Fig. 9 Drawing depicting the intercellular and cell-basement membrane junctions of retinal pigment epithelial cells. A. View of the intercellular junctions as seen from the interior of the cell. Two belt-like junctions encircle the cell near its apex. The zonula occludens (ZO) obliterates the intercellular space owing to fusion of the outer leaflets of the two contiguous cell membranes (arrow). An inset in this zonula shows the freeze-fracture appearance of this region, a network of anastomosing strands that lie in the plane of the fused membranes. The zonula adherens (ZA) is characterized by a somewhat wider intercellular space and attachment of microfilaments on the intracellular side of the membrane. The desmosome or macula adherens (MA) is a “spot-weld” multilayered structure identified by an extracellular midline, two plasma membrane domains, and two plaques at which vimentin intermediate filaments of the cytoskeleton attach. These three structures constitute the tripartite apicolateral junctional complex or terminal bar. Gap junctions (GJ) occur within the zonula occludens as well as at other sites in the lateral cell membrane. The gap junctions, as they appear in a freeze-fracture preparation, are shown at the top of the figure. They consist of an hexagonal array of proteins (connexons) that connect the adjacent cells. The basal attachment of the retinal pigment epithelial cell to the basement membrane is not through a hemidesmosome but rather through focal adhesions (FA). At focal adhesions the plasma membrane has a tiny dense plaque to which cytoplasmic microfilaments are attached. Filaments beneath the focal adhesion insert into the basement membrane. B. Transmission electron micrograph of occluding junction of human retinal pigment epithelial cells. Multiple fusions of the outer leaflets of apposed plasma membranes are seen (×96,000). (A, modified from illustration of corneal basal epithelial cell, Hogan MF, Alvarado JA, Weddell JE: Histology of the Human Eye. Philadelphia: WB Saunders, 1971. Freeze-fracture images from Hudspeth AJ, Yee AG: The intercellular junctional complexes of retinal pigment epithelia. Invest Ophthalmol Vis Sci 12:354, 1973)