Chorion frondosum

Chorion
Diagram showing earliest observed stage of human embryo.
Human fetus, enclosed in the amnion.
Details
Identifiers
Latin	chorion
MeSH	A10.615.284.473
Code	TE E5.11.3.1.1.0.3
Anatomical terminology []

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

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 transfer of nutrients from maternal blood to fetal blood.

The chorion consists of two layers: an outer formed by the trophoblast, and an inner formed by the somatic mesoderm; the amnion is in contact with the latter.

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.

The chorion undergoes rapid proliferation and forms numerous processes, the chorionic villi, which invade and destroy the uterine decidua, while simultaneously absorbing nutritive materials from it for the growth of the embryo.

The chorionic villi are at first small and non-vascular, and consist of the trophoblast only, but they increase in size and , whereas the mesoderm, carrying branches of the umbilical vessels, grows into them, and they are vascularized.

Blood is carried to the villi by the paired umbilical arteries, which branch into chorionic arteries and enter the chorionic villi as cotyledon arteries. After circulating through the capillaries of the villi, the blood is returned to the embryo by the umbilical vein. Until about the end of the second month of pregnancy, the villi cover the entire chorion, and are almost uniform in size; but, after this, they develop unequally.