Horizontal cells invest retinal capillaries in the tree shrew Tupaia belangeri

Retinal capillaries were studied by transmission electron microscopy, immun ohistochemistry and lectin histochemistry in the adult tree shrew Tupaia be langeri. In capillaries from all four vascular layers, adjacent endothelial cells were connected by tight junctions. Up to three layers of pericyte pr ocesses were embedded in the subendothelial basal lamina. However, pericyte s frequently contacted the endothelial cells. In the innermost vascular lay er (capillary layer 4), S-100-immunopositive astrocytes and the vitreal pro cesses of S-100-immunopositive Muller cells entirely ensheathed the capilla ry basal lamina. However, capillaries revealing an incomplete macroglial in vestment were observed in the outer vascular layers, predominantly in capil lary layers 1 and 2. In sections of capillaries located between the inner n uclear layer and the outer plexiform layer (capillary layer 1), these "gaps " were filled with the perikarya and electron-lucent processes of horizonta l cells that ensheathed up to approximately nine tenths of the capillary ci rcumference. Horizontal cells were identified by ultrastructural criteria. They were distinct from microglial cells by not being reactive for Griffoni a simplicifolia isolectin-B-4. In Tupaia, vessel-contacting horizontal cell s reside in a position reported to be occupied by the processes of Muller c ells in other mammals. Current concepts of retinal function and pathology, which are based on the assumption that retinal vessels are strictly isolate d from retinal neurons, at least in Tupaia, might deserve reconsideration.