Recent observations on enamel crystal formation during mammalian amelogenesis

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Abstract
Enamel mineralization taking place during amelogenesis is a unique model to investigate carbonatoapatite formation in vivo. The abundance of proteinaceous crystal growth inhibitors, in particular amelogenins, contributes significantly to the mineralization process. Their putative roles are to prevent random proliferation of crystal nuclei and to regulate the growth kinetics and orientation of the formed enamel crystals. The enamel fluid surrounding the forming enamel crystals contains high concentrations of carbonate and magnesium ions, both of which seem to modulate the mineralization process. Particularly, Mg ions can adsorb onto enamel crystal surfaces in a manner to compete with Ca ions. Enamel mineral formed during amelogenesis is featured as calcium-deficient, acid phosphate-rich carbonatoapatites. Currently the most putative stoichiometry model for enamel mineral is (Ca)5-x(HPO4)v(CO3)w(PO4)3-x (OH)1-x. Very significant changes in the morphology, stoichiometry, and solubility of enamel crystals occur during the various stages of amelogenesis. The early enamel mineralization comprises two events: the initial precipitation of the well-documented thin ribbons and the subsequent overgrowth of apatite crystals on those templates. The thin ribbons precipitated in the vicinity of the secretory ameloblasts have the highest contents of acid phosphate, particularly in the form of exchangeable species, whereas both the exchangeable and unexchangeable acid phosphate decrease concomitantly with the progress of the apatite overgrowth and the appearance of elongated hexagonal crystals in the late secretory stages. Those morphological and compositional features seem to be consistent with the formation of precursors, such as octacalcium phosphate.