[A scanning electron microscopic study on the smooth muscle cells of the human intracranial arteries--normal and...

Three-dimensional cytoarchitecture of the smooth muscles of human intracranial arteries was studied by scanning electron microscopy after removal of extra-cellular connective tissue matrices with a KOH-collagenase digestion method. The tunica media of major intracranial arteries consisted mainly of cord-shaped smooth muscle cells. In the straight portion of the major arteries in the brain base, adjacent muscle cells were often grouped into small bundles; the bundle formation was prominent in the proximal portion of the vessels, but became obscure in the periphery. These bundles were usually arranged circularly to the long axis of the vessels, but often changed their course to the oblique and longitudinal directions. The obliquely and longitudinally oriented smooth muscle bundles sometimes spanned the circularly arranged smooth muscle bundles and formed an "X" or "Y" -shaped appearance by anastomosing with one another. In the arteries about 1 mm in outer diameter, the smooth muscle cells were arranged circularly or obliquely to the vessel axis with certain regularity; wedge-shaped groups of the muscle cells appeared to interdigitate each other on the vessels. Smooth muscle cells in the bifurcation region were arranged densely as if they are encircling the crotch of the bifurcation; the facial and the dorsal sides of the bifurcation region were covered with irregularly oriented smooth muscle bundles. So called "medial defects" were found at the crotch of the bifurcations in two cases. Smooth muscle cells were tapered off toward the center of the defect, where internal elastic lamina with oval fenestrations were exposed. The aneurysms observed in the present study were present at the crotch of the bifurcation. The smooth muscle cells in these structures were arranged as if they were pushed out from inside. Smooth muscle cells with irregular shape were arranged toward the defects of the media. The internal elastic lamina was cleaved in the aneurysmal neck: the fenestrations of the elastic lamina were deformed as if the lamina was strongly enlarged along the aneurysmal neck. The etiology of the aneurysm was discussed based on the above findings.