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

Unravelling the processes of calcium phosphate formation is important in our understanding of both bone and tooth formation, and also of pathological mineralization, for example in cardiovascular disease. Serum is a metastable solution from which calcium phosphate precipitates in the presence of calcifiable templates such as collagen, elastin and cell debris. 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 phase. ACP formation is thought to proceed through prenucleation clusters--stable clusters that are present in solution already before nucleation--as was recently demonstrated for CaCO(3) (refs 15,16). However, the role of such nanometre-sized clusters as building blocks for ACP has been debated for many years. Here we demonstrate that the surface-induced formation of apatite from simulated body fluid starts with the aggregation of prenucleation clusters leading to the nucleation of ACP before the development of oriented apatite crystals.

The development of morphology and structure in hexagonal vaterite.

Inspired by the remarkable shapes and properties of CaCO(3) biominerals, many studies have investigated biomimetic routes aiming at synthetic equivalents with similar morphological and structural complexity. Control over the morphology of CaCO(3) crystals has been demonstrated, among other methods, by the use of additives that selectively allow the development of specific crystal faces, while inhibiting others. Both for biogenic and biomimetic CaCO(3), the crystalline state is often preceded by an amorphous precursor phase, but still limited information is available on the details of the amorphous-to-crystalline transition. By using a combination of cryoTEM techniques (bright field imaging, cryo-tomography, low dose electron diffraction and cryo-darkfield imaging), we show for the first time the details of this transition during the formation of hexagonal vaterite crystals grown in the presence of NH(4)(+) ions. The formation of hexagonal plate-like vaterite occurs via an amorphous precursor phase. This amorphous phase converts into the crystalline state through a solid state transformation in which order and morphology develop simultaneously. The mineral initially develops as polycrystalline vaterite which transforms into a single crystal directed by an NH(4)(+)-induced crystal plane that acts as a templating surface.