The role of prenucleation clusters in surface-induced calcium phosphate crystallization.

The surface-directed mineralization of calcium phosphate from simulated body fluid is studied by cryogenic transmission electron microscopy. Prenucleation clusters aggregate close to the surface, then amorphous calcium phosphate forms in this region, leading to the nucleation of oriented apatite crystals at the surface. Unravelling the processes of calcium phosphate formation1,2,3,4 is important in our understanding of both bone and tooth formation5,6,7, and also of pathological mineralization, for example in cardiovascular disease8,9,10. Serum is a metastable solution from which calcium phosphate precipitates in the presence of calcifiable templates such as collagen, elastin and cell debris11,12. A pathological deficiency of inhibitors leads to the uncontrolled deposition of calcium phosphate. In bone and teeth the formation of apatite crystals is preceded by an amorphous calcium phosphate (ACP) precursor phase13,14. ACP formation is thought to proceed through prenucleation clusters—stable clusters that are present in solution already before nucleation—as was recently demonstrated for CaCO3 (refs 1516). However, the role of such nanometre-sized clusters as building blocks2 for ACP has been debated for many years. Here we demonstrate that the surface-induced formation of apatite from simulated body fluid17,18 starts with the aggregation of prenucleation clusters leading to the nucleation of ACP before the development of oriented apatite crystals.