[Ultrastructural studies of vacuole formation in arterial chorionic vessels of the mature human placenta].

The so called endothelial vacuoles of the human fetal stem arteries are formed by contraction of the tunica muscularis media. The cytoplasm of the smooth muscle cells prolapses through gaps of the basal membrane, found in areas of myoendothelial junctions, into the intima. Perfusion with fixative immediately after delivery prevents the formation of the vacuoles.