Chorionic Villi - Development

Development

The chorion undergoes rapid proliferation and forms numerous processes, the chorionic villi, which invade and destroy the uterine decidua and at the same time absorb from it nutritive materials for the growth of the embryo.

They undergo several stages, depending on their composition.

Stage Description Period of gestation Contents
Primary The chorionic villi are at first small and non-vascular. End of 4th week trophoblast only
Secondary The villi increase in size and ramify, while the mesoderm grows into them. 5th week trophoblast and mesoderm
Tertiary Branches of the umbilical vessels grow into the mesoderm, and in this way the chorionic villi are vascularized. 5th - 6th week trophoblast, mesoderm, and blood vessels

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.

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