Specializations in plasma membranes of the human placenta

Two distinct plasma membranes have been partially purified and characterized from nomal human placenta. Cell membrane specializations which are likely to interact with maternal blood and humoral agents in the intervillous space were identified on the brush border membrane of the syncytiotrophoblast, which is recognized as the major site of synthetic and transport activity in the human placenta. Another placental plasma membrane fraction, which presumably contains syncytial components and is distinct from brush border membrane, has also been partially characterized. This membrane is likely to be a major site of interaction of fetal blood and humoral agents with the placenta. Both brush border and nonbrush border membranes possess different transport and receptor activities. Brush border membranes are rich in insulin receptors and in alkaline phosphatase and contain Na+-gradient--dependent amino acid transport sites (but do not contain demonstrable hormone sensitive adenylate cyclase activity). Nonbrush border placental membranes are (on the other hand) enriched in hormone sensitive adenylate cyclase activity, beta-Adrenergic receptors, Na+-K+-ATPase, [3H]ouabain binding, and an ATP-dependent Ca2+ transport system. Interposed between two separate circulatory systems the placenta has a distinct polarity of its membrane specializations thus allowing different interactions at the maternal-placental as compared to the fetal-placental interface. These interactions are likely to mediate placental metabolic, synthetic, and transport functions.