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Abstract

The formation of the hard tissues that provide support and mobility to organisms is achieved through the interplay of inorganic crystals and an organic framework composed of collagen and a small percentage of non-collagenous proteins. Despite their clinical relevance, the mechanisms governing mineralization of the extracellular matrix are still poorly understood. By using 3D electron tomography and high-resolution electron microscopy imaging and spectroscopy, it has been demonstrated that mineralization proceeds through a spherulitic-like crystal growth process. First, aggregates of disordered crystals form in the interfibrillar spaces, which lead to the mineralization of adjacent fibrils. Mineral propagates steadily through the inter- and intrafibrillar spaces of the collagen structure forming layered spherulites that grow to confluence. The structure of the collagen fibrils serves as a protein scaffold to guide the formation of a myriad of platelet-shaped crystallites that make up each of these spherulites. At their periphery, nanosized unmineralized areas remain, leading to the formation of the characteristic lacy pattern observed in the transversal cross-section of mature calcified tissues. This study provides fundamental insights into the bone formation process and represents a potential strategy for complex materials design.