Thin-section, tracer, and freeze-fracture study of the smooth-ended maturation ameloblasts in rat incisors

The morphology and functional roles of the smooth-ended maturation ameloblasts (SAs) of rat incisors were examined by means of routine thin sections, tracer experiments, and freeze-fracture replication. SAs possessed two sets of junctional complexes consisting of tight junctions (fasiae occludentes) and gap junctions at the proximal and distal ends. Neither the proximal nor the distal junctional complex formed a complete barrier around the cell; intravenously injected horseradish peroxidase (HRP) reached the developing enamel surface through SAs extracellular spaces. SA supranuclear cytoplasm included such various cytoplasmic vesicles as the multivesicular body (MVB), large and vacuoles, dense body, and coated vesicles. The HRP that reached the enamel surface was incorporated into some coated vesicles and small vacuoles via coated pits and membrane invaginations of the distal cell surface of SA. Then, in the process of time, it migrated into the MVB and large endocytic vacuoles. These results indicate that the SA layer forms an extracellular transfer route for metabolites between papillary layer and the enamel surface and that SAs absorb exogenous protein.