Bone mineralization.

This review attempts to summarize the findings made available by the literature on the mineralization of bone. The types of bone, their structures and compositions, the nature and organization of organic and inorganic matter, the organic-inorganic relationships, and the mineralization mechanism itself, are the main topics of the present review. As in other hard tissues, bone mineralization occurs in, and is conditioned by, the components of the organic matrix. Collagen fibrils have long been considered the factor that is able to induce the deposition of apatite crystallites through a process of heterogeneous nucleation. Interfibrillar non-collagenous proteins are now considered to be co-factors that permit crystallite deposition. The main components of these proteins are reviewed. It is hypothesized that two independent types of mineral are present in bone, one contained in the collagen fibrils and corresponding to the granular, electron-dense bands, and the other contained in the interfibrillar spaces and corresponding to needle- and filament-like crystals. The deposition mechanism of these mineral structures remains elusive. The formation of the crystallites through an epitaxial mechanism is discussed.