The amnion is an extraembryonic membrane that surrounds a developing amniotic embryo. It functions as a protective sac along with three other extraembryonic membranes: the chorion, the yolk sac, and the allantois.
The membranes are then further enclosed by a shell (in birds, reptiles, and some mammals) or in a uterus (in most mammals). All four membranes protect the developing embryo by providing gas exchange, nutrient delivery, and waste elimination.
What is amniotes?
The amnion is a distinctive feature of amniotes, a group of animals that includes reptiles, birds, and mammals. Amniotes are believed to have separated from non-amniotic tetrapods around 300-350 million years ago.
Amniotes are tetrapods that have evolved adaptations to live on land; Vertebrate embryos require an aquatic environment to develop, and the amniotic fluid provides that environment.
Amniotes also evolved a number of other adaptations that allowed them to move away from the water and take advantage of a larger terrestrial environment.
What is the amnion structure?
The amnion is an extraembryonic membrane that surrounds an amniotic embryo. The membrane is not part of the embryo itself, but comes from tissues that have arisen from the embryo.
The amnion consists of two germ layers: the mesoderm and the ectoderm. The ectoderm forms the inner part of the amnion, and a thin layer of mesoderm connects the amnion to the chorion.
What is amnion function?
The amnion, along with the chorion, yolk sac, and allantois, forms a series of protective barriers that provide a life-sustaining system for the developing embryo. The four membranes exchange oxygen and carbon dioxide between the embryo and the placenta, providing nutrients to the embryo and removing nitrogenous waste from the embryo.
The amnion forms a sac filled with amniotic fluid. The amniotic fluid serves as a buffer to protect the embryo from physical damage caused by mechanical shock. The amniotic fluid also helps prevent desiccation and dehydration from bathing the embryo. Amniotic fluid is released at birth when the amnion ruptures. In humans, this is the phenomenon known as maternal hydrocele.
This figure shows a chicken egg. It shows the four extraembryonic membranes that surround the embryo: the amnion, the chorion, the allantois, and the yolk sac (or vitellus).