The 82nd Annual Meeting of the American Association of Physical Anthropologists (2013)

The ultrastructure of bone: two new levels of hierarchy revealed by transmission electron microscopy (TEM)


1School of Geography and Earth Sciences, McMaster University, Hamilton ON, Canada, 2Candian Centre for Electron Microscopy, McMaster University, Hamilton ON, Canada, 3Candian Centre for Electron Microscopy, McMaster University, Hamilton ON, Canada, 4Institut de Paleontologie Humaine, Musee National d'Histoire Naurelle, Paris, France

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Bone is widely recognized to be a hierarchical structure, ranging in scale from macroscopic anatomic features (epiphyses, trabeculae, etc) through microscopic (osteons, lamellae,osteocytes, etc.) to nanometer-scale features dominated by collagen fibrils and associated mineral phases, visualized using transmission electron microscopy. The mineral component, which makes up approximately 60 wt% of bone, is generally believed to be hydroxyapatite, and is believed to be found principally inside the fibrils, in gaps between collinear collagen molecules. Using TEM images of ion-milled sections of bone, we have shown that about 80% of the mineral occurs outside the fibrils in the form of polycrystalline "mineral structures" 100's of nanometers (nm) long, and 6 nm thick. These are stacked around the fibrils and form a self-supporting network inside all bone. Using dark-field imaging we show that the component crystals of the mineral structures are themselves up to 6 nm thick, filling out the entire thickness of the mineral structures, and averaging 28 ± 19 nm long in the plane of the mineral structures. Some appear to be composite, homoaxial crystals meeting along planes parallel to the thinnest dimension of the mineral structures; such 2-3 nm thick crystals are consistent with low-angle X-ray scattering data. The mineral itself is an OH-free apatite which we show to contain hydrogen phosphate (HPO4 ) and carbonate ( CO3) ions, but which converts to hydroxyapatite when heated to 600 ºC.

Funding: Natural Sciences and Engineering Research Council of Canada

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