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

The marginal zone of the human term placenta was studied by transmission electron microscopy and immunohistochemistry using antibodies against cytoskeletal filaments, extracellular matrix molecules and endothelial markers. The marginal sinus of the intervillous space is separated from the chorionic and basal plates by a layer of cells expressing vimentin, desmin, alpha- and gamma-smooth muscle actins, and smooth muscle myosin. Also ultrastructurally, these cells share all features with smooth muscle cells. This muscular ring is continuous with the media of uteroplacental veins entering the marginal sinus. In the basal plate the muscle cells may extend far into the central parts of the placenta. The muscular ring is separated from the intervillous space by a layer of endothelial cells. They are continuous with the maternal endothelium of the marginal uteroplacental veins. Moreover this endothelium covers neighbouring parts of the chorionic and basal plates, locally extending to the surfaces of large stem villi. The data suggest (1) that the marginal zone of the intervillous space ('marginal sinus') represents the dilated and merged parts of uteroplacental veins and (2) that lateral growth of the human placenta partly takes place by expansion into the uteroplacental veins. The functional importance of this muscular ring remains unknown.