Reactions of the organic matrix in dentin caries

Abstract

A tooth consists of pulp tissues encased in hard, mineralized tissues. Dentin-forming cells (odontoblasts) are found at the border of the pulp. Odontoblasts have extensions running centripetally through narrow channels (tubules) in predentin and dentin, a bone-like tissue. Predentin is deposited by odontoblasts and consists of organic matrix. Mineralization takes place some time after the predentin deposition, thus at some distance from the odontoblast. Mineralization of the organic matrix is induced by enzymes and proteins secreted by the odontoblast extensions into the predentin at the level of the mineralization front. The tubules end in the outer part of dentin (mantle dentin), the composition of which deviates from bulk dentin. Dentin consists of mineral (70 wt%), water (10 wt%) and organic matrix (20 wt%). At the tooth's crown, dentin is lined with enamel, a mineralized tissue with far less of an organic matrix than dentin. Normally only a part of the crown is exposed to the oral cavity. Root dentin is lined with cementum, a thin layer of partially mineralized tissue containing organic material. The cementum near the root apex is of a cellular nature, but becomes increasingly acellular towards the crown. The tooth is set in the alveolar ridge of the jaw, and is encircled by the periodontal ligament, which helps absorb the mechanical forces exerted on the tooth. Above the alveolar bone, the tooth is covered by soft, gingival tissue.Dentin organic matrix. The organic matrix of dentin consists of collagen for 90 wt%. Non-collagenous compounds comprise proteoglycans, highlyphosphorylated proteins, osteonectin, osteocalcin, dentin sialoprotein, and lipids (Boskey, 1989; Butler et al., 1992; Goldberg and Septier, 1985; Linde, 1989). Several non-collagenous compounds play a role in dentin formation and mineralization.Collagen is the most prevalent protein in the human body. Nineteen types of collagen have been described. The common feature is the triple helix: three polypeptide chains (α) coiled around each other. It is essential that glycine occupies each third position in the α chains. High contents of hydroxyproline stabilize the structure. Type I collagen prevails in dentin. Its molecules have 300-nm-long rod shapes, containing triple helices (1011 residues per α chain) flanked by short non-helical ends (6-25 residues per α chain). Fibrils are evenly-spaced, linearly-aligned, cylindrical groupings of molecules. The fibrils show alternating bands in electron microscopy due to the overlapping of negatively charged 7 molecule segments stained by heavy metal ions, yielding a specific "banding pattern". Parallel fibrils are gathered into fibres (Van der Rest and Bruckner, 1993).Type V collagen has been found in dentin (Lukinmaa and Waltimo, 1992) and is involved in fibril formation. Some peculiarities have been noted for dentinal type I collagen. For example, a high content of α 1 chains in rat dentin indicates the presence of molecules composed of three α 1 chains apart from the usual (α 1 ) 2 α 2 composition...