DE VERA, ALYSSA GAIL V.

4BIO6

Gastrulation in Chick
Outline Morphogenetic movements

I. Introduction to gastrulation of Invagination: a sheet of cells

Chick (called an epithelial sheet) bends
II. Overview of Chick Blastula inward
III. Formation of Primitive Streak Ingression: individual cells leave
and Establishment of Germ an epithelial sheet and become
freely migrating mesenchyme
Layers
cells
IV. Role of Hensens node
Involution: an epithelial sheet
V. Results of Gastrulation
rolls inward to form an underlying
layer
I. Introduction to Gastrulation
of Chick Epiboly: a sheet of cells spreads
by thinning
Gastrulation is the dynamic Intercalation: rows of cells move
process during which the
major, presumptive organ-
forming areas of the blastula
become rearranged and between one another,
organized in a way which creating an array of
cells
permits their ready conversion
into the body plan of the
particular species (Nelsen,
1953).

Major events in gastrulation that is longer

reorganization of the (in one or
embryo from a simple more dimensions) but
spherical ball of cells, thinner
Convergent Extension: rows of
the blastula, into a
cells intercalate, but the
multi-layered organism intercalation is highly directional
the primary germ
layers are established II. Overview of Chick Blastula
cells at or near the
surface of the embryo
move to a new, more
interior location

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 The chick

During
blast
ula is composed of two layers,
the epiblast and hypoblast. The
area of the epiblast and
hypoblast separated from the
yolk is the area pellucida.
Epiblast and hypoblast are joined gastrulation, cells in the
together at the margin of area prospective posterior half of the
opaca. epiblast move in the plane of the
epiblast toward the midline.
The hypoblast is more complete
at the posterior end of the
blastoderm than at its extreme
anterior and anterolateral The lateral-to-medial migration
margins (Nelsen, 1953). The advances within the epiblast from
posterior to anterior eventually
involving cells located throughout
the posterior half of the
blastoderm. The result is a
marked thickening along the
posterior midline.

underlying hypoplast at the

posterior midline (Koller's sickle)
induces the formation of the
primitive streak.

III. Formation of Primitive

Streak and Establishment of
Germ Layers

Formation of Primitive Streak

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Gradually, this thickening Establishment of Germ Layers
lengthens because cells from the
anterolateral area of the epiblast Some epiblast cells move
also begin to move toward the through the primitive groove,
midline, and perhaps, too, where they ingress into the
because the entire blastoderm interior. Ingression occurs mostly
elongates front to back. This after the primitive streak has
central line of thickening reached its maximum length.
becomes an area of active Two routes of cells that leave the
invagination and ingression, as surface layers:
cells from surface layers move
inward toward the hypoblast. This 1. Some cells insert
line is called the primitive themselves all the way into
streak. the hypoblast, literally
pushing the hypoblast to
the periphery. Eventually
the entire layer beneath
the area pellucida
A
occupied by the hypoblast
becomes occupied by the
migrating cells.
depression 2. Other cells migrating
through the primitive
groove move laterally and
anteriorly in the primitive
is formed streak and establish a new
layer between the epiblast
and the remodeled
within the primitive streak called
hypoblast.
primitive groove. This structure is
analogous to the blastopore. The
anterior end of the primitive
streak is a regional thickening of
cells called primitive knot
(hensens node) which is
analogous to the dorsal lip of
blastopore.

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 Origin of Three Germ Layers
Germ layer Origin
Ectoderm Cells remaining in
the epiblast
Endoderm Cells replacing the
hypoblast
Mesoderm Cells that migrated
in the middle of
epiblast and
remodeled
hypoblast

IV. Role of Hensens Node

Hensens node is the anterior

border of the primitive streak.
It is a hub of complex
morphogenetic activities and
important cell interactions. It
has a depressed center, the
primitive pit, which is
surrounded by ridges. It
marks the position of the
node. The node is a structural
way station for passing cells.
Some of these cells come
from the lateral and
anterolateral portions of the
epiblast. Other cells
ingressing through the
anterior primitive pit
apparently also move
anteriorly and pass through
the hensens node. The node
is also a center for cell
proliferation which gradually
extends forward from the
node forming the head
process. The head process
will form the notochord. Cell

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marking experiments have
The hensens node will move
posteriorly obliterating the
primitive streak as it moves
through the blastoderm and
leaving behind the neural

plate, the notochord, and the

developing somites.
shown that the medial portion
The nodes migration
of neural plate, the notochord,
eventually brings it to an area
and the medial portions of
near the posterior margin of
somites all come from cells in
the area pellucida, where its
the hensens node.
morphogenetic movements
ceases and it becomes
indistinct.

Note: The hensens node

organizes the axis and
induces the central nervous
system.

At the end of gastrulation, the

very posterior portion of the
streak does not have well
defined germ layers. The cells

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 in this posterior portion of the References
streak will form the tail bud,
the source of muscles and Developmental Biology 3230. (n.d.).
spinal cord for the postanal Retrieved from
tail. http://courses.biology.
utah.edu/bastiani/3230/DB
V. Results of Gastrulation %20Lecture/Lectures/a7Gast.htm
Establishment of the l
three germ layers
Organization and
Nelsen, O. E. (1953). Comparative
induction of central
Embryology of the Vertebrates.
nervous system
New York: McGraw-hill Book
formation
Company, Inc.

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