Fig 2-3 Cross section of a rat incisor, illustrating mature secretory odontoblasts. (BV) Blood vessels; (CR zone) cell-rich region of the pulp containing numerous fibroblasts; (D) dentin; (OP) odontoblastic process; (PD) predentin. (Epon section [1 μm] stained with toluidine blue; original magnification × 260.)
Odontoblasts are joined and attached at their distal extremities by well-developed terminal webs and associated fascia adherens junctions (see Fig 2-2).8 Physical evidence of the strength of this bond is provided by the fact that the odontoblastic layer can be isolated relatively intact after demineralization and digestion of the dentin matrix. When observed macroscopically and histologically, the terminal web apparatus appears to form a continuous membranous structure. Early histologists called it the pulpodentinal membrane. This zone of attachment prevents the entrapment of odontoblasts in the predentin matrix and ensures that the developing surface of dentin remains relatively flat.
Although physiologic evidence suggests that a paracellular barrier to calcium exists at the distal end of the cell, no zonula occludens junction is present. Morphologic studies have revealed only a partial (fascia) occludens junction at that site. Gap junctions are formed between adjacent odontoblasts and between odontoblasts and the fibroblasts of the subodontoblast-rich zone.29,37,38
The narrow intercellular spaces between adjacent odontoblasts contain collagen fibers, aperiodic microfibrils, proteoglycans, and fibronectin.15,39–43 These intercellular fibers (von Korff fibers) follow a spiral pathway through the interodontoblastic space, passing into the predentin between adjacent odontoblasts at interruptions of the fascia occludens and fascia adherens junctions.
During odontoblast differentiation, the RER and the Golgi complex undergo hypertrophy in preparation for protein secretion. The nucleus is restricted to the pulpal end of the cell body and is characterized by an abundant euchromatic matrix, prominent nucleoli, and many nuclear pores (see Figs 2-2 and 2-3). The RER is the major cytoplasmic organelle within active odontoblasts. Parallel cisterns of RER occupy the supranuclear cytoplasm, the borders of the Golgi complex, and the cytoplasm proximal to the terminal web (see Fig 2-2).8,44–46 Mitochondria are dispersed throughout the cell body.
The Golgi complex, containing aggregates of smooth-walled vesicles and cisterns, occupies a central location (see Fig 2-2).6,45–47 Each stack of Golgi cisterns displays morphologic and functional polarity, with a forming face (the convex surface) and a mature face (the concave surface). The forming face develops from, and is continuously replenished by, fusion of small intermediate (transport) vesicles originating from the RER. Presecretory granules containing type I procollagen, glycoproteins, and glycosaminoglycans develop from the cisterns of the mature face of the Golgi apparatus.35,36,48 Phosphophoryns appear to be packaged in small, narrow vesicles.49 The complex cytoplasmic machinery operating in the Golgi complex for targeting secretory proteins to their appropriate final destination is briefly discussed later in the chapter, in the “Basic Science Correlation” section.
After their release from Golgi cisterns, the presecretory granules of the dentin matrix undergo condensation to form smaller secretory granules, approximately 300 nm long and 30 nm wide.8,35 The long axis of the secretory granule is roughly equal to the length of a type I procollagen molecule (about 280 nm long). The diameter of the granule is wide enough to contain many procollagen molecules, packaged side by side.
An essential component of the secretory machinery of the odontoblasts is its network of microtubules.50 Interference with the assembly of microtubules prevents the migration of secretory granules from the Golgi complex to the secretory pole of the odontoblast.51–53 The cytosolic motor-protein kinesin, using adenosine triphosphate (ATP) as an energy source, interacts with microtubules and the membranes of secretory granules to propel the secretory granules in an anterograde direction toward the secretory pole of the cell. Similar interactions between microtubules and cytoplasmic motor-proteins are involved in maintaining the organization of the Golgi complex and the polarized distribution of cytoplasmic organelles. Lysosomes and acid phosphatase are also present in mature odontoblasts, especially prominent in the distal portion of the cell body near the predentin.54,55
During formation of primary dentin, the internal perimeter of the pulp becomes smaller, forcing the odontoblasts into a pseudostratified organization. With further deposition of secondary dentin, some odontoblasts undergo programmed cell death. It has been reported that half of the odontoblasts in human premolar teeth are lost over 4 years.56
Dentin matrix is deposited in incremental amounts in a daily (circadian) biologic rhythm. These microscopic increments are visible in dentin as stripes running parallel to the mineralization front. In human dentin, the daily increment is about 4 μm wide. Additional periodicity occurs at roughly 5-day intervals, producing the lines of Von Ebner, spaced about 20 μm apart. Circadian rhythms may contain further oscillations, which produce ultradian increments. In dentin from rodent incisors, three ultradian lines are spaced about 8 μm apart within the wider 20-μm circadian incremental lines.57
Various explanations have been put forth to explain these rhythmic patterns of matrix deposition. Feeding and/or sleeping patterns were originally suggested to be the most likely causes of variation in secretory function. Fluctuating levels of hormones and growth factors regulated by central neural activity are the probable cause of these patterns.
Mature odontoblasts express parathyroid hormone receptors. Parathyroid hormone has an anabolic effect on odontoblasts, increasing the level of cyclic adenosine monophosphate and alkaline phosphatase.58
Composition of the Dentin Matrix
The organic matrix of dentin contains collagen, non-collagenous proteins (proteoglycans, phosphophoryns, and glycoproteins), phospholipids, and growth factors.
Collagen
Type I collagen is the major protein of dentin matrix. Lesser amounts of types III, V, and VI collagen are also found in dentin matrix.
Electron microscopic autoradiographic studies with tritiated proline and immunocytochemical studies have shown that the procollagen of dentin matrix is secreted mainly from the predentinal segment of the odontoblastic process (Figs 2-4 to 2-6).35,49