Nucleation and growth of calcium phosphate crystals in fibrillar collagen structures are important processes for the formation of mammalian mineralized tissues. In collagen, mineralization occurs both at the nanoscale, in intrafibrillar spaces, and at the microscale, in extrafibrillar spaces. However, the differences in the initial stages of mineralization between the two sites have not been fully explored because of the complex fibrillar structure and the lack of in situ analytical tools. Utilizing in situ small/wide angle X-ray scattering measurements, we studied the nucleation, growth, and transformation of calcium phosphate minerals within fibrillar collagen matrices. A series of ex situ complementary techniques, including electron microscopies and Raman spectroscopy, was also used to evaluate morphological changes of minerals after nucleation in simulated body fluid. Nucleation sites, whether in intra- or extrafibrillar space, were governed by acidic polymeric molecules such as polyaspartic acid. These molecules acted as nucleation inhibitors in the extrafibrillar space, but they enhanced intrafibrillar mineralization. We also found that nucleation pathways and kinetics were significantly different depending on the nucleation sites. In extrafibrillar mineralization, the formation of amorphous calcium phosphate minerals via aggregation of prenucleation clusters is an important step; however, they did not form in the nanoscale intrafibrillar space. Consequently, crystallization to a hydroxyapatite-like phase was faster during intrafibrillar mineralization than extrafibrillar mineralization.

Nucleation, growth, and transformation of calcium phosphate minerals in multi-scale nucleation sites of collagen fibrils

253rd National Meeting (2017)

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