Allantois

This sac-like structure, whose name is Greek for sausage (from ἀλλαντοειδής allantoeidḗs, in reference to its shape when first formed){{cite web|title=Allantois|url=http://www.dictionary.com/browse/allantois?s=t|publisher=dictionary.com}} 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 mammals, except the egg-laying monotremes, the allantois is one of the fetal membranes, and is part of and forms an axis for the development of the umbilical cord.

While the function of the allantois remains conserved, there is a divergence in the characteristics of the allantois among mammalian species. Structural variations of the allantois include differences in size and shape.{{Cite journal |last=Hassan |first=Waad |last2=Viebahn |first2=Christoph |date=May 2017 |title=A correlative study of the allantois in pig and rabbit highlighting the diversity of extraembryonic tissues in four mammalian species, including mouse and man: HASSAN AND VIEBAHN |url=https://onlinelibrary.wiley.com/doi/10.1002/jmor.20657 |journal=Journal of Morphology |language=en |volume=278 |issue=5 |pages=600–620 |doi=10.1002/jmor.20657}}

In mammals, the emergence of the allantois varies during early development. In humans, the allantois appears during the presomitic stages of mid-gastrulation. The mouse allantois appears before the first somite pair as well; however, it occurs shortly after gastrulation. In both pigs and rabbits, the allantois arises at early somite stages.

The human allantois is a caudal out-pouching of the yolk sac, which becomes surrounded by the mesodermal connecting stalk known as the body-stalk. The vasculature of the body-stalk develops into umbilical arteries that carry deoxygenated blood to the placenta.{{cite book |last1=Moore |first1=Keith |last2=Persaud |first2=T.V.N. |last3=Torchia |first3=Mark G. |title=The Developing Human: Clinically Oriented Embryology |date=2016 |publisher=Elsevier |location=Philadelphia |isbn=978-0-323-31338-4 |page=58 |edition=10th}} It is externally continuous with the proctodeum and internally continuous with the cloaca. The embryonic allantois becomes the fetal urachus, which connects the fetal bladder (developed from cloaca) to the yolk sac. The urachus removes nitrogenous waste from the fetal bladder.First AID for the USMLE Step 1 2008 After birth the urachus becomes obliterated as the median umbilical ligament.

The mouse allantois consists of mesodermal tissue, which undergoes vasculogenesis to form the mature umbilical artery and vein.{{cite journal |last1=Downs |first1=KM |title=The Murine Allantois |journal=Curr Top Dev Biol |date=1998 |volume=39 |pages=1–33 |doi=10.1016/s0070-2153(08)60451-2 |pmid=9475996}}

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. Koalas and wombats also demonstrate allantoic fusion with the chorion.{{Cite journal |last=Carter |first=Anthony M. |date=July 2021 |title=The role of mammalian foetal membranes in early embryogenesis: Lessons from marsupials |url=https://onlinelibrary.wiley.com/doi/10.1002/jmor.21140 |journal=Journal of Morphology |language=en |volume=282 |issue=7 |pages=940–952 |doi=10.1002/jmor.21140 |issn=0362-2525}}

File:Monotreme Egg Diagram.svg egg showing allantois at #4]]

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 reptiles and birds, the allantois consists of extraembryonic endoderm enclosed in mesodermal tissue. A chorioallantoic membrane (CAM) is later formed from the chorion and allantois.

During the third week of embryonic development, the allantois protrudes into the area of the urogenital sinus. During fetal development, the allantois becomes the urachus, a duct between the bladder and the yolk sac. A patent allantois can result in a urachal cyst.

Because the allantois can be cultured ex vivo, it has utility as a model system for studying the formation of blood vessels as well as considerable usefulness in drug screening.{{Cite journal |last=Arora |first=Ripla |last2=Papaioannou |first2=Virginia E. |author-link2=Virginia Papaioannou|date=2012-09-27 |title=The murine allantois: a model system for the study of blood vessel formation |url=https://doi.org/10.1182/blood-2012-03-390070 |journal=Blood |volume=120 |issue=13 |pages=2562–2572 |doi=10.1182/blood-2012-03-390070 |issn=0006-4971 |pmc=3460680 |pmid=22855605}}

File:Gray21.png|Section through the embryo

File:Gray26.png|Diagram showing later stage of allantoic development with commencing constriction of the yolk-sac

File:Catfetus1.jpg|Opened uterus with cat fetus in midgestation: 1 umbilicus, 2 amniotic sac (chorion and amnion), 3 allantois

• [https://web.archive.org/web/20060227204317/http://www.uta.edu/biology/westmoreland/classnotes/1442/Chapter_47_files/image007.gif Diagram at University of Texas - Arlington]

{{Extraembryonic and fetal membranes}}

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Category:Amniotic sac

Category:Animal reproductive system