Poorly crystalline apatites: evolution and maturation in vitro and in vivo

Poorly crystalline apatites (PCA) are the major mineral component of mineralized tissues in vertebrates. Their physical-chemical properties are, however, not very well known due to their relative instability and the difficulties to characterize nanocrystalline compounds. Several studies using spectroscopic techniques (Fourier transform infrared [FTIR]; 31P nuclear magnetic resonance [NMR]) have demonstrated the existence, both in precipitated and biological PCA, of labile non-apatitic environments of the mineral ions. These environments are involved in the high surface reactivity and evolution ability of PCA and they are believed to form a hydrated layer at the surface of the nanocrystals in aqueous media. The extent of the hydrated layer may vary considerably depending on the conditions of precipitation and maturation time. As PCA age, the decrease of the non-apatitic environments proportion is associated with a decrease of intracrystalline disorder and an increase of stable apatitic domains. For synthetic and biological apatites, the carbonation rate of the mineral and the uptake of essential or toxic trace elements can be related to the maturation processes. The mineral ions of the hydrated layer can be easily and reversibly substituted by other ions which can either be included in the growing stable apatite lattice during maturation or remain in the hydrated layer. In addition, the non-apatitic environments seem to be involved in the binding of soluble non-collagenic proteins. This phenomenon could be related to calcium phosphate formation; we showed that, at an albumin concentration close to that in human serum, this protein has an inhibitory effect on octacalcium phosphate crystallization on collagen in vitro.