Pericyte coverage of retinal and cerebral capillaries

We performed electron microscopic morphometric analyses on capillaries from macular and peripheral retinas of five adult cynomolgous monkeys and three elderly human subjects. Measurements from the monkey retinal capillaries were compared to those made on capillaries from frontal, temporal, parietal, and occipital cerebral cortex of the same animals. We measured the percent coverage of the endothelial lining of the capillaries by pericyte processes, as well as the ratio of the cytoplasmic areas of pericytes and endothelial cells. In addition, we compared the thickness of the capillary basement membranes in three regions: overlying pericytes; overlying endothelial cells; and interposed between pericytes and endothelial cells. In both monkey and human retinas, pericyte processes covered greater than 85% of the circumference of the capillary endothelial tube, whereas pericyte coverage of monkey cerebral capillaries was highly significantly (P less than 0.001) less than that of capillaries in either the macular or peripheral retina. The ratio of pericyte to endothelial cell cytoplasmic areas also was lower in the monkey cerebral cortex than in the retina, though the statistical significance was less than that of the length measurements. In all tissues measured, both from monkeys and humans, the portions of the capillary basement membranes interposed between pericytes and endothelial cells were highly significantly (P less than 0.0001) thinner than the regions of capillary basement membranes covering pericytes and endothelial cells. Considering functions that have been proposed for pericytes, these measurements suggest that regional control of microcirculatory flow and of blood-tissue barrier integrity, as well as control of endothelial cell proliferation, should be much greater in the retina than in the cerebral cortex. Thinner basement membranes between pericytes and endothelial cells may permit more cell membrane contacts between these cells, thus facilitating such control.