placental membrane

Definition: the semipermeable layer of fetal tissue separating the maternal from the fetal blood in the placenta; composed of: 1) endothelium of the fetal vessels in the chorionic villi, 2) stromata of the villi, 3) cytotrophoblast (negligible after the fifth month of gestation), and 4) syncytiotrophoblast covering the villi; the placental membrane acts as a selective membrane regulating passage of substances from the maternal to the fetal blood.

Synonym(s): placental barrier