Amnion

The amnion is a thin but resilient membrane that encloses and protects the embryos of amniotes—reptiles, birds and mammals. By forming the amniotic sac filled with amniotic fluid, it provides a controlled and cushioned environment essential for embryonic and fetal development. Its appearance marks a fundamental evolutionary distinction between amniotes and anamniotes (fishes and amphibians), which lack this structure and therefore rely on aquatic external environments for development.

Structure and Development

The amnion originates from two extraembryonic layers: the somatic mesoderm externally and the ectoderm or trophoblast internally. Together, these form a membrane that first appears as a cavity within the inner cell mass of the early human embryo. The roof of this cavity consists of flattened amniotic ectoderm, while the floor comprises the columnar ectoderm of the embryonic disc. The structure is externally lined by mesoderm that is continuous with the somatopleure and connected via the body stalk to the chorion.
During the fourth and fifth weeks of human development, the accumulation of amniotic fluid begins. As fluid volume increases, the amnion expands and eventually conforms to the inner surface of the chorion, eliminating the extraembryonic coelom. Amniotic fluid continues to increase until the sixth or seventh month of pregnancy, then decreases slightly before birth. By term, the volume is typically around one litre.
The fluid consists largely of water with less than two per cent solids, including urea, salts, small quantities of protein and occasional traces of glucose. The fetus swallows some of this fluid, as evidenced by the presence of epidermal cells and lanugo in its gastrointestinal tract. The amniotic cavity facilitates fetal movement and cushions the developing fetus against external mechanical shocks.

Etymology

The term amnion has traditionally been linked to the Greek word for “little lamb”, interpreted in relation to birth and the protection of offspring. A competing etymological theory associates the word with Amnias, an epithet of the Cretan childbirth goddess Eileithyia, suggesting a ritual or mythological connection to her sanctuary at Amnisos, Crete.

The Amnion in Humans

Although the earliest phases of its formation have not been directly observed in humans, embryological evidence shows that the amnion appears early as a closed sac. Its rapid expansion separates it from the embryo, and as the amount of fluid increases, the amnion presses against and adheres to the chorion. Clinically, pathology involving the amnion is rare, but extra-amniotic pregnancy—resulting from rupture of the amnion—may occur, causing development to proceed within the extraembryonic coelom.

The Amnion in Other Vertebrates

The development of the amnion varies among amniotes. In reptiles, birds and many mammals, an amniotic fold at the junction of the embryonic gut and yolk sac rises and envelops the embryo dorsally. This type of development is termed pleuroamnion, formed by folding of the somatopleure, in contrast to schyzoamnion, which forms by delamination. The outer layer of the fold becomes the serous membrane or false amnion, while the inner layer forms the true amnion, with the intervening space constituting the extraembryonic coelom.
Among certain mammals, the amnion may enclose the fetus at birth, as commonly observed in kittens and puppies. The mother opens and consumes the membrane immediately after parturition. In elephants, the amnion is continuous with the umbilical cord and the allantoic structures. The allantois lies between the chorion and amnion, preventing direct attachment of the amnion to the placental surface.

Biological Role

Across amniotes, the amnion serves crucial developmental functions. It maintains hydration, buffers mechanical forces and isolates the fetus from maternal tissues. Its mechanical strength and elasticity allow expansive growth throughout gestation. By enabling terrestrial reproduction, the evolution of the amnion facilitated the diversification of vertebrates into dry environments.

Clinical and Therapeutic Applications

Human amniotic membrane has found increasing use in regenerative medicine. Harvested from placentas following caesarean delivery, the membrane is processed under sterile conditions and screened for infectious pathogens before being used as a biological dressing. Its inherent properties—low immunogenicity, anti-inflammatory action, and promotion of epithelialisation—make it effective in treating otherwise non-healing wounds, including those associated with systemic sclerosis.
Commercially prepared amniotic membrane products must meet rigorous sterility and safety standards, including endotoxin testing. These applications represent a modern extension of the amnion’s natural protective roles into therapeutic settings.

Significance in Vertebrate Evolution

The presence of an amnion defines the clade Amniota, a major evolutionary innovation enabling embryos to develop in self-contained aquatic environments while residing within terrestrial habitats. This adaptation underpins the reproductive strategies of reptiles, birds and mammals and marks a key divergence from ancestral anamniote lineages that rely on external aquatic media for embryonic survival.