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.
Details
Days 16
Precursor yolk sac
Gives rise to Umbilical cord
Identifiers
Latin Aallantois
MeSH A10.615.284.147
Code TE E6.0.1.2.0.0.2

Anatomical terminology

Allantois (/əˈlæntɔɪs/; plural allantoides or allantoises) is a sac-like structure that forms part of a developing amniote's 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, and other mammals, as amniotes. Other amniotes include reptiles, dinosaurs, and birds. 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, and also as a means for oxygenation of the embryo. Oxygen is absorbed by the allantois through the egg shell.

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.

Clinical significance

During the third week of development, the allantois protrudes into the area of the urogenital sinus. Between the 5th and 7th week of development, the allantois will become the urachus, a duct between the bladder and the yolk sac. A patent allantois can result in urachal cyst.

Etymology

The word comes from the Greek words allanto-, meaning sausage, and eidos, meaning shape or similarity.

Additional images

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
  3. http://www.answers.com/topic/allantois

External links

This article is issued from Wikipedia - version of the 10/29/2016. The text is available under the Creative Commons Attribution/Share Alike but additional terms may apply for the media files.