The human placenta is encircled by a ring of smooth muscle cells

The marginal zone of the human term placenta was studied by transmission el ectron microscopy and immunohistochemistry using antibodies against cytoske letal filaments, extracellular matrix molecules and endothelial markers. Th e marginal sinus of the intervillous space is separated from the chorionic and basal plates by a layer of cells expressing vimentin, desmin, alpha- an d gamma-smooth muscle actins, and smooth. muscle myosin. Also ultrastructur ally, these cells share all features with smooth muscle cells. This muscula r ring is continuous with the media of uteroplacental veins entering the ma rginal sinus. In the basal plate the muscle cells may extend far into the c entral parts of the placenta. The muscular ring is separated from the inter villous space by a layer of endothelial cells. They are continuous with the maternal endothelium of the marginal uteroplacental veins. Moreover this e ndothelium covers neighbouring parts of the chorionic and basal plates, loc ally extending to the surfaces of large stem villi. The data suggest (1) th at the marginal zone of the intervillous space ('marginal sinus') represent s the dilated and merged parts of uteroplacental veins and (2) that lateral growth of the human placenta partly takes place by expansion into the uter oplacental veins. The functional importance of this muscular ring remains u nknown. (C) 2000 Harcourt Publishers Ltd.