Figure 1517

Electron micrograph of odontoblasts. The plasma membrane of one odontoblast has been marked with arrows. The cell contains a large amount of rough endoplasmic reticulum and a large Golgi apparatus. The odontoblast processes are not included in this image; one process would extend from the apical pole of each cell (top). The black objects in the Golgi region are abacus bodies. The tissue has been treated with pyroantimonate, which forms a black precipitate with calcium, x 12,000.

fibers). The odontoblasts differentiate from cells at the periphery of the dental papilla. The progenitor cells have the appearance of typical mesenchymal cells; i.e., they contain little cytoplasm. During their differentiation into odontoblasts, the cytoplasmic volume and organelles characteristic of collagen-producing cells increase. The cells form a layer at the periphery of the dental papilla, and they secrete the organic matrix of dentin, or predentin, at their apical end (away from the dental papilla) (Fig. 15.19). As the predentin thickens, the odontoblasts move or are displaced centrally (see Fig. 15.12). A wave of mineralization follows the receding odontoblasts; this mineralized product is the

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