chorion

The chorion is a membrane that forms around the embryo during development. In mammals, it develops from the trophoblast (the outer layer of the blastocyst) together with extraembryonic mesoderm. It’s one of the fetal (extraembryonic) membranes, alongside the amnion, yolk sac, and allantois. In amniotes — reptiles, birds, and mammals — the chorion is a key “outer” membrane that supports the embryo as it grows.

In mammals like humans, the chorion plays a vital part in forming the placenta. Tiny finger-like projections called chorionic villi grow out of the chorion and burrow into the uterine wall, establishing a connection for nutrient exchange between mother and fetus. This is why doctors sometimes perform chorionic villus sampling (CVS) — a prenatal test where they take a small sample from these villi to check for genetic abnormalities.

Initially, the chorion is a simple membrane, but it becomes increasingly complex as pregnancy progresses. It contributes to the formation of the placenta and is part of the boundary where the mother’s body and the developing fetus interact. In birds and reptiles, the chorion typically works together with the allantois (forming the chorioallantoic membrane) to support respiration — facilitating gas exchange through the eggshell.

In other fields, the same word can show up with a broader meaning: a protective outer layer of an egg. For example, in entomology it can refer to the outer “shell” of an insect egg, and in some aquatic animals it’s used for the egg envelope as well.

So, while you won't find "chorion" in everyday conversation, it's a fundamental structure in developmental biology, essential for successful reproduction across many species. It's a testament to how life finds ways to nurture and protect its beginnings, whether inside an egg or within the warmth of a mother's womb.