The Material Composition of Bone – A Brief Overview of Old, New, and Emerging Concepts

Abstract

Bone is a complex biological system where the interplay between composition and structure governs the overall function. In addition to the constituents of the mineralised compartment (i.e., the extracellular matrix), the unmineralised compartment comprises vascular spaces, the osteocyte lacuno-canalicular network, and nanoporosities that contain water. Dehydration causes embrittlement. The mineralised collagen fibril is the basic unit of structure and function. Recent findings reveal that micrometre-sized marquise-shaped motifs bridge the length scales between individual mineralised collagen fibrils and the interwoven mesh-like architecture. With little evidence of hydroxyl (OH^-) groups, the bone mineral must be viewed as carbonated apatite with various ion substitutions such as CO₃^2-, Mg²⁺, and HPO₄^2-, rather than as hydroxy(l)apatite. This mineral is found both around the fibril (i.e., extra-fibrillar mineral) and within the fibril (i.e., intra-fibrillar mineral). Finally, the extra-fibrillar mineral takes the form of curved, polycrystalline lamellae that closely wrap around individual collagen fibrils in addition to uncurved (flat) lamellae.