Pericyte form and distribution in rat retinal and uveal capillaries

Ultrastructural morphometric techniques were used to assess differences in endothelial cells and in pericyte structure and distribution in rat retinal and uveal capillaries. Retinal capillaries were significantly smaller than those in the three different uveal vascular beds, all of which were similar in size. Approximately 10% of the capillaries in the retina and choroid were formed by three endothelial cells, compared with 30% and 46% of capillaries sampled from ciliary processes and iris, respectively. The percentage of the capillary circumference covered by pericytes (46-58%) and the percentage of capillary sections with pericyte nuclei (12-16%) were similar in retina, iris, and ciliary processes. Corresponding data for the choriocapillaris indicated that pericyte coverage of these capillaries was approximately 50% of that observed in the other eye microcirculations. The number of pericyte processes per capillary varied markedly in the different vasculatures, with an average of three for capillaries in the retina and choriocapillaris and nine to eleven for capillaries in the iris and ciliary processes. These marked differences in capillary dimensions are consistent with the well-known capillary hemodynamic and functional differences of these tissues; however, the significance of the differences in pericyte shape, frequency and distribution in the different vasculatures of the eye is less clear.