Dentin lies deep to the enamel and cementum. It contains less hydroxyapatite than enamel, about 70%, but more than is found in bone and cementum. Dentin is secreted by odontoblasts that form an epithelial layer over the inner surface of the dentin, i.e., the surface that is in contact with the pulp (Fig. 16.16). Like ameloblasts, odontoblasts are columnar cells that contain a well-developed rER, a large Golgi apparatus, and other organelles associated with the synthesis and secretion of large amounts of protein (Fig. 16.17). The apical surface of the odontoblast is in contact with the forming dentin; junctional complexes between the odontoblasts at that level separate the dentinal compartment from the pulp compartment.
primordium of enamel primordium of pulp dental papilla dental papilla dental pulp
FIGURE 16.11 • Diagram showing the cellular relationships during enamel formation. In the initial secretory stage, dentin is produced first by odontoblasts. Enamel matrix is then deposited directly on the surface of the previously formed dentin by secretory-stage ameloblasts. The secretory-stage ameloblasts continue to produce enamel matrix until the full thickness of the future enamel is achieved. (Adapted with permission from Schour I. The neonatal line in the enamel and dentin of the human deciduous teeth and first permanent molar. JADA 1936;23:1946. Copyright (c) 1936 American Dental Association. All rights reserved.)
The layer of odontoblasts retreats as the dentin is laid down, leaving odontoblast processes embedded in the dentin in narrow channels called dentinal tubules (see Fig. 16.15). The tubules and processes continue to elongate as the dentin continues to thicken by rhythmic growth. The rhythmic growth of dentin produces certain “growth lines” in the dentin (incremental lines of von Ebner and thicker lines of Owen) that mark significant developmental times such as birth (neonatal line) and when unusual substances such as lead are incorporated into the growing tooth. Study of growth lines has proved useful in forensic medicine.
Predentin is the newly secreted organic matrix, closest to the cell body of the odontoblast, which has yet to be mineralized. Although most of the proteins in the organic matrix are similar to those of bone, predentin contains two unique proteins:  Dentin phosphoprotein (DPP), a 45-kilodalton highly acidic phosphorylated protein, which is rich in aspartic acid and phosphoserine and binds large amounts of calcium.
DPP is involved in the initiation of mineralization and in control of mineral size and shape.  Dentin sialoprotein (DSP), a 100-kilodalton proteogly can which is rich in aspartic and glutamic acids, serine, glycine, and chondroitin 6-sulfate. DSP is also involved in the mineralization process.
An unusual feature of the secretion of collagen and hydroxy-apatite by odontoblasts is the presence, in Golgi vesicles, of arrays of a formed filamentous collagen precursor. Granules believed to contain calcium attach to these precursors, giving rise to structures called abacus bodies (Figs. 16.17 and 16.18). Abacus bodies become more condensed as they mature into secretory granules.
Dentin is produced by odontoblasts.
Dentin is the first mineralized component of the tooth to be deposited. The outermost dentin, which is referred to as mantle dentin, is formed by subodontoblastic cells that produce small bundles of collagen fibers (von Korff ’s fibers). The odontoblasts differentiate from cells at the