The allantois is a hollow sac-like structure filled with clear fluid that forms part of a developing amniote's conceptus. It helps the embryo exchange gases and handle liquid waste. The allantois, along with the amnion and chorion, identify humans and other mammals as well as reptiles as amniotes. Of the vertebrates, only the anamniotes lack this structure.
Function
This sac-like structure, whose name is the New Latin equivalent of "sausage" 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.
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 eggshell.
In most marsupials, the allantois is avascular, having no blood vessels, but still serves the purpose of storing nitrogenous 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, 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.
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. 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 to the yolk sac. The urachus removes nitrogenous waste from the fetal bladder.
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.
