The early development of the chick begins with fertilization and cleavage inside the hen. Cleavage occurs only in the small blastodisc atop the large yolk, forming a single-layered blastoderm. During blastoderm formation, some cells delaminate to form the hypoblast layer below the epiblast. Gastrulation then occurs through the formation of the primitive streak, with cells ingressing through the streak to form the mesoderm and endoderm lineages. FGF signaling guides the migration and patterning of these lineages as the embryo develops its basic body plan.

1. Early Development of
Chick

3. Fertilization and Cleavage
• Fertilization of the chick egg occurs in the oviduct, before the albumen and shell are secreted to
cover it.
• Cleavage occurs during the first day of development, while the egg is still inside the hen, during
which time the embryo progresses from a zygote through late blastula stages.
• The chick egg is telolecithal, with a small disc of cytoplasm-the blastodisc-sitting on top of a
large yolk.
• The yolky eggs of birds undergo discoidal meroblastic cleavage.
• Cleavage occurs only in the blastodisc, which is about 2-3 mm in diameter and is located at the
animal pole of the egg.
The first cleavage furrow appears centrally in the blastodisc; other cleavages follow to create a
Single layered blastoderm.

4. Blastoderm Formation (Blastulation)
• Thereafter, equatorial and vertical cleavages divide the blastoderm into a tissue 5-6 cell layers
thick, and the cells become linked together by tight junctions.
• Between the blastoderm and the yolk of avian eggs is a space called the subgerminal cavity,
which is created when the blastoderm cells absorb water from the albumen ("egg white") and
secrete the fluid between themselves and the yolk.
• At this stage, the deep cells in the center of the blastoderm appear to be shed and die, leaving
behind a 1-cell-thick area pellucida; this part of the blastoderm forms most of the actual embryo.
The peripheral ring of blastoderm cells that have not deep cshed their ells constitutes the area
opaca. Between the area pellucida and the area opaca is a thin layer of cells called the marginal
zone.

7. Epiblast – Hypoblast
Delamination

8. Entire Embryo developed from Epiblast.
Hypoblast helps epiblast cells in migration by providing signals.
Epiblast + Koller’s
sickle cells
Epiblast --Primitive streak
Ingression

9. (A-C) Events prior to laying of the shelled egg,
(A) Stage X embryo, where islands of hypoblast
cells can be seen, as well as a congregation of
hypoblast cells ls around Koller's sickle.
(B) By stage XII, just prior to primitive streak
formation, the hypoblast island cells have
coalesced to form the primary hypoblast layer,
which meets endoblast cells and primitive
streak cells at Koller's sickle.
(C) By stage XIII, the secondary hypoblast cells
migrate anteriorly.
(D) By stage 2 (6--7 hours after the egg is laid),
the primitive streak cells form a third layer that
lies between the hypoblast and epiblast
cells.
(E) By stage 3 (up to 13 hours post laying). The
primitive
streak has become a definitive region of the
epiblast, with cells
migrating through it to become the mesoderm
and endoderm.

10. Gastrulation in avian (chick) embryo
• By the time a hen has laid an egg, the blastoderm contains some 20,000 cells.
• Most of the cells of the area pellucida remain at the surface, forming an "upper layer,"
called the epiblast (upper layer).
• A number of area pellucida cells delaminate and migrate into the subgerminal cavity
where they form the polyinvagination islands (primary hypoblast), a loose collection
(archipelago of disconnected clusters) of cell groups containing 5-20 cells each.
Shortly, after the egg is laid, a small thickening of the epiblast, called Koller’s sickle,
is formed at edge of the area pellucida which is now marked as the posterior end of the
embryo. Koller’s sickle is a belt like region of cells called the posterior marginal
zone (PMZ).

11. • Next, a sheet of cells originating from a local thickening at the posterior margin of the
blastoderm (Koller's Sickle) migrate anteriorly and push the primary hypoblast cells in
front of them, thereby forming the secondary hypoblast or endoblast.
• The resulting two-layered blastoderm (epiblast and hypoblast) is joined together at the
marginal zone of the area opaca, and the space between the layers forms a blastocoel.
• The avian embryo comes entirely from the epiblast; the hypoblast does not contribute
any cells to the developing embryo.
• Rather, the hypoblast cells form portions of the external membranes, especially the yolk
sac and the vitelline duct- the stalk linking the yolk mass to the digestive tube. Hypoblast
cells also provide chemical signals that specify the migration of epiblast cells.

12. Development of Primitive Streak
• The major structural characteristic of amniotes (avian, reptilian, and mammalian)
gastrulation is the primitive streak.
• Primitive streak cells arise in the posterior marginal region.
• The streak is first visible as cells accumulate in the middle layer, followed by a
thickening of the epiblast at the posterior marginal zone, just anterior to Koller's
sickle.

13. • As cells converge to form the primitive streak, a depression forms within the streak. This
depression is called the primitive groove, and it serves as an opening through which migrating
cells pass into the
• deep layers of the embryo (blastocoel). Thus, the primitive groove is homologous to the amphibian
blastopore, and the primitive streak is homologous to the blastopore lips.
• At the anterior end of the primitive streak is a regional thickening of cells called Hensen's node
(also known as tile primitive knot). The center of Hensen's node contains a funnel-shaped
depression (sometimes called the primitive pit) through which cells can migrate into the blastocoel.
• Hensen's node is the functional equivalent of the dorsal lip of the amphibian blastopore (i.e., the
organizer).
• The first cells to enter the primitive streak are endodermal progenitors (endoblast cells) from the
epiblast. These cells go through an epithelia mesenchymal transition.

14. Cell movements of the primitive
streak and fate map of the chick
embryo.
(A-C) Dorsal view of the formation
and elongation of the primitive
streak. The blastoderm is seen at
(A) 3-4 hours, (8) 7-8 hours, and
(C) "1 5-16 hours after fertilization.
(O-F) Formation of notochord and
mesodermal somites as the
primitive streak regresses,
shown at (D) 1 9-22 hours, (E) 23-
24 hours, and (F) the four-somite
stage.
Fate maps of the chick epiblast
are shown for two stages, the
definitive primitive streak stage (C)
and neurulation IF). In (F), the
endoderm has already ingressed
beneath the epiblast, and
convergent extension is seen in
the midline. The movements of
the mesodermal precursors
through the primitive streak at (C)
are shown.

15. • When the primitive streak is formed it defines the axes of the embryo. It stretches
from anterior to posterior; gastrulating cells ingress from its dorsal side and
move to the ventral side; and it divides the left part of the embryo from the right.
• At Hensen’s node and throughout the primitive streak, embryo release scatter
factor, a 190 kDa protein secreted by the cells as they enter the streak. Scatter
factor can convert epithelial sheets into mesenchymal cells in several ways, and
is probably involved both in downregulating E-Cadherin expression and in
preventing E-Cadherin functioning.

16. • Migration of endodermal and mesodermal cells through the primitive streak
Deep lateral migrating cells form
Endoderm displace hypoblast.
Shallow cells form mesodermal
Mesenchyme.
FGF8 expression primitive streak
repel migrating cell away. FGF 4
produced by chordamesoderm
attract migrating mesoderm cells.

17. FGF Signaling
• The movement of the prospective mesoderm through the anterior primitive streak
and its condensation into chordamesoderm seems to be regulated by complex
chemoattractive and chemorepulsive processes, both involving FGF signaling.
• FGF 8 is expressed in the primitive streak and repels migrating cells away
from the streak, while the same cells are attracted to FGF 4 secreted from the
developing chordamesoderm.

18. Fate of cells ingressing through the primitive streak
• As soon as the streak has developed, epiblast cells begin to move through it into the blastocoel
to form mesoderm and endoderm. Thus the primitive streak is formed by an always changing
cell population. With respect to mesodermal components , the anterior end of the streak
(Hensen’s Node) gives rise to prechordal plate mesoderm, notochord and rostral somites.
• Cells migrate through middle part of the streak give rise to somites, heart and kidneys.
• Cells coming from the posterior part of the streak make the lateral plate and
extraembryonic mesoderm.

20. Characteristic Features of Avian Gastrulation
• Formation of hypoblast and its important role on formation of the axis and orientation of
embryo.
• Presence of the cells of all three germinal areas in the epiblast.
• Formation of the primitive streak and its regression in later stages of gastrulation.
• Absence of archenteron formation. Gastrular cell movements include ingression,
involution, convergence, divergence and epiboly.
• Cephalo-caudal differentiation of axial structures and growth of the embryo during
gastrulation.

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