FIGURE 16.12 • Schematic diagrams of a partially formed tooth showing details of amelogenesis. a. The enamel is drawn to show the enamel rods extending from the dentinoenamel junction to the surface of the tooth. Although the full thickness of the enamel is formed, the full thickness of the dentin has not yet been established. The contour lines within the dentin show the extent to which the dentin has developed at a particular time, as labeled in the illustration. Note that the pulp cavity in the center of the tooth becomes smaller as the dentin develops. (Based on Schour I, Massler M. The neonatal line in the enamel and dentin of the human deciduous teeth and first permanent molar. J Am Dent Assoc 1936;23:1948.) b. During amelogenesis, enamel formation is influenced by the path of the ameloblasts. The rod produced by the ameloblast forms in the wake of the cell. Thus, in mature enamel, the direction of the enamel rod is a record of the path taken earlier by the secretory-stage ameloblast. c. At the apical pole of the secretory-stage ameloblasts are Tomes’ processes, surrounded by the developing enamel. Junctional complexes at the apical pole are also shown. Note the numerous matrix-containing secretory vesicles in the cytoplasm of the processes.
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. 16.19). As the predentin thickens, the odontoblasts move or are displaced centrally (see Fig. 16.12). A wave of mineralization follows the receding odontoblasts; this mineralized product is the dentin. As the cells move centrally, the odontoblastic
FIGURE 16.13 • Enamel organ cells and odontoblasts in a developing tooth. This photomicrograph of an unstained plastic thick section viewed with the phase contrast microscope shows enamel organ cells and odontoblasts as they begin to produce enamel (E) and dentin (D), respectively. Young enamel is deposited by secretory-stage ameloblasts (SA) onto the previously formed dentin. The enamel appears dark in the illustration. At the top, the enamel surface displays a characteristic picket-fence pattern because of the sharp contrast between the lightly stained Tomes’ processes (TP) of the secretory-stage ameloblasts and the darkly stained young enamel product that partly surrounds the cell processes. The nuclei (N) at the right belong to cells of the stratum intermedium. The nuclei (N) on the left belong to odontoblasts located in the basal part of the cells. The odontoblast cytoplasm extends to the dashed line. At this point, cytoplasmic processes (OP) extend into the dentin. 85.