Origin and Tissue Structure
The allantois arises as an outpocketing of the ventral wall of the hindgut. In terms of origin and tissue composition, it is largely similar to the yolk sac.
The wall of the organ is always formed by two germ layers:
- Inner layer — represented by extraembryonic endoderm. Notably, this is derived specifically from the gut endoderm, rather than the hypoblast.
- Outer layer — formed by the visceral layer of extraembryonic mesoderm (derived from embryonic mesoderm).
- This bilaminar structure is characteristic of both birds and mammals, providing the structural basis for the physiological functions of the organ during embryogenesis.
Developmental Features in Birds
In birds, the allantois exhibits marked growth dynamics. During development, the organ rapidly increases in size. Its growth direction is strictly determined by the topography of surrounding structures: the allantois grows into the coelomic space located between the amnion and the serosa (chorion).
As it expands, it takes on two critically important functions:
- Excretion: the organ serves as a capacious reservoir for accumulating excretory products (embryonic metabolic waste).
- Gas Exchange: due to its large size and extensive vascularization, it facilitates efficient embryonic respiration.
Development and Functions in Mammals
In mammals, the allantois retains its finger-like shape and general origin analogous to that of birds, but it has a different growth topography — it grows into the connecting stalk (amniotic stalk).
Its functional role in mammals has specific characteristics:
- Excretory function is transient. The organ collects metabolic waste only in early stages, before the placenta becomes fully functional.
- Vascular function ("Vector"): This is a crucial role of the allantois in establishing the maternal-fetal interface. The organ acts as a pathway along which blood vessels grow from the embryonic body to the chorion.
Once the allantois completes its tasks of vascular delivery and temporary excretion, its fate is sealed. Much like the yolk sac, the organ undergoes complete regression during early embryonic development.