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Abstract
Previous studies of the retinal pigment epithelium have suggested that its cells are interconnected by typical arrays of intercellular junctions. Using both conventional and freezefracture electron microscopy, we have found on the contrary that pigment epithelial cells in a wide variety of vertebrates (goldfish, frog, newt, mudpuppy, turtle, chicken, mouse, cat, and monkey) in fact have unusual junctional complexes. In all these species, each cell is completely girdled by a zonula occludens (tight junction) and a zonula adherens (intermediate junction). In addition, very large, macular gap junctions occur within the region occupied by the zonula occludens. The zonula adherens partially overlaps the zonula occludens, and thereby modifies the membrane cleavage properties of the latter junction. This yields a novel freeze-fracture pattern; the strands and grooves normally encountered on cleavage faces at zonulae occludentes are disrupted and fragmented in the region of the zonula adherens. Physiologic experiments demonstrate that, as might be expected from the presence of numerous gap junctions, pigment epithelial cells are electrically coupled. Our results confirm that pigment epithelial cells are joined by extensive zonulae occludentes (which form the "R" membrane) and zonulae adherentes. In addition, they show that gap junctions occur widely in pigment epithelia and that, as a result, pigment epithelial cells are functionally in electrical continuity with one another