Tracer, cytochemical, and freeze-fracture study on the mechanisms whereby secretory ameloblasts absorb exogeneous...

To investigate the mechanisms whereby secretory ameloblasts absorb and digest exogenous proteins, rat incisors were examined by means of the tracer, cytochemical, and freeze-fracture methods. In the tracer experiment, intravenously injected horseradish peroxidase (HRP) rapidly penetrated into the intercellular spaces of the ameloblast layer through the proximal junctional complexes and further diffused beyond the distal junctional complexes. Freeze-fracture replicas indicated various spaces among the strands of the distal tight junctional complex, which nonetheless completely sealed the cell. The HRP was taken into the cytoplasm from Tomes' process by large coated vesicles and tubulovesicular structures. In replica images, many small pits on the P face and corresponding bumps on the E face of the cell membranes were observed in Tomes' process. These pits and bumps are thought to correspond to coated pits representing initial images of micropinocytosis in thin sections. Following absorption, the HRP migrated into endocytic vacuoles or large phagosomes in the supranuclear cytoplasm. Both structures were positive for acid phosphatase reaction and are, therefore, taken to represent phagolysosomes. These results suggest that the secretory ameloblast is able to absorb and digest enamel protein actively from the developing matrix.