Chondrichthyan cartilage: new discoveries in morphology and the composition of the extracellular matrix

Many chondrichthyans, or cartilaginous fishes with skeletons made entirely of cartilage and known for their regenerative abilities, inhabit high-hydrostatic-pressure environments. We hypothesise that basic research on chondrichthyan cartilage will fill knowledge gaps currently holding back progress in cartilage tissue engineering. Here, we qualitatively and quantitatively characterise the cellular and extracellular matrix of native cartilage in two species of chondrichthyans (a shark and a ray) using immunohistochemical markers (collagen-I, collagen-II, Ki67, elastin, Sox9, Gata1, Lyve-1, α-SMA and vWF), histological staining (Haematoxylin and Eosin, Safranin O, Masson’s Trichome and Picrosirius Red) and scanning electron microscopy. We reveal a network of canals akin to vascular capillaries, which may act as chondrogenic and hematopoietic centres mediating growth and regeneration. The implications of the existence of specialised capillaries within hyaline cartilage are far-reaching, and uncovering their specific roles may fill knowledge gaps about the mechanisms underlying adult chondrogenesis.