ULTRASTRUCTURE AND MINERALOGY OF MODERN AND FOSSIL BONE

Bone and dentin are composite materials in which hydroxyapatite (HA) is intergrown with protein, principally collagen, as well as other non-collagenous proteins. The collagen occurs in fibrils, 50 nm diameter bundles of triple-helix molecules. Conventionally, HA is believed to occur inside the fibrils. Using transmission electron microscopy (TEM) of ion-milled sections of cortical bone, we have shown that the HA is mostly external to the fibrils and in the form of plates 5 nm thick elongated parallel to the axis of the fibril. The HA within these plates (“mineral structures”) appears to be in the form of flakes <1 nm thick and 10’s of nm across. This structural complex is observed over a wide range of vertebrate taxa including fish. During diagenesis, the mineral structures are conserved although the internal flakes may grow larger. Using Fourier transform infrared spectroscopy (FTIR) we observe changes in diagenetic changes in atomic coordination in HA, and also suggestions that H in primary bone is mainly present in hydrogen phosphate (HPO₄) rather than OH.