Scanning electron microscopy of rat maturation ameloblasts.

Post-secretory, maturation-phase ameloblasts were studied by scanning electron microscopy of freeze-fractured or dry-dissected rat incisors. These cells are in contact with the enamel which they secreted at an earlier time and which undergoes a process of continuing mineralization. The lateral intercellular compartment between maturation ameloblasts is sometimes continuous with the intercellular space of the papillary layer of the enamel organ, but often closed by basal ring contacts which correspond to terminal bars seen in transmission electron microscopy. The distal poles of the cells sometimes possess striated borders. Lateral cell surfaces may show longitudinal gutter-like depressions between ridges from which numerous intercellular connections arise; or a maze of lateral folds and ridges; or they may have mostly microvillous surface projections bordering a minimal intercellular space compartment. Preliminary correlations of groupings of basal, lateral and distal cell features indicate that "basal-closed plus distal striated border" cells may show every type of lateral surface. Cell without a striated border, whether open or closed basally, have ridge or maze lateral surfaces bordering a wide intercellular compartment. "Basal-open plus striated border" cells have microvillous or maze-like surfaces. These combinations of features are encountered a few times along the length of the maturation zone of individual incisors and suggest tha existence of cyclical changes in the type of activity of maturation ameloblasts.