noun, plural: chorionic villi
Chorionic villus, as the name implies, is a villus that grows out of the chorion. The chorion, which is the extraembryonic membrane surrounding the embryo of amniote vertebrates, consists of two layers, i.e. the outer layer formed by the trophoblast and the inner layer formed by the mesoderm. The chorion forms numerous projections called chorionic villi.
The chorionic villi are at first trophoblastic but as they grow in size and branch out they become vascularized. Branches of the umbilical vessels, which are of mesoderm origin, grow into them, constituting the chorionic vessels. The chorionic vessels carry blood through the chorion in the feto-placental circulation. The chorionic artery that branches off the umbilical artery supplies blood to the capillaries of the chorionic villi. Following the circulation through the capillaries in the chorionic villi, blood retuns to the embryo via the umbilical veins. Thus, the chorionic villi are essential in maximizing the contact area with maternal blood and serve as a border between maternal and fetal blood.1
The chorionic villi may be classified as either floating villi or anchoring villi. The floating villi float freely in the intervillous space whereas the anchoring villi are anchored and provide support to the mechanical integrity of the placental-maternal interface. 1
1 Chorionic villi. Retrieved from [].