Figure 159

Structure of young enamel, a. This electron micrograph shows enamel rods cut obliquely. Arrows indicate the boundaries between adjacent rods, x 14,700. b. Parts of two adjacent rods are seen at higher magnification. Arrows mark the boundary between the two rods. The dark needle-like objects are young hydroxyapatite crystals;.

the substance between the hydroxyapatite crystals is the organic matrix of the developing enamel. As the enamel matures, the hydroxyapatite crystals grow, and the bulk of the organic matrix is removed. x60,000.

eosinophilic staining of this region in hematoxylin and eosin (H&E)-stained paraffin sections (Fig. 15.13). Adjacent to the mitochondria is the nucleus; in the main column of cytoplasm are the rER, Golgi, secretory granules, and other cell elements. Junctional complexes are present at both apical and basal parts of the cell. They maintain the integrity and orientation of the ameloblasts as they move away from the dentoenamel junction. Actin filaments joined to these junctional complexes are involved in moving the secretory-stage ameloblast over the developing enamel. The rod produced by the ameloblast follows 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.

At their base, the secretory-stage ameloblasts are adjacent to a layer of enamel organ cells called the stratum intermedium (see Fig. 15.10, b, c, and g). The plasma membrane of these cells, especially at the base of the ameloblasts, contains alkaline phosphatase, an enzyme active in calcification. Stellate enamel organ cells are external to the stratum intermedium and are separated from the adjacent blood vessels by a basal lamina.

Maturation-stage ameloblasts transport substances needed for enamel maturation

The histologic feature that marks the cycles of maturation-stage ameloblasts is a striated or ruffled border (Fig.

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