Octacalcium phosphate formation in vitro: implications for bone formation.

Octacalcium phosphate (OCP, Ca8H2(PO4)6 . 5H2O) has been found in kidney stones and dental calculi and proposed by Brown et al. [1] to be a precursor of hydroxyapatite in bone and teeth formation. As saliva and urine often have acidic pH which favors OCP formation, the question remains then whether OCP can form in the more basic extracellular fluid in the bone milieu. This paper shows that calcium phosphate crystal phases obtained from neutral and pH 7.4 solution mixtures containing [CaCl2] = 0.1-10.0 mM and [Na2HPO4] = 0.1-90.0 mM with 100-300 mosM at 37 degrees C include brushite (CaHPO4 . 2H2O), OCP, and hydroxyapatite (Ca5(OH)(PO4)3). In some solutions OCP transforms into hydroxyapatite after 1 or more days, but brushite has not been observed to transform into OCP. Assuming the extracellular bone fluid has [Ca2+] = 1.0 mM, 300 mosM, and pH 7.4, this work suggests that hydroxyapatite crystals would only form when the ambient [Pi] greater than 5 mM, and that octacalcium phosphate may be a precursor.