Horizontal cells invest retinal capillaries in the tree shrew Tupaia belangeri

Retinal capillaries were studied by transmission electron microscopy, immunohistochemistry and lectin histochemistry in the adult tree shrew Tupaia belangeri. In capillaries from all four vascular layers, adjacent endothelial cells were connected by tight junctions. Up to three layers of pericyte processes were embedded in the subendothelial basal lamina. However, pericytes frequently contacted the endothelial cells. In the innermost vascular layer (capillary layer 4), S-100-immunopositive astrocytes and the vitreal processes of S-100-immunopositive Muller cells entirely ensheathed the capillary basal lamina. However, capillaries revealing an incomplete macroglial investment were observed in the outer vascular layers, predominantly in capillary layers 1 and 2. In sections of capillaries located between the inner nuclear layer and the outer plexiform layer (capillary layer 1), these "gaps" were filled with the perikarya and electron-lucent processes of horizontal cells that ensheathed up to approximately nine tenths of the capillary circumference. Horizontal cells were identified by ultrastructural criteria. They were distinct from microglial cells by not being reactive for Griffonia simplicifolia isolectin-B-4. In Tupaia, vessel-contacting horizontal cells reside in a position reported to be occupied by the processes of Muller cells in other mammals. Current concepts of retinal function and pathology, which are based on the assumption that retinal vessels are strictly isolated from retinal neurons, at least in Tupaia, might deserve reconsideration.