Quantitative ultrastructure of capillaries in the rat retina.

The quantitative ultrastructure of capillaries in the rat retina was studied in order to establish control parameters for experimentally induced vascular disorders. A near perfect correlation teas found between the number of tight junctions and the mean lumen area of retinal capillaries. Comparisons between female pigmented and albino rats of the same age showed no significant differences in all parameters measured. In contrast, comparisons between the nerve fiber layer, inner plexiform layer, and outer plexiform layer within the groups showed significant differences in the mean cross-section area, lumen area, cytoplasmic area of endothelial and mural cells, and basement membrane thickness. Light microscopic studies of aralditeembedded tissue showed a highly significant difference in the frequency of capillaries per unit area between retinal layers; approximately one half of the total number of retinal capillaries occurs in the outer plexiform layer of this species. Since the variability between retinal layers is greater than the variability between animals and between different strains, care should be taken to restrict comparisons to capillaries of the same retinal layer in control and experimental animals. Variation due to differences in the number of tight functions can be minimized by restricting comparisons to capillaries with the same number of functions.