Thermodynamic and Kinetic Interpretation of Enamel Mineralization Octacalcium Phosphate (OCP) as an Inevitable Intermediate Phase

Abstract

Much attention has been given to the growth mechanism of enamel mineral by which the unique features (i. e., morphology, size and alignment) of enamel crystallites in mammals are achieved. In this paper, we report our experimental and theoretical approaches to a long-debated and unsettled issue, i. e., the involvement and role of an acidic precursor in enamel mineralization. Analysis of nucleation events, which were induced in neutral media at 37°C and 1.8% partial CO2 pressure, substantiated the involvement of octacalcium phosphate (OCP) in precipitation reactions. We also developed a new crystal growth model using miniaturized columns in tandem, in which OCP itself was proved to be equally competent, in comparison with hydroxyapatite (HA), as a template for crystal precipitation and as an adsorbent for interaction with enamel proteins. Simulation of the precipitation process taking place in situ with calcium transport (its increment) into enamel fluid also allowed us to predict that OCP is more kinetically favorable than HA, if Ca ions are supplied through the cellular activity of ameloblasts. The results obtained support the theory that OCP is involved as an inevitable intermediate phase to yield large enamel carbonatoapatite during amelogenesis.