Stuart R.  Stock, Dept. of Cell & Developmental Biology, Feinberg School of Medicine, & Simpson Querrey Inst., Northwestern Univ., Chicago, IL, USA

Micro- and nano-scale structural quantification of shark mineralized cartilage

The skeletons of elasmobranchs, which include sharks and rays, consist of cartilage (type 2 collagen). Some elasmobranch skeletal cartilage is mineralized with a bioapatite related to hydroxyapatite, and the centra (i.e., vertebral bodies) of sharks possess remarkable resistance to large in vivo strains exceeding 4%. It appears that these centra’s fracture resistance is related to a hierarchy of structures, and this talk focuses on the bioapatite micro- and nano-organization of the tissue. Studying these very different size scales requires different techniques, especially considering that structural gradients in centra of shark orders Carcharhiniformes and Lamniformes can span tens of millimeters.

Results of lab microCT on entire centra provide a 3D quantitative picture of the macrostructure and of microstructure down to the 20 micrometer scale. In blocks cut from centra, synchrotron microCT reveals that the centra tissue consists of closely spaced, mineralized trabeculae; these 3D characteristics (trabecular thickness and spacing) have been quantified with volume element (voxels) sizes below one micrometer. Nanostructure plays an important role in functionality of mineralized tissue based on collagen-bioapatite composite material. High-energy, position-resolved diffraction and small angle scattering with monochromatic x-rays furnishes precise data on various quantities: from the former, bioapatite lattice parameters, nanoparticle dimensions and crystallographic texture; from the later, values of the collagen D-period and sharpness of texture.

Finally, energy dispersive diffraction mapping of entire centra reveals anatomically-related, 3D variation of crystallographic texture of the mineral phase; the first results on in situ compressive loading of centra are also covered briefly. Speculation on how these different components contribute to in vivo mechanical performance concludes the talk.