Embryology

1. Extra Embryonic Membranes
Dr. Gauri Haval
Assistant Profrssor
Abasaheb Garware College Pune

2. The amniote egg allowed vertebrates to
• roam on land, far from existing ponds. Whereas most
amphibians must return to water to lay their eggs,
• the amniote egg carries its own water and food supplies.
• It is fertilized internally and contains yolk to nourish the
developing embryo.
• the amniote egg contains four sacs: the yolk sac, which
stores nutritive proteins; the amnion, which contains the
fluid bathing the embryo; the allantois, in which waste
materials from embryonic metabolism collect; and the
chorion, which interacts with the outside environment,
selectively allowing materials to reach the embryo.
• The entire structure is encased in a shell that allows the
diffusion of oxygen but is hard enough to protect the
embryo from environmental assaults and dehydration.

3. The Extra-embryonic Membranes
In reptiles, birds, and mammals, embryonic development has
taken a new evolutionary direction. Reptiles evolved a
mechanism for laying eggs on dry land, To accomplish this, the
embryo produces four sets of extra-embryonic membranes to
mediate between it and the environment.
Most mammals have evolved placentas instead of shells, the
basic pattern of extra-embryonic membranes remains the same.
In developing reptiles, birds, and mammals, there initially is no
distinction between embryonic and extra-embryonic domains.
However, as the body of the embryo takes shape, the epithelia
at the border between the embryo and the extra-embryonic
domain divide unequally to create body folds that isolate the
embryo from the yolk and delineate which areas are to be
embryonic and which extra-embryonic.

4. These membranous folds are formed by the extension of ectodermal
and endodermal epithelium underlain with lateral plate mesoderm.
The combination of ectoderm and mesoderm, often referred to as
the somatopleure, forms the amnion and chorion; the combination
of endoderm and mesoderm the splanchnopleure forms the yolk sac
and allantois. The endodermal or ectodermal tissue supplies
functioning epithelial cells, and the mesoderm generates the
essential blood supply to and from this epithelium.

7. The amnion and chorion
The first problem of a land-dwelling egg is desiccation. Embryonic
cells would quickly dry out if they were not in an aqueous
environment. This environment is supplied by the amnion. The
cells of this membrane secrete amnionic fluid; thus,
embryogenesis still occurs in water. This evolutionary adaptation
is so significant and characteristic that reptiles, birds, and
mammals are grouped together as the amniote vertebrates, or
amniotes.
The second problem of a land-dwelling egg is gas exchange. This
exchange is provided for by the chorion, the outermost
extraembryonic membrane. In birds and reptiles, this membrane
adheres to the shell, allowing the exchange of gases between the
egg and the environment. In mammals, as we have seen, the
chorion has developed into the placenta, which has evolved
endocrine, immune, and nutritive functions in addition to those of
respiration

8. The allantois and yolk sac
The third problem for a land-dwelling egg is waste disposal. The
allantois stores urinary wastes and also helps mediate gas
exchange. In reptiles and birds, the allantois becomes a large
sac, as there is no other way to keep the toxic by-products of
metabolism away from the developing embryo. In some amniote
species, such as chickens, the mesodermal layer of the allantoic
membrane reaches and fuses with the mesodermal layer of the
chorion to create the chorioallantoic membrane. This extremely
vascular envelope is crucial for chick development and is
responsible for transporting calcium from the egg shell into the
embryo for bone production

9. The fourth problem that a land-dwelling egg has to solve is
nutrition. The yolk sac is the first extraembryonic membrane to
be formed, as it mediates nutrition in developing birds and
reptiles. It is derived from splanchnopleural cells that grow over
the yolk to enclose it. The yolk sac is connected to the midgut by
an open tube, the yolk duct, so that the walls of the yolk sac and
the walls of the gut are continuous. The blood vessels within the
mesoderm of the splanchnopleure transport nutrients from the
yolk into the body, for yolk is not taken directly into the body
through the yolk duct. Rather, endodermal cells digest the
protein into soluble amino acids that can then be passed on to
the blood vessels within the yolk sac. Other nutrients, including
vitamins, ions, and fatty acids, are stored in the yolk sac and
transported by the yolk sac blood vessels into the embryonic
circulation. In these ways, the four extraembryonic membranes
enable the amniote embryo to develop on land.

10. • Reference:
• Developmental Biology By Gilbert
• Embryology By Balinsky