Figure 1. The human placental barrier at term.

The functional unit of the human placenta is the trophoblast villous tree (VT), containing fetal blood vessels and covered by the syncytiotrophoblast (ST), the multinuclear transporting and hormone producing epithelium of the human placenta, which is generated from mononuclear cytotrophoblast cells (CT). The syncytiotrophoblast is directly exposed to maternal blood entering the intervillous space (IVS) from the spiral arteries (SA). At term, the syncytiotrophoblast and the fetal capillary (FC) endothelial cells (EC) are the only largely continuous cell layers between maternal and fetal blood. The syncytiotrophoblast represents the primary barrier for movement of most solutes from the maternal to the fetal circulations. Specifically, transfer across the two polarized plasma membranes of the syncytiotrophoblast, the apical or microvillous plasma membrane (MVM) directed toward maternal blood in the intervillous space and the basal plasma membrane (BM) facing the fetal capillaries constitute the limiting steps for net flux from maternal to fetal circulations. UC, umbilical cord; N, nucleus, M; mitochondrion.