Shark and ray cartilage canals show blood vessels can exist in mature cartilage without endochondral bone formation

Cartilage in adult tetrapod skeletons is known to lack blood vessels. In young birds/mammals, however, a dense network of vasculature-containing tunnels—cartilage canals—perforates the growing skeleton, nourishing cartilage, and developing ossification centers for later bone. As the canals and their vascular network regress as the animals age, healthy cartilage in adults is typically regarded as avascular. We use diverse tissue characterization and visualization (e.g., light/electron microscopy, microCT) to show skeletons of adult rays/sharks (elasmobranchs) possess persistent cartilage canals. Elasmobranch canal morphology and tissue composition support a structural analogy with mammalian cartilage canals, suggesting that early jawed vertebrates had vasculature-invaded cartilage. However, the anatomical location of canals—extending away from, not toward, mineralized tissue—and the lack of endochondral ossification in ray/shark cartilage suggest functions divergent from amniotes. We describe distinctive features and variation in elasmobranch cartilage canals, discuss their potential for tissue mineralization, and implications for their presence in an animal clade with continuously growing cartilaginous skeletons.