Roles of Fluoride on Octacalcium Phosphate and Apatite Formation on Amorphous Calcium Phosphate Substrate

As fluoride is known to affect the biomineralization of calcium phosphates, we examined how the growth at 37 °C of octacalcium phosphate (OCP) and hydroxyapatite (HAp) on an amorphous calcium phosphate substrate is affected by low-dose fluoride (0–2 ppm) in metastable calcium phosphate solution. In the absence of fluoride, highly oriented plate-like OCP crystals grow on the substrate. With 0.7–0.8 ppm fluoride, OCP crystal formation was greatly modulated (the crystals were smaller and their orientation was lower); moreover, tiny plates and needles appeared on the OCP plates. There were two critical fluoride concentrations: (1) at 0.9 ppm, the OCP plates were completely replaced by HAp nanorods; (2) at 1 ppm, the structure of the HAp nanorod assembly changed from an open one to a tightly assembled one. The fluoride suppressed OCP formation, induced HAp nanorod formation, and promoted tight nanorod assembly. The relationship between the fluoride concentration and the formation of OCP and HAp provides insight into the structural design of an OCP/HAp coating layer and should be useful in the application of fluoride to an OCP/HAp coating on any substrate

Tooth Enamel Proteins Enamelin and Amelogenin Cooperate To Regulate the Growth Morphology of Octacalcium Phosphate Crystals

To examine the hypothetical cooperative role of enamelin and amelogenin in controlling the growth morphology of enamel crystals in the postsecretory stage, we applied a cation selective membrane system for the growth of octacalcium phosphate (OCP) in the truncated recombinant porcine amelogenin (rP148) with and without the 32 kDa enamelin fragment. Enamelin alone inhibited the growth in the c-axis direction more than rP148, yielding OCP crystals with the smallest aspect ratio of all conditions tested. When enamelin was added to the amelogenin “gel-like matrix”, the inhibitory action of the protein mixture on the growth of OCP in the c-axis direction was diminished, while that in the b-axis direction was increased. As a result, the length to width ratio (aspect ratio) of OCP crystal was markedly increased. Addition of enamelin to amelogenin enhanced the potential of amelogenin to stabilize the amorphous calcium phosphate (ACP) transient phase. The ratio of enamelin and amelogenin was crucial for stabilization of ACP and the growth of OCP crystals with larger aspect ratio. The cooperative regulatory action of enamelin and amelogenin was attained, presumably, through coassembling of enamelin and amelogenin. These results have important implications in understanding the growth mechanism of enamel crystals with large aspect ratio