Percolation networks inside 3D model of the mineralized collagen fibril

Bone is a hierarchical biological material, characterized at the nanoscale by a recurring structure mainly composed of apatite mineral and collagen, i.e. the mineralized collagen fibril (MCF). Although the architecture of the MCF was extensively investigated by experimental and computational studies, it still represents a topic of debate. In this work, we developed a 3D continuum model of the mineral phase in the framework of percolation theory, that describes the transition from isolated to spanning cluster of connected platelets. Using Monte Carlo technique, we computed overall 120 × 106 iterations and investigated the formation of spanning networks of apatite minerals. We computed the percolation probability for different mineral volume fractions characteristic of human bone tissue. The findings highlight that the percolation threshold occurs at lower volume fractions for spanning clusters in the width direction with respect to the critical mineral volume fractions that characterize the percolation transition in the thickness and length directions. The formation of spanning clusters of minerals represents a condition of instability for the MCF, as it could be the onset of a high susceptibility to fracture. The 3D computational model developed in this study provides new, complementary insights to the experimental investigations concerning human MCF.

Intermolecular channels direct crystal orientation in mineralized collagen

The mineralized collagen fibril is the basic building block of bone, and is commonly pictured as a parallel array of ultrathin carbonated hydroxyapatite (HAp) platelets distributed throughout the collagen. This orientation is often attributed to an epitaxial relationship between the HAp and collagen molecules inside 2D voids within the fibril. Although recent studies have questioned this model, the structural relationship between the collagen matrix and HAp, and the mechanisms by which collagen directs mineralization remain unclear. Here, we use XRD to reveal that the voids in the collagen are in fact cylindrical pores with diameters of ~2 nm, while electron microscopy shows that the HAp crystals in bone are only uniaxially oriented with respect to the collagen. From in vitro mineralization studies with HAp, CaCO3 and γ-FeOOH we conclude that confinement within these pores, together with the anisotropic growth of HAp, dictates the orientation of HAp crystals within the collagen fibril.

Intermolecular Channels Direct Crystal Orientation in Mineralized Collagen

ABSTRACTThe mineralized collagen fibril is the basic building block of bone, commonly pictured as a parallel array of ultrathin carbonated hydroxyapatite (HAp) platelets distributed throughout the collagen. This orientation is often attributed to an epitaxial relationship between the HAp and collagen molecules inside 2D voids within the fibril. Although recent studies have questioned this model, the structural relationship between the collagen matrix and HAp, and the mechanisms by which collagen directs mineralization remain unclear. Here, we use XRD to reveal that the voids in the collagen are in fact cylindrical pores with diameters of ∼2 nm, while electron microscopy shows that the HAp crystals in bone are only uniaxially oriented with respect to the collagen. From in vitro mineralization studies with HAp, CaCO3 and γ-FeOOH we conclude that confinement within these pores, together with the anisotropic growth of HAp, dictates the orientation of HAp crystals within the collagen fibril.

Visualization of Collagen–Mineral Arrangement using Atom Probe Tomography

Bone is a complex, hierarchical structure comprised of two distinct phases: the organic, collagen– rich phase and the inorganic mineral–rich phase. This collagen–mineral arrangement has implications for bone function, aging, and disease. However, strategies to extract a single mineralized collagen fibril to investigate the interplay between components with sufficient resolution have been mostly confined to in vitro systems that only approximate the biological environment or transmission electron microscopy studies with lower spatial and chemical resolution. Therefore, there is extensive debate over the location of mineral with respect to collagen in in vivo mineralized tissues as visualization and quantification of the mineral in a living system is difficult or impossible. Herein, we have developed an approach to artificially extract a single mineralized collagen fibril from bone to analyze its composition and structure atom-by-atom with 3D resolution and sub-nanometer accuracy using atom probe tomography. This enables, for the first time, a method to probe fibril-level mineralization and collagen–mineral arrangement from an in vivo system with both the spatial and chemical precision required to comment on collagen– mineral arrangement. Using atom probe tomography, 4D (spatial + chemical) reconstructed volumes of leporine bone were generated with accuracy from correlative scanning electron microscopy. Distinct, winding collagen fibrils were identified with mineralized deposits both encapsulating and incorporated into the collagenous structures. This work demonstrates a novel fibril-level detection method that can be used to probe structural and chemical changes of bone and contribute new insights to the debate on collagen–mineral arrangement in mineralized tissues such as bones, and teeth.