An ultrastructural study of cell junctions and the cytoskeleton in epithelial cells of the molluscan integument

Cell junctions and the cytoskeleton of integumental epidermal cells from six bivalves, four gastropods, and two cephalopods were studied by transmission electron microscopy. In all species examined, the junctions in supporting cells presented the following similar pattern: an apical-lateral adhesion belt (occluding junctions were not observed); (b) a lateral complex of septate junctions and smooth septate junctions, with interdigitations between adjacent cells while the gap junctions were not constantly present, and a basal complex with hemidesmosomes, focal contacts, and sometimes basolateral adherent junctions. Desmosomes were never observed. Microfilamentous and microgranular material were present throughout the cells, as bundles of microfilaments within microvilli and the terminal web, within interdigitations, and as cytoplasmic plaques forming part of the adherent junctions, hemidesmosomes, and focal contacts. Bundles of intermediate filaments that originated from basal hemidesmosomes were located close to and oriented parallel with the lateral plasma membrane and terminated within the terminal web. In cells of Aplysia depilans, intermediate filaments converged apically to terminate in hemidesmosome-like structures at the bases of the microvilli. In the cephalopods, hemidesmosomes were never observed and intermediate filaments made direct contact with the basal cell membrane. Some functional interpretations and hypotheses were also discussed.