Dentin mineralization and the role of odontoblasts in calcium transport

Dentin is formed by two simultaneous processes, in which the odontoblasts are instrumental-the formation of the collagenous matrix, and mineral crystal formation in this matrix. This pattern of formation is similar to that of bone, another mineralized connective tissue. Dentin and bone also have chemical compositions which are similar but, with distinct differences. It is of fundamental importance to understand how the ions constituting the inorganic phase are transported from the circulation to the site of mineral formation and how this transport is regulated. For dentinogenesis, calcium is essentially the only ion for which data are available. Recent evidence suggests that a major portion of the Ca2+ ions are transported by a transcellular route, thus being under cellular control. The cells maintain a delicate Ca2+ ion balance by the concerted action of transmembraneous transport mechanisms, including Ca-ATPase, Na+/Ca2+ exchangers and calcium channels, and of intracellular Ca2+-binding proteins. The net effect of this is a maintenance of a submicromolar intracellular Ca2+ activity, and an extracellular accumulation of Ca2+ ions in predentin, at the mineralization front. Predentin can be regarded as a zone of formation and maturation of the scaffolding collagen web of the dentin organic matrix. In addition to collagen, it contains little but proteoglycan. Simultaneous with mineral formation, additional non-collagenous macromolecules are added to the ext;acellular matrix of dentin, these presumably being transported within the odontoblast process. Among these are highly phosphorylated dentin phosphoprotein (phosphophoryn) and another pool of proteoglycan. The functionality of this may be explained by the fact that polyanionic macromolecules are capable of inducing the formation of hydroxyapatite at ionic conditions resembling those in vivo. They can also inhibit mineral growth and regulate crystal size.