Pericyte coverage is greater in the retinal than in the cerebral capillaries of the rat

Using electron microscopic morphometry, we demonstrate here that, in the albino rat, the pericytes of the retinal microcirculation over the capillary circumference much more extensively than in five widely separate regions of cerebral cortex. The ratio of pericyte plasma membrane length in contact with the vascular circumference, to the outer circumference of the endothelial cell tube, was 0.41 +/- 0.02 (SEM) for the retinal microvessels, and ranged from 0.22 +/- 0.02 to 0.30 +/- 0.03 for the five regions of cerebral cortex. The values of this ratio for the retina differed significantly from those for the various cortical regions. Differences of comparable magnitude were obtained between ratios of pericyte to endothelial cell areas in retina and cerebral cortex. The substantial quantitative difference in pericyte coverage of the microvessel circumference between the retinal and cerebral microcirculations may have important implications for normal and pathological microcirculatory dynamics in these two organs.