Ultrastructural, biochemical, and immunologic evidence of receptor-mediated endocytosis in the crystalline lens.

The lens epithelium is essentially the basal layer of the crystalline lens of the eye, an uncommon stratified epithelium. Ions and metabolites present in the aqueous humor gain access to the lens epithelium by diffusion through the lens capsule (the basement membrane of the lens epithelium). Then, it is presumed, the underlying lens fiber cells obtain necessary ions, metabolites, and nutrients through gap junctions conjoining the apical surfaces of the lens epithelial cells from the basal layer with the apical surfaces of elongating fiber cells from upper strata. In this report, correlative morphologic, biochemical, and immunochemical evidence is presented that both lens epithelial and fiber cells use endocytotic and/or transcytotic processes rather than being solely dependent on gap junctions for metabolic cooperation. Freeze-fracture analysis of the apicoapical interface between lens epithelial and elongating fiber cells (epithelial-fiber cell interface [EFI]) revealed protrusions and pits of two distinct sizes (average diameters, 46 and 126 nm). Gap junctions with tight particle packing were only rarely observed at the EFI. Gap junctions with loose particle packing were never observed at the EFI. "Orthogonal arrays" of intramembrane particles (OAPs) were not uncommon at the EFI. Thin-sections taken perpendicular to the EFI confirmed the existence of micropinocytotic and clathrin-coated vesicles in both lens epithelial and elongating fiber cells. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) of separate preparations of lens epithelial and fiber cells, specifically enriched for clathrin-coated vesicles, showed a 180-kD protein. Western blot analysis of this protein revealed selective cross-reactivity with polyclonal anticlathrin antibodies. These results strongly suggest that transcytotic processes provide a primary route for the entry and egress of macromolecules in the lens.