Exploring the Organization of Uncalcified Vertebral Shark Cartilage

Elasmobranch vertebral cartilage is a sophisticated hierarchical composite, integrating a collagenous organic framework with bioapatite mineral, proteoglycans, and a complex suite of non-collagenous proteins. While the mineralized skeletal tissues have been characterized, the structural motifs and biomechanical contributions of the unmineralized regions remain poorly understood. This study investigates the micro- and nanoscale architecture of unmineralized nurse shark (Ginglymostoma cirratum) cartilage to infer the structure-property relationships governing its biological function. By combining synchrotron nearfield X-ray holotomography (nanoCT), synchrotron X-ray fluorescence (XRF) microscopy, transmission electron microscopy (TEM), and polarized light microscopy (PLM), we develop an in-depth understanding of the soft-hard tissue interfaces and 3D collagenous organization. The unmineralized phase consists of a dense extracellular matrix (ECM), distinct collagen structures, and graded organic-inorganic interfaces. NanoCT imaging revealed a complex network of cartilage channels and plate-like collagenous lamellae. Quantitative analysis by AI-assisted machine learning segmentation identified differences in fiber directionality, exhibiting both collinear and perpendicular orientations relative to the mineralized front. PLM intensity profiles corroborated optical anisotropy across extended regions. Ultrastructural analysis via TEM confirmed highly oriented type-II collagen fibers with distinct d-banding periodicity, supporting the structural motifs observed in 3D nanoCT volumes. Elemental XRF mappings identified a heterogeneous distribution of mineral species within the ECM and collagenous domains, highlighting localized intensity gradients that define the transition between phases. This structural hierarchy, characterized by tailored stiffness gradients and lamellar reinforcement, suggests an evolutionary optimization for mechanical load distribution, serving as a blueprint for the design of advanced, bio-inspired composites and biomimetic joint replacement materials.