Placenta

The human placenta is composed of both maternal and fetal tissues. Visible in this section are the decidua basalis, which is derived from the maternal endometrial lining, and the chorionic villi, which emerge from the chorion that surrounds the embryo. The villi invade decidua's blood-filled lacunae to facilitate exchange between the fetus and mother. Nutrients, gases, waste products, and macromolecules are transferred between the maternal blood in the lacunae and the tiny vessels in villi.