Cell and Developmental Biology of the Retinal Pigment Epithelium Epithelial and endothelial cells are joined to one another through a junctional complex that includes an adherens junction and an occludens or tight junction.
Tight junctions encircle the cell in the lateral membrane near the apical cell surface.
In freeze fracture, tight junctions appear as anastomosing strands or fibrils immediately beneath the cell surface. Tight junctions serve as a semi-permeable barrier, allowing diffusion of some solutes but not others. In addition to a paracellular diffusion barrier, tight junctions also play a crucial role in the development and maintenance of cell polarity, forming a membranous fence restricting diffusion of lipids and proteins between one cell domain and another. Since its identification, however, ZO-1 has also been identified in adherens junctions of cells that never form tight junctions. Occludin and claudins both localize to the tight junction strands seen in freeze-fracture and it is currently believed that both molecules significantly participate in the creation of the paracellular barrier.