Tracer, cytochemical, and freeze-fracture study on the mechanisms whereby secretory ameloblasts absorb exogeneous proteins
Sasaki, T.
Acta Anatomica 118(1): 23-33
1984
ISSN/ISBN: 0001-5180 PMID: 6702405 Document Number: 223347
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, i.v. injected horseradish peroxidase (HRP) rapidly penetrated into the intercellular spaces of the ameloblast layer through the proximal junction 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 tubovesicular structures. In replica images, many small pits on the P face of the cell membranes were obseved 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. The secretory ameloblast is apparently able to absorb and digest enamel protein actively from the developing matrix.