Biomineralization of collagenous tissues, like bone and dentin, is a complex process which involves the secretion, assembly and organization of matrix molecules which predominantly suppress mineral formation. Only after the processing and modification of collagen and non-collagenous proteins (NCPs) mineral deposition will occur and is, under healthy conditions, highly specific to nucleation site, crystal size and orientation with regards to collagen fibril direction. Incorrect fibril assembly or deficient expression of specific NCPs can severely affect biological mineralization. A model suggesting NCPs predominantly act as nucleators in mineralizing collagenous matrices. Recently a number of studies have shown the ability of polymeric systems to promote mineralization of individual collagen fibrils and matrices leading to the hypothesis that in this in-vitro system charged residues stabilize an amorphous precursor of apatite in solution and facilitate the release of mineralizing ions at site-specific locations on collagen Ⅰ fibrils, transfuse into the entire volume of the fibrils, to subsequently transform into apatite nanocrystals with their c-axes oriented perpendicular to the fibril long axis. Hence biological mineralization as it occurs in-vivo may follow different mechanisms as observed during mineralization of biological tissues in-vitro. This paper discusses the origin of such differences and presents further evidence on the importance of the type of collagen for facilitating intrafibrillar mineralization.
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