Diagram illustrating a chicken egg in its 9th day with all extraembryonic membranes
Sectional plan of the gravid human uterus in the third and fourth months of pregnancy
|Gives rise to||Umbilical cord|
The allantois (plural allantoides or allantoises) is a hollow sac-like structure filled with clear fluid 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 extraembryonic membranes), identify humans and other mammals as well as reptiles (including birds) as amniotes. Of the vertebrates, only the anamniotes (amphibians and non-tetrapod fish) lack this structure.
This sac-like structure, whose name is the New Latin equivalent of "sausage" (in reference to its shape when first formed) 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
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.
- 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 known as the body-stalk. The body-stalk forms the umbilical vasculature. In other words, the allantois is a caudal diverticulum (out-pouching) of the yolk-sac. 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. The allantois is vestigial and may regress, yet the homologous blood vessels persist as the umbilical arteries and veins connecting the embryo with the placenta.
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.