Name = Allantois
GraySubject = 12
GrayPage = 54
Caption = Diagram illustrating early formation of allantois and differentiation of body-stalk.
Caption2 = Sectional plan of the gravid uterus in the third and fourth month.
Days = 16
MeshName = Allantois
MeshNumber = A10.615.284.147
DorlandsPre = a_24
DorlandsSuf = 12121253
Allantois (plural "allantoides" or "allantoises") is a part of a developing animal
conceptus(which consists of all embryonic and extra-embryonic tissues). It helps the embryo exchange gases and handle liquid waste.
The allantois, along with the
amnionand chorion(other embryonic membranes), identify humans as amniotes, along with reptiles, dinosaurs, birds, and other mammals.
This sac-like structure is primarily involved in
nutritionand excretion, and is webbed with blood vessels.
The function of the allantois is to collect liquid waste from the embryo, as well as to exchange gases used by the embryo.
In reptiles, birds, and monotremes
The structure first evolved in
reptilesand birdsas a reservoir for nitrogenous waste, but also as a means for oxygenation of the embryo. Oxygenis absorbed by the allantois through the egg shell. The allantois functions similarly in monotremes, which are egg-laying mammals.
In most marsupials
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.
In placental mammals (Eutheria)
placentalmammals, the allantois is part of and forms an axis for the development of the umbilical cord.
mouseallantois consists of mesodermal tissue, which undergoes vasculogenesis to form the mature umbilical artery and vein. [Downs, K.M. 1998. "The Murine Allantois". "Current Topics in Developmental Biology" vol. 39, pp 1-33.]
humanallantois is an endodermal evagination of the developing hindgutwhich becomes surrounded by the mesodermalconnecting 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, which removes nitrogenous waste from the fetal bladder [First AID for the USMLE Step 1 2008] .
A patent allantois can result in
The word comes from the Greek word for
sausage, which the allantois resembles.
* [http://www.uta.edu/biology/westmoreland/classnotes/1442/Chapter_47_files/image007.gifDiagram at University of Texas - Arlington]
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