tertiary chorionic villus

Tertiary chorionic villi are distinguished from earlier villous stages by the presence of embryonic blood vessels within them. Those vessels develop from mesenchymal cells in the loose connective tissue core of the villus, and they connect with vessels forming in the chorion and the connecting stalk. Once that continuity is established, embryonic blood begins to circulate through the villous capillaries at about the third week of development in humans. The tertiary stage therefore marks the point at which the villus becomes a functioning vascular unit rather than a purely epithelial and mesenchymal outgrowth, since exchange with the surrounding maternal blood requires a fetal capillary bed inside the villous core.