Scanning electron microscopic studies of the medial smooth muscles in human major intracranial arteries.

The three-dimensional arrangement of the medial smooth muscle cells of human major intracranial arteries was studied by scanning electron microscopy after removal of extracellular connective tissue matrices with a KOH-collagenase digestion method. In the straight portion of the major arteries, the smooth muscle cells were arranged roughly circularly, whereas the arrangement was somewhat random in the vertebral, basilar, and internal carotid arteries. Groups of the longitudinal muscle cells were also found in the vertebral, basilar, and internal carotid arteries, but were absent in the anterior, middle, and posterior cerebral arteries. At a higher magnification, smooth muscle cells in these arteries formed anastomosed bundles about 5-30 micrograms in diameter. The smooth muscle cells in the dichotomous branching and the uniting portions were arranged circularly, but multidirectionally and longitudinally oriented smooth muscle cell groups were present in the facial and dorsal walls of the forked vessels. These multidirectional muscle cell groups were small in number in the portion where the major arteries gave off lateral branches at a right angle. So called "medial defects" were found at the crotch of the bifurcating region in two cases out of four. Smooth muscle cells near the defects tapered off toward the center of the defect where internal elastic lamina with oval fenestrations were exposed. However, no special arrangement of the smooth muscle cells was observed around the "medial defects" as compared with that in the ordinary bifurcating region.