Fig. 22. Inset. A gap junction between pigmented and nonpigmented epithelial cells (NPC) in Macaca mulatta. The tracer horseradish peroxidase, injected in vivo, has filled the small gap between the cell plasma membranes. Main figure. Gap junctions (GJ) at apex of nonpigmented epithelial cells are rounded aggregates of 8- to 9-nm particles (arrows) on the inner leaflet (P-face) of the plasmalemmal membrane. Zonula occludens-like strands are frequently seen at the periphery (arrowheads). (Freeze-fracture, X 34,000; Inset, X 168,000; Courtesy of Dr. Guiseppina Raviola)