Diagram illustrating early formation of allantois and differentiation of body-stalk.
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|MeSH
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Allantois (plural allantoides or allantoises) is a part of a developing animal conceptus (which consists of all embryonic and extra-embryonic tissues). The word comes from the Greek word for sausage, which the allantois resembles. 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.
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.
In most marsupials, the allantois is avascular, having no blood vessels, but still serves the purpose of storing nitrogenous 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.
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. 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.
References
Bagnell, C. 2005. "Animal Reproduction". Rutgers University Department of Animal Sciences.
Downs, K.M. 1998. "The Murine Allantois". Current Topics in Developmental Biology vol. 39, pp 1-33.
