Intercellular Junctions of the Secretory Ameloblasts

Abstract

In A study of the ultrastruncture of the secretory ameloblasts, fifty neonatal rats were perfused with Karnovsky's fixative or fixative containing tannic acid and glutaraldehyde. The distribution and structural features of cell junctions in the ameloblasts were studied by thin sectioning and freeze-fracture technique.The longitudinal sections of fractured ameloblasts were observed as long, narrow cells, which were divided by single or double ridges into 2-3 faces on the lateral surface. In thin sections and freeze-fracture replicas, gap junctions and desmosomes were found on the basal surfaces, and gap junctions of various sizes and outlines were found on the lateral surfaces of the ameloblasts. In replica images, the focal linear ridges of macular tight junctions were seen on the lateral surfaces, and closely associated with small clusters of the aggregated particles of gap junctions.In thin sections and replicas, the junctional complexes which consist of tight and gap junctions and desmosomes were observed at both proximal and distal ends of the secretory ameloblasts. The proximal junctional complex consisted of 1-4 tight junctional strands, and in completely sealed the paracellular spaces of the ameloblasts. In contrast, the well-developed meshwork of tight junctions occupied the distal end of the cell body. This distal junctional complex was more extensive than that of proximal, and completely occluded the paracellular spaces of the ameloblasts.However the discontinuous and free-ending strands of tight junctions were commonly observed in the junctional complex. Discontinuous strands are thought to indicate the incomplete fusion of particles in tight junctional strands. Leaky structures of these junctional complexes must he concerned with the penetration of calcium and the other molecules throughout the paracellular spaces in the ameloblast layer into the enamel.