Origin and tissue structure
The allantois originates as an evagination of the ventral wall of the hindgut. In its origin and tissue composition, it is largely similar to the yolk sac.
The wall of the organ is always formed by two germ layers:
- Internal layer — represented by extraembryonic endoderm. It is important to emphasize that this is a derivative of intestinal endoderm specifically, rather than the hypoblast.
- External layer — formed by the visceral leaflet of the extraembryonic mesoderm (a derivative of the embryonic mesoderm).
This bilayered structure is characteristic of both birds and mammals, providing the structural basis for the organ's physiological functions during embryogenesis.
Developmental features in birds
In birds, the allantois exhibits marked growth dynamics. During development, this organ actively increases in size. The direction of its growth is strictly determined by the topography of surrounding structures: the allantois grows into the slit-like space located between the amnion and the serosa.
As its volume increases, it assumes two critically important functions:
- Excretion: the organ serves as a capacious reservoir for the accumulation of excreta (embryonic metabolic waste products).
- Participation in gas exchange: due to its size and position, it ensures an efficient respiratory process for the embryo.
Development and functions in mammals
In mammals, the allantois retains its finger-like shape and general origin analogous to that in birds, but has a different growth topography — it grows into the connecting stalk (body stalk).
Its functional role in mammals has specific characteristics:
- Excretory function is temporary in nature. The organ collects metabolic waste products only at early stages, until the placenta becomes fully functional.
- Vascular function ("Vector"): this is a crucial role of the allantois in establishing the maternal-fetal connection. The organ acts as a pathway along which blood vessels grow from the embryo's body to the chorion.
After the allantois fulfills its tasks of vascular conduction and temporary excretion, its fate is sealed. Much like the yolk sac, the organ completely regresses at early stages of embryonic development.
Briefly: answers to topical questions
Which specific blood vessels grow along the allantois within the umbilical cord?
Umbilical arteries and umbilical veins grow along the allantois within the umbilical cord. The vascular component of the umbilical cord is represented by the following structures:
- Umbilical arteries — two arteries that carry deoxygenated blood from the fetus to the chorionic villi (placenta).
- Umbilical veins — two veins are laid down in early ontogenesis, but they subsequently fuse, leaving a single umbilical vein that carries oxygenated blood from the placenta to the fetus.
To avoid confusing the origin of the internal layer of the allantois, remember its shape: it is an outpocketing of the primitive gut. Consequently, its inner lining is formed by intestinal endoderm specifically, rather than the hypoblast.
Class and exam questions
What are the external and internal layers of the allantois formed by?
The internal layer is formed by extraembryonic endoderm (a derivative of the intestinal endoderm), and the external layer by extraembryonic mesoderm.
Into what space does the allantois grow in birds?
In birds, it actively grows into the space (slit) between the amnion and the serosa.
What does the "vector" function mean in mammals?
This means that the allantois serves as a pathway (guide structure) along which blood vessels grow from the embryo to the chorion.
What happens to the organ in mammals subsequently?
It regresses at early stages of development immediately after the placenta begins to function.
What else to study on this topic
- Formation of the connecting stalk
- The relationship of the allantois to chorionic development
- Topography of the space between the amnion and the serosa
- Mechanisms of regression of extraembryonic organs
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