Chorionic villi

Chorionic villi are villi that sprout from the chorion to provide maximal contact area with maternal blood.Section through the embryo.Transverse section of a chorionic villus.Primary chorionic villi. Diagrammatic.Secondary (Tertiary? Vessels are present.) chorionic villi. Diagrammatic.Human embryo of about 28 days, with yolk-sac. Chorionic villi are villi that sprout from the chorion to provide maximal contact area with maternal blood. They are an essential element in pregnancy from a histomorphologic perspective, and are, by definition, a product of conception. Branches of the umbilical arteries carry embryonic blood to the villi. After circulating through the capillaries of the villi, blood returns to the embryo through the umbilical vein. Thus, villi are part of the border between maternal and fetal blood during pregnancy.