Brain aging in normotensive and hypertensive strains of rats. III. A quantitative study of cerebrovasculature.

Quantitative histological techniques were used to evaluate age-related alterations in the vascularity and thickness of the cerebral cortex in normotensive and hypertensive strains of rats. In both strains, aging was accompanied by a statistically significant increase in mean minimal luminal diameter (MMLD) of capillaries in all cortical laminae. While the overall depth of the cerebral cortex was significantly decreased in aged rats of both strains, the total number of capillaries in a strip of cortex extending from the pia mater to the underlying white matter did not change between 3 months and 23-24 months. The greatest vascularity was seen in the field occupying 41-60% of the depth below the pial surface. Few differences in the laminar variations in capillary density were observed. The number and MMLD of venules were increased in aged WKY, but the aged hypertensive rats exhibited a decrease in number and MMLD of venules. No significant changes in either the number or MMLD of arterioles was observed in aged and/or hypertensive rats. The total brain weight of the hypertensive rats was significantly less than that of normotensive rats at 3 and 12 months of age but no difference was observed between the aged groups. Hypertensive rats demonstrated a significantly decreased relative brain weight (brain weight per 100 g body weight) in all age groups.