The amnion is the thin but durable innermost membrane that surrounds and protects a developing embryo, and later the fetus, during pregnancy. It is a component of the larger amniotic sac, which encloses the fetus and amniotic fluid. This structure is present in mammals, birds, and reptiles, and it creates a contained and stable environment for fetal development.
Formation and Structure of the Amnion
The amnion begins to form around the second week after fertilization from epiblast cells. As the embryo grows, this membrane expands to envelop it, creating the amniotic cavity to hold the amniotic fluid. The membrane also extends from the embryo to eventually cover the umbilical cord.
Structurally, the amnion is a thin, transparent, yet tough tissue lacking its own blood supply. It is composed of two layers that provide flexibility and durability. The inner layer consists of a single sheet of epithelial cells, while the outer layer is a thicker, fibrous connective tissue called the stroma. The stroma provides support and connects to the chorion, the sac’s outermost membrane.
Primary Functions During Development
The amnion’s most recognized function is producing and containing amniotic fluid, which is necessary for healthy fetal development. The amnion is metabolically active, helping regulate the fluid’s volume and composition. This fluid serves as a cushion, absorbing shock from sudden movements and safeguarding the umbilical cord from compression.
The controlled environment allows the fetus to move freely, which is required for proper musculoskeletal development. The fluid also prevents parts of the embryo from adhering to the amnion itself. It helps maintain a constant temperature and pressure, shielding the fetus from external fluctuations and potential infections.
Amniotic fluid also acts as a reservoir of nutrients from the mother, containing proteins, electrolytes, and vitamins. It provides the space and growth factors for the normal development of internal organs, including the lungs and gastrointestinal system.
The Amnion’s Role in Labor and Delivery
Toward the end of pregnancy, the amnion and the attached chorion form the “bag of waters.” The rupture of this sac, often called “water breaking,” is a common indicator that labor is starting. This event releases amniotic fluid from the uterus.
The released fluid helps lubricate the birth canal, facilitating the baby’s passage. After delivery, the amnion, placenta, and chorion detach from the uterine wall. These tissues are then expelled from the body as the afterbirth.
Clinical Significance and Medical Applications
The amnion and the fluid it contains have uses in modern medicine. During pregnancy, the fluid can be sampled in a procedure called amniocentesis. This prenatal test involves collecting a small amount of amniotic fluid to analyze fetal cells for genetic abnormalities and to assess fetal lung maturity.
Beyond pregnancy, the amniotic membrane is used in regenerative medicine. Because the membrane has low immunogenicity and properties that reduce inflammation and scarring, it serves as a biological dressing. It is frequently used in ophthalmic surgery to help heal the surface of the eye and can also be applied as a graft to treat chronic wounds, burns, and other skin injuries.