The association of amorphous mineral deposits with the plasma membrane of pre- and young odontoblasts and their relationship to the origin of dentinal matrix vesicles in rat incisor teeth.

Young and preodontoblasts and matrix vesicles which occur in the presecretory region of incisor teeth of growing rats were examined in stained and unstained ultrathin sections in order to characterize sites involved in the initial mineralization of dentin. Common to pre- and young odontoblasts in the presecretory region were hemispherical membrane-associated amorphous densities, measuring 5-35 nm in diameter after fixation in glutaraldehyde-osmium tetroxide or glutaraldehyde only. Amorphous densities were associated also with the limiting membranes of some vesicles in the extracellular matrix. Other vesicles in the extracellular matrix contained needle-like crystalline deposits typical of dentinal matrix vesicles. Fully differentiated odontoblasts in more incisal regions of the tooth lacked plasma membrane-associated amorphous densities. Neither amorphous nor crystalline densities were associated with any other cellular or subcellular structures in cells of the presecretory region. Flotation of ultrathin sections on solutions of EDTA or EGTA removed the amorphous densities from the plasma membranes, suggesting that the amorphous densities are calcium-containing mineral deposits. Amorphous deposits were associated with the membrane of vesicular structures protruding from the surfaces of pre- and young odontoblasts, suggesting that vesicles found in the extracellular matrix arise by budding from the plasma membranes of pre- and young odontoblasts. The occurrence of amorphous mineral deposits in association with the limiting membrane of some vesicles in the extracellular matrix, and the occurrence of needle-like mineral crystals within other matrix vesicles, suggest that an amorphous-to-crystalline phase transformation of mineral takes place within the matrix vesicle. The results of this study suggest that calcium-binding sites associated with plasma membranes of pre- and young odontoblasts act as nucleating centers for primary mineral deposition in tooth dentin.