Structure and formation mechanism of calcium phosphate concretions formed in simulated body fluid

Synthetic calcium phosphate (CaP) concretions in the form of hemispheres formed under carefully controlled conditions (pH 7.4, 37 °C, no evaporation) from stagnant simulated body fluid on anionic polymeric substrate (laminin) were observed by scanning and transmission electron microscopy and atomic force microscopy. The hemispheres with diameter between approximately 50 and 200 μm were composed of closely connected round spherical and elliptical objects of diameter varying from 70 to 120 nm with surface layer composed of tightly packed spherical objects of diameter 25-30 nm. The phase composition of concretions consisting of amorphous material was uniform. The concretions were formed by aggregation of CaP clusters (Posner's clusters Ca9(PO4)3 or [Ca3(PO4)2] n ) generated in the solution by perikinetic coagulation, their settling onto a substrate and subsequent accumulation through surface migration (surface nucleation) followed by accretion of nanoparticles arriving from surrounding solution. The similarity of ultrafine structures of these synthetic CaP concretions and the compact phosphatic phase appearing in some phosphate calculi indicates that analogous mechanisms could be active at the formation of the latter in the kidneys.