Figure 1515

Electron micrograph of Sharpey's fibers. Sharpey's fibers extend from the periodontal ligament (right) into the cementum. They consist of collagen fibrils. Sharpey's fibers within the cementum are mineralized; those within the periodontal ligament are not mineralized, x 13,000.

Sharpey's fibers cementum contact with the pulp (Fig. 15.16). 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. 15.17). 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.

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. 15.15). 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), which is rich in aspartic acid and phosphoserine and binds large amounts of calcium. DPP is involved in initiation of mineralization and in control of mineral size and shape.

• Dentin sialoprotein (DSP), which is rich in aspartic acid, serine, glutamic acid, and glycine and is also involved in the mineralization process.

An unusual feature of the secretion of collagen and hy-droxyapatite 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. 15.17 and 15.18). 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

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[gfieflgMtini predentin]

- dental pulp i." Jr blood vessels.

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Odontoblasts

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