allantois

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
Days	16
Precursor	yolk sac
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). It helps the embryo exchange gases and handle liquid waste.

The allantois, along with the amnion and chorion (other embryonic membranes), identify humans as amniotes, along with reptiles, dinosaurs, birds, and other mammals. Of the vertebrates, only Ichthyopsidas (fish and amphibians) lack this structure.

Function

This sac-like structure is primarily involved in nutrition and 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 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

In most marsupials, the allantois is avascular, having no blood vessels, but still serves the purpose of storing nitrogenous (NH₃) 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)

In placental mammals, the allantois is part of and forms an axis for the development of the 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 definitively becomes the urachus, which removes nitrogenous waste from the fetal bladder^[2].

Pathology

A patent allantois can result in urachal cyst.

Etymology

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

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)

References

1. ^ Downs, K.M. 1998. "The Murine Allantois". Current Topics in Developmental Biology vol. 39, pp 1-33.
2. ^ First AID for the USMLE Step 1 2008

External links

• Diagram at University of Texas - Arlington

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