The characterization of cell junctions in normal tissues has increased rapidly in the past several years. Junctional structures can now be separated into five major groups, each with significant structural pleomorphism. Cell junctions have been described in developing tissues, and their appearance can often be associated with significant morphogenetic events. Recently, the process of junctional membrane differentiation has been examined in sea urchin embryogenesis and rat testis organogenesis. Both studies indicate that dramatic membrane assembly processes are involved in cell junction development (maturation).