Transient OCP-apatite epitaxy controls bone mineralization. An X-ray Total Scattering Study

Despite the biological relevance and the unique mechanical properties of bone tissue, the pathway leading to the formation of the main mineral component – specifically, the mechanism responsible for the formation of extremely thin apatite nanoplatelets – remains unclear. In this work, the presence and evolution of an elusive octacalcium phosphate / apatite (OCP/Ap) heterostructure is identified through the combination of small- and wide-angle X-ray total scattering experiments. These experiments were performed in situ during collagen mineralization, and the structure, morphology, and epitaxy of these nanocrystalline composites are described using atomistic models. The epitaxial growth model proposed by Xin and coworkers in 2006 is validated by experimentally supported structural and morphological considerations, whereas others accepted models are analyzed and dismissed. By applying the Debye Scattering Equation to our high-quality synchrotron X-ray scattering datasets, a quantitative estimate of the size and shape of the nanoplates, as well as their temporal evolution, is achieved. The transformation of this transient heterostructure is revealed through confocal Raman microscopy to occur within the intrafibrillar spaces of the collagen fibers. As mineralization progresses, these spaces gradually cluster, resulting in a ca. 60% increase in the lateral density of the purely organic molecular bundles. In contrast, in dry collagen, the mineralization process induces a (marginal) decrease in bundles density. Despite their apparent opposite, we present an explanation that reconciles both effects.