chorion

The chorion is the outermost fetal membrane in amniotes. In mammals, it develops from the trophoblast and surrounding extraembryonic mesoderm and envelops the amnion and yolk sac. It contributes to placental formation, with the fetal portion called the chorionic plate. Chorionic villi extend from this surface into the maternal decidua basalis to establish the fetal–maternal exchange interface within the intervillous spaces.

Chorionic villi are the branching structures of the villous chorion (chorion frondosum) that contact maternal blood, forming the main exchange surface. The remaining, smoother portion of the chorion is termed the chorion laeve. As pregnancy progresses, some villi regress in certain regions, leaving a mixed villous and smooth surface on the fetal side of the placenta.

In non-mammalian amniotes such as birds and reptiles, the chorion still surrounds the embryo and, together with the allantois, forms the chorioallantoic membrane. This membrane plays a key role in gas exchange and waste handling in eggs, often within the protective eggshell.

Clinical relevance includes obstetric procedures such as chorionic villus sampling (CVS), which obtains chorionic villi for early fetal genetic testing. Abnormal development or function of the chorion and its villi can contribute to placental insufficiency and pregnancy complications, including miscarriage or fetal growth restriction.