FIGURE 1.

Schematic representation of a healthy functional unit of the human placenta. The placenta is a specialized organ that is primarily involved in the exchange of metabolites between the mother and her fetus. In the early trimester, the trophoblast differentiates to form primary syncytium that further divides into the cytotrophoblast and syncytiotrophoblast. The syncytiotrophoblast will then expand to form a continuous layer of tree-like villous that are in contact with maternal blood in the intervillous spaces. These villous trees greatly increase the surface areas available for the exchange of metabolites. Cytotrophoblasts can further differentiate into extravillous trophoblasts that invade the decidua, which is the maternal uterine tissue, to promote immune tolerance between the mother and her fetus. The extravillous trophoblasts also migrate up the maternal spiral arteries and promote spiral artery remodeling, to form large vessels of low resistance that is required to sustain a healthy pregnancy.