The distribution of fibronectin in developing zebrafish (Danio rerio) cartilage

The extracellular matrix (ECM) plays a complex and vital role throughout the process of cartilage formation. Fibronectin is a large ECM glycoprotein with an important role in various developmental processes, including skeletogenesis. Taking advantage of the known sequence of cartilage development in zebrafish and using an immunohistochemical stain for collagen type II to identify differentiation phase cartilage, we evaluate the distribution of fibronectin in various cartilaginous elements of the zebrafish (elements of the splanchnocranium, and of the dorsal, caudal, pelvic and pectoral fins). Contrary to what is observed in tetrapods, our data on zebrafish indicate the apparent lack of fibronectin during the condensation phase of cartilage development. This lack is possibly linked to the high developmental rate of the zebrafish and the small size of the condensations, which brings different needs for the extracellular environment to ensure cell survival. Furthermore, the fin disk cartilage of the pectoral fin develops an ECM with a strong fibronectin signal, whereas other cartilage elements show only a weak fibronectin signal in early differentiation, which gradually disappears. Thus, the pectoral fin disk cartilage is unique not only because of its specific way of development (subdivision of a continuous plate into four elements, the proximal radials), but also because of its strong fibronectin-positive ECM.