MOLECULAR MECHANISMS OF DENTAL ENAMEL FORMATION

Tooth enamel is a unique mineralized tissue in that it is acellular, i s more highly mineralized, and is comprised of individual crystallites that are larger and more oriented than other mineralized tissues. Den tal enamel forms by matrix mediated biomineralization. Enamel crystall ites precipitate from a supersaturated solution within a well-delineat ed biological compartment. Mature enamel crystallites are comprised of non-stoichiometric carbonated calcium hydroxyapatite. The earliest cr ystallites appear suddenly at the dentino-enamel junction (DEJ) as rap idly growing thin ribbons. The shape and growth patterns of these crys tallites can be interpreted as evidence for a precursor phase of octac alcium phosphate (OCP). An OCP crystal displays on its (100) face a su rface that may act as a template for hydroxyapatite (OHAp) precipitati on. Octacalcium phosphate is less stable than hydroxyapatite and can h ydrolyze to OHAp. During this process, one unit cell of octacalcium ph osphate is converted into two unit cells of hydroxyapatite. During the precipitation of the mineral phase, the degree of saturation of the e namel fluid is regulated. Proteins in the enamel matrix may buffer cal cium and hydrogen ion concentrations as a strategy to preclude the pre cipitation of competing calcium phosphate solid phases. Tuftelin is an acidic enamel protein that concentrates at the DEJ and may participat e in the nucleation of enamel crystals. Other enamel proteins may regu late crystal habit by binding to specific faces of the mineral and inh ibiting growth. Structural analyses of recombinant amelogenin are cons istent with a functional role in establishing and maintaining the spac ing between enamel crystallites.