New ultrastructure of rat RPE cells: basal intracytoplasmic tubules.

Retinal pigment epithelial cells have prominent basal folds facing Bruch's membrane. In addition to folds I have observed intracytoplasmic tubules 60-90 nm in diameter in the basal cytoplasm of rat pigment epithelial cells. The tubules arise from the basal plasma membrane and open to the extracellular space. The tubules are most evident when intravenous horseradish peroxidase is used as a tracer. The tracer leaks out of the fenestrated choriocapillaris, into the extracellular space of the pigment epithelium and into the tubules. Electron microscopy at 1000 KV (High Voltage Electron Microscopy) confirms the tubular nature of these structures and their continuity with folds or the plasma membrane facing Bruch's membrane. The tubules are also observed in tissue not infiltrated with peroxidase. Morphometry shows that the tubules occupy about 21% of the surface area of the basal plasma membrane. Tubules appear plentiful where folds are reduced, and reduced where folds are plentiful; the tubules may be a different conformation of the normally slit-like fold extracellular space. The tubules are observed in all quadrants of the retina; centrally and peripherally; in young and adult rats and in pigmented and albino rats. The tubule's function may be linked to that of the folds, from which many of them arise.