Allantois

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Allantois

Diagram illustrating early formation of allantois and differentiation of body-stalk.

Sectional plan of the gravid uterus in the third and fourth month.
Gray's	subject #12 54
Gives rise to	Umbilical cord
MeSH	Allantois

Allantois (plural allantoides or allantoises) is a part of a developing animal conceptus (which consists of all embryonic and extra-embryonic tissues).

1 Function
2 Etymology
3 Additional images
4 References
5 External links

[edit] Function

This sac-like structure is primarily involved in respiration and excretion, and is webbed with blood vessels.

It is primarily found in the blastocyst stage of early embryological development, and its purpose is to collect liquid waste from the embryo.

[edit] In reptiles, birds, and monotremes

The structure first evolved in reptiles and birds as a reservoir for nitrogenous waste, but also as a means for oxygenation of the embryo.

Oxygen is absorbed by the allantois through the egg shell. The allantois functions similarly in monotremes, which are egg-laying mammals.

[edit] In most marsupials

In most marsupials, the allantois is avascular, having no blood vessels, but still serves the purpose of storing nitrogenous (N2) waste.

Also, most marsupial allantoises do not fuse with the chorion. An exception is the allantois of the bandicoot, which has a vasculature, and fuses with the chorion.

[edit] In placental mammals

In placental mammals, the allantois is the precursor of the mature umbilical cord.

The mouse allantois consists of mesodermal tissue, which undergoes vasculogenesis to form the mature umbilical artery and vein.^[1]

The human allantois is an endodermal evagination of the developing hindgut which becomes surrounded by the mesodermal connecting stalk. The connecting stalk forms the umbilical vasculature. These endodermal and mesodermal tissues together form the human umbilical cord. The allantois later becomes the urachus.

[edit] Etymology

The word comes from the Greek word for sausage, which the allantois resembles.

[edit] Additional images

Section through the embryo.	Diagram showing later stage of allantoic development with commencing constriction of the yolk-sac.	Diagram showing the expansion of amnion and delimitation of the umbilicus.	Model of human embryo 1.3 mm. long.
Tail end of human embryo from fifteen to eighteen days old.	Cloaca of human embryo from twenty-five to twenty-seven days old.	Tail end of human embryo twenty-five to twenty-nine days old.	Tail end of human embryo thirty-two to thirty-three days old.
Opened uterus with cat fetus in midgestation: 1 umbilicus, 2 amnion, 3 allantois, 4 Yolk sac, 5 developing marginal hematoma, 6 maternal part of placenta (endometrium)