 Matrix production, or secretory stage. In the formation of mineralized tissues of the tooth, dentin is produced first. Then, partially mineralized enamel matrix (Fig. 16.11) is deposited directly on the surface of the previously formed dentin. The cells producing this partially mineralized organic matrix are called secretory-stage ameloblasts. As do osteoblasts in bone, these cells produce an organic proteinaceous matrix by activity of the rough endoplasmic reticulum (rER), Golgi apparatus, and secretory granules. The secretory-stage ameloblasts continue to produce enamel matrix until the full thickness of the future enamel is achieved.
 Matrix maturation. Maturation of the partially mineralized enamel matrix involves the removal of organic material as well as continued influx of calcium and phosphate into the maturing enamel. Cells involved in this second stage of enamel formation are called maturation-stage ameloblasts. Maturation-stage ameloblasts differentiate from secretory-stage ameloblasts and function primarily as a transport epithelium, moving substances into and out of the maturing enamel. Maturation-stage ameloblasts undergo cyclical alterations in their morphology that correspond to cyclical entry of calcium into the enamel.
Secretory-stage ameloblasts are polarized columnar cells that produce enamel.
The secretory-stage ameloblast lies directly adjacent to the developing enamel. At the apical pole of each ameloblast is a process, Tomes’s process, which is surrounded by the developing enamel (Fig. 16.12). A cluster of mitochondria in the base of the cell accounts for the eosinophilic staining of this region in hematoxylin and eosin (H&E)– stained paraffin sections (Fig. 16.13). 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. 16.10, 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. 16.14) Maturation-stage ameloblasts with a striated border occupy approximately 70% of a specific cycle, and those that are smooth-ended, approximately 30% of a specific cycle. There is no stratum intermedium in the enamel organ during enamel maturation; stellate papillary cells are adjacent to the maturation-stage ameloblasts.
The maturation-stage ameloblasts and the adjacent papillary cells are characterized by numerous mitochondria. Their presence indicates cellular activity that requires large amounts of energy and reflects the function of maturation-stage ameloblasts and adjacent papillary cells as a transporting epithelium.