Bone ultrastructure -: Collagen-reinforced mineral matrix or interpenetrating network of hydroxyapatite crystals and collagen molecules?

We develop and verify three different continuum-micromechanics representations of the collagen-mineral interaction in the elasticity of mineralized tissues: (i) mineral foam matrix with collagen inclusions [5], (ii) interpenetrating network of hydroxyapaptite crystals and collagen molecules, (iii) composite of fibrils (collagenhydroxyapatite network) embedded in a collagen-free extrafibrillar mineral foam matrix. The most advanced concept (iii), with the best predictive capabilities, integrates the two others into a consistent whole. There, the collagen is clearly represented as the governing element in inducing tissues' the anisotropy, by (i) the anisotropy of molecular collagen itself, (ii) the anisotropy of the fibrils, and (iii) the oriented morphology of the cylindrical fibrils in the isotropic extrafibrillar space.