The amnion and chorion membranes are two distinct, thin layers that form the amniotic sac, which provides a protective enclosure for a developing fetus. Originating from the trophoblast layer of the blastocyst, an early stage of embryonic development, the amniotic sac, often called the “bag of waters,” surrounds the embryo and later the fetus. It functions as a fluid-filled cushion, shielding the developing baby from external forces.
Anatomy and Development
The amnion is the innermost layer of the fetal membranes, directly in contact with the amniotic fluid, the fetus, and the umbilical cord. It is a transparent and avascular membrane, lacking blood vessels or nerves. The amnion is composed of two layers: an inner layer from ectoderm and an outer layer from mesoderm.
Surrounding the amnion is the chorion, the outer membrane that separates the amnion from the maternal decidua, the modified uterine lining during pregnancy. The chorion is a thicker, more fibrous layer containing fetal blood vessels. It develops from extraembryonic mesoderm and two layers of trophoblasts. Chorionic fluid initially separates the amnion from the chorion, but as pregnancy progresses and amniotic fluid accumulates, the amnion passively attaches to the chorion. The chorion also features finger-like projections called chorionic villi, which aid in nutrient transfer from the mother to the fetus.
Essential Roles in Fetal Development
The amnion and chorion membranes work together to create a stable environment for fetal growth. The amnion forms and contains the amniotic fluid, which fills the amniotic sac. This fluid provides physical protection, cushioning the fetus from external shocks.
The amniotic fluid also helps maintain a consistent temperature for the fetus. The fluid allows the fetus to move freely, which is important for proper musculoskeletal development. The chorion, through its interaction with the maternal decidua, contributes to nutrient and waste exchange, facilitating oxygen and nutrient transfer and waste removal. This system of membranes acts as a barrier, protecting against infections and fluid loss from the embryo.
Common Conditions Affecting the Membranes
Several health issues can impact the amnion and chorion membranes during pregnancy. Premature rupture of membranes (PROM) occurs when the amniotic sac ruptures and amniotic fluid leaks before the onset of labor. When this happens before 37 weeks of pregnancy, it is known as preterm premature rupture of membranes (PPROM). PPROM occurs in about 3% of pregnancies and accounts for approximately one-third of preterm births.
A concern with PROM and PPROM is the risk of chorioamnionitis, an infection of the placenta and the amniotic fluid. This infection can arise from ascending bacterial infections, often through the cervix. Chorioamnionitis can lead to complications for both the mother, such as postpartum infection and hemorrhage, and the baby, including preterm birth, neonatal sepsis, respiratory distress syndrome, and a higher risk of cerebral palsy. Another condition, amniotic band syndrome, involves fibrous bands forming within the amniotic sac, which can entangle and constrict fetal body parts.
Therapeutic Uses in Medicine
Beyond pregnancy, the amniotic membrane has found applications in various medical treatments due to its properties. It has anti-inflammatory, anti-scarring, and regenerative potential. It contains growth factors and cytokines that promote wound healing and reduce inflammation.
In ophthalmology, amniotic membrane transplantation is used for ocular surface reconstruction, treating conditions like dry eye, corneal burns, and ulcers. It serves as a biological bandage, supporting the growth of epithelial cells and reducing scar formation. Its healing and anti-inflammatory properties also make it valuable in general wound care and for treating burns, where it can act as a biological dressing.