Perfused capillary morphometry in the senescent brain.

This study compared quantitative indices of capillary morphometry in the perfused vs. total capillary bed of conscious young (8-10 month old) and senescent (28-33 month old) rat brains. The total capillary bed was identified through alkaline phosphatase staining of tissue sections cut from specific brain regions. The perfused capillary bed was identified by the presence of fluorescein isothiocyanate (FITC) dextran in the microvessels. Average capillary volume fraction, VV (mm3/mm3, mean +/- S.E.M.) was 0.040 +/- 0.002 and 0.033 +/- 0.001 in the total capillary bed of the young and old animals, respectively. These values were not statistically different. Perfused VV averaged 0.020 +/- 0.001 and 0.017 +/- 0.001 mm3/mm3 in the young and old animals, respectively. Average perfused VV was 50% in the young and 49% in the senescent rat brains. Young and senescent brains utilize similar proportions of their "capillary reserves." In the areas examined in the brains of young animals, differences were found in the percentage of capillary volume/mm3, VV, and surface area/mm3, SV, perfused which were not present in old animals. The structural and neurochemical changes noted by others in the brain during aging were not related to alterations in indices of average and regional total or perfused cerebral capillary bed morphometry. However, differences in percent perfused VV, SV, length, LV, and number, Na, present in the younger rat brains were not present in the senescent rat brains.