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Gastrulation in Frogs
Cells located where the gray crescent was begin to invaginate into the
blastocoel, forming the blastopore. This area is now known as the dorsal
lip of the blastopore and is a key player in the future development of
tissue. Tissue rolls over the dorsal lip of the blastopore as it moves into
the embryo. This process is called involution.The new cavity is called the
archenteron (primitive gut) and will eventually become the digestive
tract.The blastopore will become the anus in the frog. The mouth will
form elsewhere (“second the mouth”)
Early on, the dorsal lip of the blastopore forms due to the contraction of
bottle cells. The blastopore continues to develop from the early "frown"
until it can be observed as a complete circular ring of involuting cells.
Convergent extension closes the blastopore at the yolk plug and elongates
the embryo along the anterior--posterior axis. The posterior end of the
embryo is pointed at you. How does the the blastopore lip form? A small
group of cells change shape, narrowing at the exterior edge of the
blastula. This change in cell shape, called apical constriction, creates a
local invagination, which pushes more interior cells upwards and begins
to roll a sheet of cells towards the interior. The constricted cells are called
bottle cells, due to their shape
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Mammalian Gastrulation
In mammals, gastrulation is achieved in a very different way. This is
because the outer layer of the mammalian blastocyst, which is roughly
equivalent to the blastula, becomes the mammalian placenta and is not
available to create the primary germ layers of the embryo. Instead, the
inner cell mass, or ICM, has to form the three primary germ layers of the
mammalian embryo after the blastocyst has implanted in the uterine wall.
Instead of forming a hollow ball and invaginating to create the gut tube
and the three cell layers, the ICM of the mammalian embryo forms a
single layer of cells called the epiblast. As these epiblast cells proliferate,
some of them migrate out of the epiblast to form the endoderm, which is
this layer here, and then after that more cells migrate out to form the
mesoderm, which is this cell layer here. The cells that remain in the
epiblast
form
the
ectoderm.
What the Different Germ Layers Become
Gastrulation is accomplished when the hollow blastula invaginates to
form an inner layer of cells called the endoderm and an outer layer of
cells called the ectoderm. A third, middle layer of cells called the
mesoderm is formed as cells begin to populate the space between the
endoderm and the ectoderm. These three layers, the endoderm, the
ectoderm and the mesoderm, are called the primary germ layers. With the
formation of the three primary germ layers, the embryo has begun the
process of tissue differentiation. Each of the three primary germ layers
will form a different set of specific organs and tissue types in the animal.
The ectoderm will form the nervous system, the epithelial cells of the
mammary glands, the lens of the eyes, hair, pigment cells and the
epidermis of the skin. The mesoderm will form skeletal muscle, smooth
muscle, the heart, blood vessels, blood cells, kidney, spleen, fat cells, the
skeleton, most connective tissues and most of the urogenital system. The
endoderm will form the entire epithelial lining of the gastrointestinal
tract, as well as the liver, pancreas, gall bladder, thyroid, the epithelial
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lining of the trachea and the respiratory surface of the lungs.
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