Nanocrystal Compressive Residual Stresses: A Strategy to Strengthen the Bony Spines of Osteocytic and Anosteocytic Fish

Bone is a living tissue in which communicating cells, osteocytes, are assumed to be vital for tissue turnover and adaptation. Interestingly however, most advanced teleost fish do not possess osteocyte-mediated porosity, prompting intriguing questions about alternative material-strategies for these bones to cope with damage. Using advanced imaging techniques, including phase-contrast enhanced (PCE) microtomography (µCT) and nanotomography (nanoCT), X-ray fluorescence (XRF), and diffraction (XRD) tomography, the micro- and nano-architectures of osteocytic zebrafish are compared with anosteocytic medaka fishbone. PCE µCT and Zernike phase-contrast nanoCT showed a lack of porosity in medaka bone and 0.75 – 2.26% osteocytic porosity in zebrafish. Both fish species have similar mineralized collagen fibril arrangements containing calcium (Ca) and traces of strontium (Sr) with increased zinc (Zn) localized on the outer bone regions. Medaka bones exhibit reduced apatite nanocrystal lattice spacings on the outer surfaces. Indeed we find higher compressive residual strains (-0.100 ± 0.02) compared to zebrafish (-0.071 ± 0.03). We propose that medaka bone evolved to replace the mechanosensitive osteocytic network by entrapping protective residual strains between collagen nanofibers and mineral crystals. These strains may enhance fracture toughness while making this nanocomposite well-suited for sustaining repeated loading cycles, thus reducing the metabolic costs associated with housing a large network of cells.