ELECTRON MICROSCOPY OF ION MILLED SECTIONS REVEALS SINGLE CRYSTAL HA FIBERS IN BONE

The form of the crystals in mineralized bone has been studied for decades without resolution. Hydroxyapatite (HA) is widely believed to occur as thin, plates although X-ray scattering studies suggest needle-like forms. Direct imaging by transmission electron microscopy (TEM) in fully mineralized bones is impaired by the difficulty of sectioning this hard material. Ion milling is well-suited to this task, and allows us to visualize the intact, undistorted structure at the nanometer scale. All bone (cortical, trabecular) displays the 67 nm repeat of collagen fibrils, with HA along gap zones. In addition, we see dense sheaves of 5 - 9 nm thick fibers, constructed of single crystals in segments 10's of nm long, oriented parallel to the collagen axes. The HA fibers may have hollow cores, suggesting nucleation around collagen mini-fibrils. Rotation of samples in TEM confirms that these are not edge-on views of plates. Non-dispersive XRF analyses of fibers gives HA stoichiometry. The fiber + fibril pattern is seen in bones of mice, humans, cows and 10 ka-old mammoth.