Formation of tight junctions in differentiating and secretory ameloblasts of rat molar tooth germs.

Forty newborn rats were perfused with Karnovsky fixative and the tight junctions in differentiating and secretory ameloblasts were examined by conventional electron microscopy and freeze-fracture replications. Pre-ameloblasts were divided into types I, II and III based on morphology. Initial indications of tight-junction formation appeared as linear aggregations of particles in type II. The apparent tight junctional strands were observed in type III and in secretory ameloblasts. Though the junctional strands were numerous and long, no complete barrier between pre-ameloblasts at their distal ends was present. Complete zonular tight junctions were first observed at the distal ends of secretory ameloblasts; at this stage, proximal tight junctions incompletely sealed the paracellular spaces around the ameloblasts. Throughout their formative processes, the tight junctional strands were engaged in forming gap junctions. The structural features of tight junctions were considered to be closely associated with the cytodifferentiation of ameloblasts and permeability in the ameloblast layer.