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Home > E. Pathology by systems > Reproductive system > Fetus and annexes > Placenta > chorion


Sunday 23 January 2022


Definition : The chorion is the outermost fetal membrane around the embryo in mammals, birds and reptiles (amniotes). It develops from an outer fold on the surface of the yolk sac, which lies outside the zona pellucida (in mammals), known as the vitelline membrane in other animals.

In humans and other mammals (excluding monotremes), the chorion is one of the fetal membranes that exist during pregnancy between the developing fetus and mother. The chorion and the amnion together form the amniotic sac.

In humans, it is formed by extraembryonic mesoderm and the two layers of trophoblast that surround the embryo and other membranes.

The chorionic villi emerge from the chorion, invade the endometrium, and allow the transfer of nutrients from maternal blood to fetal blood.

The chorion forms chorionic villi that allow the transfer of blood and nutrients from mother to fetus.


The chorion consists of two layers:
- an outer formed by the trophoblast
- an inner formed by the somatic mesoderm

The trophoblast is made up of an internal layer of cubical or prismatic cells, the cytotrophoblast or layer of Langhans, and an external layer of richly nucleated protoplasm devoid of cell boundaries, the syncytiotrophoblast.

See also

- choriogenesis