Quantitative Regional Determination of Morphometric Indices of the Total and Perfused Capillary Network in the Rat Brain

A new method has been developed to simultaneously and regionally determine various morphometric indices of the perfused and total capillary network in rat brain on a quantitative basis. A high molecular weight dextran, labeled with fluorescein isothiocyanate (FITC), was injected into the femoral vein of a barbiturate anesthetized rat. After 20 seconds, the animal was decapitated and the head frozen in liquid nitrogen. The anterior cortex, hypothalamus, thalamus, lenticulate nuclei, substantia nigra, hippocampus, posterior cortex, cerebellum, pons, and medulla were isolated and mounted in a microtome cryostat. Sections, 2 μm thick, were photographed with a fluorescent microscope to detect the perfused capillaries. The sections then were stained for alkaline phosphatase to visualize the total capillary network. Standard morphometric techniques were employed to determine the total and perfused volume, surface area, length, diameter, and number per mm3 from the photographs. The method was validated in brain, muscle, and omentum to show that the technique did not alter the capillary network, that both the total and perfused capillary network were visualized, and that there was little tissue shrinkage. The study found no significant differences in the total capillary network on a regional basis in rat brain in comparisons of any of the above indices in the 10 large brain areas. Similarly, it was found that the perfused capillary network was not different on a regional basis. Within the cortex, both total and perfused indices were greater in grey than white matter. The perfused capillary network ranged from 50.1% to 56.9% of total for the various indices determined. The percentage of the capillary network perfused increased significantly in a series of asphyxiated rats. Diameter of the capillaries in the control series averaged 6.05 ± 1.09 μm (mean ± SD) and was not significantly greater in the portion of the bed which was perfused.

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