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The chorion is the outermost of the four membranes surrounding the developing fetus, consisting of trophoblast and underlaying extraembryonic mesoderm. It is continuous with the the lining of the uterine wall (endometrium) superficially and is in contact with the amnion deep to it. During development, the chorion and amnion will attach together to form the amniotic sac

Chorionic villi emerge from the chorion as it develops. The villi located adjacent to the decidua capsularis (the part of the decidua overlaying the embryo) will degenerate to produce the smooth part of the chorion termed the chorion laeve. Whereas the villi adjacent to the decidua basalis (the part of the decidua between the chorion and uterine wall) persist and increase in size to produce the part of the chorion termed the chorion frondosum, also known as the fetal portion of the placenta. These villi will invade and anchor to the endometrium, allowing for the exchange of nutrients, gases and wastes between maternal and fetal blood.

Terminology English: Chorion
Latin: Chorion
Parts Chorion leave: adjacent to decidua capsularis; lacks villi
Chorion frondosum: adjacent to decidua basalis, rich in villi 
Function Surrounds fetus for protection and nourishes via chorionic villi

Learn more about fetal development in this study unit:

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