Cell membrane structure of vascular smooth muscle of circle of Willis.

Cell membranes of vascular smooth muscles of the circle of Willis were studied in thin sections and freeze-replicas. The cell membranes were differentiated into a caveolae intracellulares zone and caveolae-free zone, both of which were generally arranged in an alternate manner and parallel to the major axis of the smooth muscle cell. In the former zone, the caveolae intracellulares, about 600 A in diameter, were neatly oriented in one to several rows running parallel to the longitudinal axis of the muscle cell with a center-to center distance of about 800 A. The latter zone was of variable width and smooth, apart from membrane particles or scattered caveolae, and corresponded mainly to the dense area and partially to the myofibril area beneath the cell membrane. Membrane particles were generally more numerous on face A than on face B, and their average number per micronm2 was about twice as many inside the rows of the caveolae as outside. Rosette formations of membrane particles were often evident at the stomal rims of the caveolae. Adherentes and gap junctins were occasionally found on the caveolae-free areas which often protruded externally. Tight junctions appeared as a collection of scattered strands, which frequently showed free ends and were parallel to each other and also to the major axis of the smooth muscle cell.