Meeting Abstract
Although cartilage is typically described as avascular, this is not always true. In developing mammal/bird skeletons, particularly regions of endochondral ossification, hyaline cartilage is invested by a dense network of tubules called cartilage canals. These canals carry vasculature and undifferentiated mesenchymal cells, are lined by Type I collagen, nourish cartilage, and develop ossification centers. The canals and their vascular network are typically obliterated as animals age. We use a range of tissue characterization/visualization techniques to show that cartilaginous fishes (rays and relatives) possess cartilage canals which persist throughout life. Elasmobranch skeletons are comprised largely of a hyaline-like cartilage sheathed in mineralized geometric tiles (tesserae). Cartilage canals were observed in species from disparate groups, starting in the outer fibrous perichondrium, perforating the tesserae layer in large circular pores, and penetrating the uncalcified cartilage. As in other vertebrates, canals carried vasculature, were either unbranched or bifurcated blunt tubules, but never extended completely through skeletal elements. We demonstrate that Type I collagen lines canals, the same lining canals and forming the perichondrium in mammals/birds. Fluorescence microscopy of DAPI-stained samples suggests canals contain polyphosphates, stable apatite nucleation precursors for controlled distribution to mineralization sites. The morphology and tissue composition of elasmobranch cartilage canals argues for homology with mammal/bird canals and an ancient invasion of bone-like collagen (Type I) into cartilage (Type II collagen). However, anatomical location —heading from mineralized tissue not toward it— and the lack of endochondral ossification in elasmobranch cartilage points to alternative roles for these canals in more basal vertebrate skeletal types.
