Development in terrestrial animals, avian embryology and the fate of mesoderm
Development in
terrestrial environments:
Reptiles, Aves
and mammals develop on land and required protection from desiccation which is
provided by extra embryonic membranes. Longer developmental periods of these
animals reflect their lack of independent larval stages.
Avian
embryology:
Egg: The yellow
portion of the chicken egg is single cell produce in chicken ovary. This egg is
released into the oviduct where fertilization may occur. Following
fertilization, membranes and fluids collect around the egg. A vitelline
membrane covers the surface of true egg. The ‘white’ consists of water and a
protein called albumen. This watery environment protects the egg from
mechanical damage and drying. Albumen is a source of nutrients and is
eventually consumed during development.
Two denser strands of albumen (called
chalazas) attach to the inside of the shell and to the egg and suspend the egg
in the centre of watery albumen. The shell is made up of calcium carbonate
impregnated with protein. Thousands of tiny pores in the shell permit gas
exchange between the embryo and the outside. On the inside of the shell are two
shell membranes. An air pocket forms between these membranes at the rounded end
of the shell. The air pocket enlarges during development as air moves through
pores in the shell to replace water loss as hatching approaches the chick
penetrates the air pocket with its beak, the lungs inflate and the chick begins
to breathe from the air sac while still exchanging gases across vascular extra
embryonic membranes.
Early cleavages
and Gastrulation:
Cleavage of
chicken egg is meroblstic. A small disc blastoderm of about sixty thousand
cells at the animal end of the egg develops. The blastoderm is raised off the
yolk, leaving a fluid filled space analogous to blastocoel of the amphibian
blastula. Proliferation and movement of blastoderm cell sort the cells into two
layers. The epiblast is the outer layer of cells and the hypoblast is the inner
layer. The movements of blastoderm cells are the beginning of gastrulation. The
female reproductive tract releases the egg at about this time. A medial, linear
imagination called the primitive streak, gradually extends anteriorly. A
depression called Henson’s node, forms at the anterior margin of the primitive
streak and marks the beginning of inward migration of epiblast cells,
comparable to involution of amphibian gastrula.
The primitive
streak is therefore analogous toe the dorsal lip of the blastopore. The
migration occurs during a dramatic posterior movement of Henson’s node.
Migrating cells form mesoderm, what is left of the epiblast forms the
endodermal lining of the gut tract. Three germ layers are now arranged above
the surface of the yolk.
Following
gastrulation, notochordal cells separate from the overlying neural ectoderm and
the neural tube forms. In addition mesoderm which originally formed as solid
blocks of cells, organizes into somites and splits to form the coelom.
The embryo lifts
off the yolk when the margins of the embryo grow downward and meet below the
embryo. A connection between the embryo and the yolk is retained and called the
yolk stalk. Blood vessels develop in the yolk stalk and carry nutrients from
the yolk to the embryo.
Development of
extra embryonic membranes:
Extra embryonic
membranes of amniotes include the yolk sac, the amnion, the chorion and the
allantois. Reptiles and birds have large quantity of yolk that becomes enclosed
by yolk sac. Yolk sac develops from proliferation of endoderm and mesoderm
around the yolk. The yolk sac is highly vascular and distributes nutrients to
the developing embryo.
Amnion and
Chorion:
Following the
neural tube stage the ectoderm and mesoderm on both sides of the embryo lift
off the yolk and grow dorsally over the embryo. As these membranes meet
dorsally they fuse and form inner amnion and outer chorion. Amnion encloses the
embryo in a fluid filled sac. This amniotic cavity protects against shock and
drying. Chorion is nearer the shell, becomes highly vascular and aids in gas
exchange.
Allantois:
The immediate
breakdown product of proteins is highly toxic ammonia. This ammonia is
converted to uric acid which is excreted and stored in the allantois, a ventral
outgrowth of the gut tract. Uric acid is semi solid and thus little water is
wasted. Allantois gradually enlarges during development to occupy the region
between amnion and chorion. In addition allantois become, highly vascular and
functions with chorion in gas exchange.
Fate of
Mesoderm:
Following
gastrulation in birds, reptiles and mammals all three primary germ layers have
been formed of the three layers the face of mesoderm is the most complex.
Mesoderm forms all of the supportive tissues of vertebrates including
connective tissues (bone, cartilage and blood) and muscle. These supportive
tissues are frequently associated with derivates of other primary germ layers.
For example the inner lining of the gut is endodermal but mesodermally derived
structures such as smooth muscles, blood and blood vessels make up the bulk of
that system.
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