The membranous structure that surrounds the developing fetus and forms the amniotic cavity is derived from fetal tissue and is composed of two layers: the amnion (inner layer) and the chorion (outer layer). The amnion is a translucent structure adjacent to the amniotic fluid, which provides necessary nutrients to the amnion cells . The chorion is a more opaque membrane that is attached to the decidua (ie, maternal tissue that lines the uterus during pregnancy). The amnion and chorion are separated by the exocelomic cavity until approximately three months gestation, when they become fused. Intact, healthy fetal membranes are required for an optimal pregnancy outcome.
Extracellular matrix (ECM) proteins synthesized by several cell types within the amnion and chorion confer both strength and elasticity to the fetal membranes . During or just prior to labor, the breakdown of these proteins is regulated by matrix metalloproteinases (MMPs) and their inhibitors . These biochemical changes in the fetal membranes reduce their integrity and elasticity, make them more vulnerable to rupture, and may contribute to the initiation of parturition .
Inspection of the fetal membranes following delivery reveals amnion that is mildly adherent to the fetal side of the chorion. Small amounts of maternal decidual tissue can be observed attached to the outer, maternal side of the chorion.
Cellular anatomy — Fetal membranes, as the name implies, are genetically identical to the fetus . The membranes contain many cell types, but are avascular and without nerve cells . The cells appear columnar where the membranes are attached to the placenta, but become more flattened or cuboidal adjacent to the decidua [3,4].
Amnion — The amnion is loosely composed of three layers of cells: