Bioapatite in shark centra studied by wide-angle and by small-angle X-ray scattering

A Nanoscale View of the Structure and Deformation Mechanism of Mineralized Shark Vertebral Cartilage

Swimming kinematics and macroscale mechanical testing have shown that the vertebral column of sharks acts as a biological spring, storing and releasing energy during locomotion. Using synchrotron X-ray nanotomography and deep-learning image segmentation, we studied the ultrastructure and deformation mechanism of mineralized shark vertebrae from Carcharhinus limbatus (Blacktip shark). The vertebral centrum con regions: the corpus calcareum, a hypermineralized double cone, and the intermediale, blocks of mineralized cartilage interspersed by unmineralized arches. At the micron scale, mineralized cartilage has previously been described as a 3D network of interconnected mineral plates that vary in thickness and spacing. The corpus calcareum consists of stacked, interconnected, curved mineralized planes permeated by a network of organic occlusions. The mineral network in the intermedialia resembles trabecular bone, including thicker struts in the direction opposite to the predominant biological strain. We characterized collagenous fiber elements winding around lacunar spaces in the intermedialia, and we hypothesize the swirling arrangement and elasticity of the fibers to be distributing stress. With little permanent deformation detected in mineralized structures, it is likely that the soft organic matrix is crucial for absorbing energy through deformation, irreversible damage, and viscoelastic behavior. In the corpus calcareum, cracks typically terminate toward thick struts along the mineral planes, resembling the microscale crack deflection and arrest mechanism found in other staggered biocomposites, such as nacre or bone. Using transmission electron microscopy (TEM), we observed preferentially oriented, needlelike bioapatite crystallites and d-band patterns of collagen type-II fibrils resulting from intrafibrillar mineralization.

Revealing chemistry-structure-function relationships in shark vertebrae across length scales

Shark cartilage presents a complex material composed of collagen, proteoglycans, and bioapatite. In the present study, we explored the link between microstructure, chemical composition, and biomechanical function of shark vertebral cartilage using Polarized Light Microscopy (PLM), Atomic Force Microscopy (AFM), Confocal Raman Microspectroscopy, and Nanoindentation. Our investigation focused on vertebrae from Blacktip and Shortfin Mako sharks. As typical representatives of the orders Carcharhiniformes and Lamniformes, these species differ in preferred habitat, ecological role, and swimming style. We observed structural variations in mineral organization and collagen fiber arrangement using PLM and AFM. In both sharks, the highly calcified corpus calcarea shows a ridged morphology, while a chain-like network is present in the less mineralized intermedialia. Raman spectromicroscopy demonstrates a relative increase of glucosaminocycans (GAGs) with respect to collagen and a decrease in mineral-rich zones, underlining the role of GAGs in modulating bioapatite mineralization. Region-specific testing confirmed that intravertebral variations in mineral content and arrangement result in distinct nanomechanical properties. Local Young's moduli from mineralized regions exceeded bulk values by a factor of 10. Overall, this work provides profound insights into a flexible yet strong biocomposite, which is crucial for the extraordinary speed of cartilaginous fish in the worlds' oceans. STATEMENT OF SIGNIFICANCE: Shark cartilage is a morphologically complex material composed of collagen, sulfated proteoglycans, and calcium phosphate minerals. This study explores the link between microstructure, chemical composition, and biological mechanical function of shark vertebral cartilage at the micro- and nanometer scale in typical Carcharhiniform and Lamniform shark species, which represent different vertebral mineralization morphologies, swimming styles and speeds. By studying the intricacies of shark vertebrae, we hope to lay the foundation for biomimetic composite materials that harness lamellar reinforcement and tailored stiffness gradients, capable of dynamic and localized adjustments during movement.

Micrometer-scale structure in shark vertebral centra

The vertebral centra of sharks consist of cartilage, and many species' centra contain a bioapatite related to that in bone. Centra microarchitectures at the 0.5-50 µm scale do not appear to have been described previously. This study examines centrum microarchitecture in lamniform and carcharhiniform sharks with synchrotron microComputed Tomography (microCT), scanning electron microscopy and spectroscopy and light microscopy. The analysis centers on the blue shark (carcharhiniform) and shortfin mako (lamniform), species studied with all three modalities. Synchrotron microCT results from seven other species complete the report. The main centrum structures, the corpus calcareum and intermedialia, consist of fine, closely-spaced, mineralized trabeculae whose mean thicknesses and spacings range from 4.5 to 11.2 µm and 4.5 to 15.6 µm, respectively. A significant (p = 0.00001) positive linear relationship between and exists for multiple positions within one mako centrum. Carcharhiniform species' and exhibit an inverse linear relationship (p = 0.005) while in lamniforms these variables tend toward a positive relationship which does not reach statistical significance (p = 0.099). In all species, the trabeculae form an uninterrupted, interconnected network, and the unmineralized volumes are similarly interconnected. Small differences in mineralization level are observed in trabeculae. Centrum growth band pairs are found to consist of locally higher /lower mineral volume fraction. Within the intermedialia, radial canals and radial microrods were characterized, and compacted trabeculae are prominent in the mako intermedialia. The centra's mineralized central zones were non-trabecular and are also described. STATEMENT OF SIGNIFICANCE: This study's novel result is the demonstration that the mineralized cartilage of sharks' vertebral bodies (centra) consists of a fine 3D array of interconnected plates (trabeculae) and an interpenetrating network of unmineralized tissue. This microstructure is radically different from that in tesserae or in teeth, the other main mineralized shark tissues. Using volumetric synchrotron microComputed Tomography, numerical values of mean trabecular thickness and spacing and their relationship were measured for nine species. Scanning electron microscopy added a higher resolution view of the microstructures, and histology provided complementary information on cartilage and cells. The present results suggest centra microstructure helps accommodate the very large in vivo strains and may prevent damage accumulation during millions of cycles of swimming-induced loading.

Three-dimensional mapping of mineral in intact shark centra with energy dispersive x-ray diffraction

The centra of shark vertebrae consist of cartilage mineralized by a bioapatite similar to bone's carbonated hydroxyapatite, and, without a repair mechanism analogous to remodeling in bone, these structures still survive millions of cycles of high-strain loading. The main structures of the centrum are an hourglass-shaped double cone and the intermedialia which supports the cones. Little is known about the nanostructure of shark centra, specifically the relationship between bioapatite and cartilage fibers, and this study uses energy dispersive diffraction (EDD) with polychromatic synchrotron x-radiation to study the spatial organization of the mineral phase and its crystallographic texture. The unique energy-sensitive detector array at beamline 6-BM-B, the Advanced Photon Source, enables EDD to quantify the texture within each sampling volume with one exposure while constructing 3D maps via specimen translation across the sampling volume. This study maps a centrum from two shark orders, a carcharhiniform and a lamniform, with different intermedialia structures. In the blue shark (Prionace glauca, Carcharhiniformes), the bioapatite's c-axes are oriented laterally within the centrum's cone walls but axially within the wide wedges of the intermedialia; the former is interpreted to resist lateral deformation, the latter to support axial loads. In the shortfin mako (Isurus oxyrinchus, Lamniformes), there is some tendency for c-axis variation with position, but the situation is unclear because one dimension of the sampling volume is considerably larger than the thickness and spacing of the intermedialia's radially-oriented lamellae. Because elastic modulus in collagen plus bioapatite mineralized tissues varies significantly with both volume fraction of bioapatite and crystallographic texture, the present 3D EDD-derived maps should inform future 3D numerical models of shark centra under applied load.

Microstructure and energy dispersive diffraction reconstruction of 3D patterns of crystallographic texture in a shark centrum

Purpose: Tomography using diffracted x-rays produces reconstructions mapping quantities such as crystal lattice parameter(s), crystallite size, and crystallographic texture, information quite different from that obtained with absorption or phase contrast. Diffraction tomography is used to map an entire blue shark centrum with its double cone structure (corpora calcerea) and intermedialia (four wedges). Approach: Energy dispersive diffraction (EDD) and polychromatic synchrotron x-radiation at 6-BM-B, the Advanced Photon Source, were used. Different, properly oriented Bragg planes diffract different x-ray energies; these intensities are measured by one of ten energy-sensitive detectors. A pencil beam defines the irradiated volume, and a collimator before each energy-sensitive detector selects which portion of the irradiated column is sampled at any one time. Translating the specimen along X , Y , and Z axes produces a 3D map. Results: We report 3D maps of the integrated intensity of several bioapatite reflections from the mineralized cartilage centrum of a blue shark. The c axis reflection's integrated intensities and those of a reflection with no c axis component reveal that the cone wall's bioapatite is oriented with its c axes lateral, i.e., perpendicular to the backbone's axis, and that the wedges' bioapatite is oriented with its c axes axial. Absorption microcomputed tomography (laboratory and synchrotron) and x-ray excited x-ray fluorescence maps provide higher resolution views. Conclusion: The bioapatite in the cone walls and wedges is oriented to resist lateral and axial deflections, respectively. Mineralized tissue samples can be mapped in 3D with EDD tomography and subsequently studied by destructive methods.