitive
notochord.
C. Schematic view
showing the definitive
notochord.
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The cloacal membrane is formed at
the caudal end of the embryonic disc
This membrane, which is similar in
structure to the buccopharyngeal
membrane, consists of tightly
adherent ectoderm and endoderm
cells with no intervening mesoderm.
allantoenteric diverticulum, or
Allantois: the posterior wall of the yolk
sac forms a small diverticulum that
extends into the connecting stalk. This
diverticulum appears around the 16th
day of development . in some lower
vertebrates, It serves as a reservoir
for excretion products of the renal
system. In humans, it remains
rudimentary but may be involved in
abnormalities of bladder development
.
Fate map for epiblast cells
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Notochord-n-: Epiblast cells migrating
at the cranialmost part of the node .
Paraxial mesoderm( somitomeres and
somites ) -pm- : Epiblast cells
migrating at the lateral edges of the
node and the cranial end of the streak
.
Intermediate mesoderm( urogenital
system ) -im- epiblast cells migrating
trough the midstreak region
Lateral plate mesoderm ( body wall ) :
–lpm-epiblast cells migrating through
the more caudal part o the streak
extraembryonic mesoderm (eem;
chorion): epiblast cells migrating
through the most caudal part . the
other source of this tissue is the
primitive yolk sac (hypoblast).
Growth of the embryonic disc
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The embryonic disc, initially flat and round, gradually
becomes elongated, with a broad cephalic and a narrow
caudal end because of the continuous migration of cells from
the primitive streak region in a cephalic direction.
In the cephalic part, germ layers begin their specific
differentiation by the middle of the third week, whereas in the
caudal part, differentiation begins by the end of the fourth
week. This causing the embryo to develop cephalocaudally .
Invagination of surface cells in the primitive streak and their
subsequent migration forward and laterally continues until the
end of the fourth week. At that stage, the primitive streak
shows regressive changes and soon disappears.
Clinical Correlates
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Teratogenesis Associated with Gastrulation: The initiation of
gastrulation at the beginning of the 3rd week is a highly
sensitive stage for teratogenic insult. At this time, fate maps
can be made for various organ systems, such as the eyes
and brain anlage, and these cell populations may be
damaged by teratogens )e.g. high doses of alcohol ).
Tumors Associated with Gastrulation : Sometimes, remnants
of the primitive streak persist in the sacrococcygeal region.
These clusters of pluripotent cells proliferate and form
tumors, known as sacrococcygeal teratomas, that commonly
contain tissues derived from all three germ layers . This is the
most common tumor in newborns. These tumors may also
arise from primordial germ cells (PGCs) that fail to migrate to
the gonadal ridge.
Trophoblast (villous structure)
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the trophoblast is characterized by primary villi that consist of a cytotrophoblastic
core covered by a syncytial layer .
During further development, mesodermal cells penetrate the core of primary villi and
grow toward the decidua. The newly formed structure is known as a secondary villus
.
By the end of the third week, mesodermal cells in the core of the villus begin to
differentiate into blood cells and small blood vessels, forming the villous capillary
system . The villus is now known as a tertiary villus or definitive placental villus.
Trophoblast
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cytotrophoblastic cells in the
villi penetrate progressively
into the overlying syncytium
until they reach the maternal
endometrium. Here they
establish contact with similar
extensions of neighboring
villous stems, forming a thin
outer cytotrophoblast shell .
This shell gradually surrounds
the trophoblast entirely and
attaches the chorionic sac
firmly to the maternal
endometrial tissue.
Trophoblast
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Stem or anchoring villi: those Villi extend
from the chorionic plate to the decidua
basalis (decidual plate: the part of the
endometrium participate in the formation of
the placenta ).
Free (terminal) villi : Those villi that branch
from the sides of stem villi . Exchange of
nutrients occur through these villi .
Chorionic cavity
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The chorionic cavity,
meanwhile, becomes
larger, and by the 19th or
20th day, the embryo is
attached to its
trophoblastic shell by a
narrow connecting stalk .
The connecting stalk later
develops into the umbilical
cord, which forms the
connection between
placenta and embryo.
Thank You
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Embryonic peroid