SCANNING ELECTRON-MICROSCOPIC STUDIES OF THE MEDIAL SMOOTH MUSCLES INHUMAN MAJOR INTRACRANIAL-ARTERIES

The three-dimensional arrangement of the medial smooth muscle cells of human major intracranial arteries was studied by scanning electron mi croscopy after removal of extracellular connective tissue matrices wit h a KOH-collagenase digestion method. In the straight portion of the m ajor arteries, the smooth muscle cells were arranged roughly circularl y, whereas the arrangement was somewhat random in the vertebral, basil ar, and internal carotid arteries. Groups of the longitudinal muscle c ells were also found in the vertebral, basilar, and internal carotid a rteries, but were absent in the anterior, middle, and posterior cerebr al arteries. At a higher magnification, smooth muscle cells in these a rteries formed anastomosed bundles about 5-30 mu m in diameter. The sm ooth muscle cells in the dichotomous branching and the uniting portion s were arranged circularly, but multidirectionally and longitudinally oriented smooth muscle cell groups were present in the facial and dors al walls of the forked vessels. These multidirectional muscle cell gro ups were small in number in the portion where the major arteries gave off lateral branches at a right angle. So called ''medial defects'' we re found at the crotch of the bifurcating region in two cases out of f our. Smooth muscle cells near the defects tapered off toward the cente r of the defect where internal elastic lamina with oval fenestrations were exposed. However, no special arrangement of the smooth muscle cel ls was observed around the ''medial defects'' as compared with that in the ordinary bifurcating region.