ULTRASTRUCTURAL OBSERVATIONS ON BIFURCATIONS IN RAT CEREBRAL ARTERIES

The bifurcation pads ("valve-like projections") in the anterior cerebral arteries of rats with long-term hypertension were studied electron microscopically. The rats ranging in age from 2 to 10 months were sacrificed after bilateral renal artery constriction at 5-week-old. In the bifurcation pads (intimal pads) of hypertensive rats, smooth muscle cell damage first appeared in the roots of the pads as well as underlying media in the form of granular, vesicular, and somewhat tubular structures (200–1500 Å in diameter). Thereafter, with a lapse in time after the operation, these abnormal substances increased around the smooth muscle cells which were arranged in the marginal zones of the pads. Deeply arranged intimal smooth muscle cells which were embedded in an abundance of ground substances were only damaged in rats with very long-term hypertension. The medial smooth muscle cells beneath the pads manifested lesions which were more marked and diffuse than in the intima. These smooth muscle cells were bizarrely atrophied and abnormal substances which might be derived from necrotic smooth muscle cells were found around them. Basement membrane-like substances, either thick or multilaminated, were also present around them.