CARBONATOAPATITE FORMATION IN ENAMEL MINERALIZATION

牙釉质矿物在发育和爆发的牙齿是最充分的描述

Enamel mineral in developing and erupted teeth is most adequately described as a carbonatoapatite. The present proposal aims at: a) determining the stoichiometry and solubility product constants of the carbonatoapatite in the enamel tissue at various developmental stages; b) assessing the driving force for precipitation of the enamel mineral during amelogenesis; and, as a long-term objective, c) investigating possible evolutionary changes in the mechanism of carbonatoapatite formation in hard tissues. The work will entail a combination of chemical analyses, electron microscopy, FTIR and FTIR microscopy, x-ray diffraction and protein analyses. Such a combination of procedures will be applied toward the following specific goals: (i) Determination of the solubility of enamel mineral under controlled partial pressures of CO2. Equilibration of each sample will be conducted in solutions having ionic composition similar to that of the enamel fluid. (ii) Determination of the stoichiometry of enamel mineral and development of new solubility models taking into account most of the lattice constituents: Ca2+, PO43-, HPO42-, CO32-, Mg2+, Na+, OH-, and F-. For this purpose, the total contents of these species and the exchangeable pools of carbonate, HPO42-, Mg2+, and Na+ on the crystal surfaces will be assessed separately. At the end, solubility product constants of the enamel mineral at the various stages of porcine amelogenesis will be determined on the basis of the stoichiometry and the solubility model that fit the best the experimental data. (iii) Preparation of dahllite- and francolite-type carbonatoapatites containing HPO42-, Mg2+ and Na+ as lattice constituents. The effects of magnesium, sodium, and fluoride ions on the CO32- and HPO42- incorporation into the lattice, the sites of CO32- substitution, and the morphology of the crystallites will be assessed. The synthetic products, better prototypes for enamel crystals than hydroxyapatite, will be used in solubility, protein adsorption, and crystal growth studies. (iv) Investigation of the effects of fluoride on carbonatoapatite formation in vivo and in vitro. Fish enameloid provides a unique model, since the fluoride incorporation into the mineral is related to the phylogeny of fishes. Attention will be focused on species- specific carbonate substitution in the crystal lattice and morphology of fish enameloid crystals at various stages of development. Synthetic media will be used to ascertain the existence of a critical concentration of fluoride in solution, which causes significant changes in the precipitation process (i.e., involvement of precursors or de novo formation of apatite crystals) and in the properties of formed carbonated apatites, such as morphology and solubility. (v) Elucidation of a possible regulatory mechanism of carbonatoapatite formation through the enamel proteins-crystal interaction. Carbonatoapatites synthesized to yield a range of chemical and physical properties will be used to study the effects of mineral composition and crystal-surface properties on the adsorption of enamel proteins and inhibitory activity of the adsorbed proteins on apatite crystal growth. The overall proposal is designed to gain a new insight into the mechanism of biological carbonatoapatite formation and its regulation during amelogenesis. The information obtained is valuable to predict the stability of dental enamel in normal and cariogenic environments or under fluoride regimes.