Characterization of the trunk neural crest in the bamboo shark, Chiloscyllium punctatum.

The neural crest is a population of mesenchymal cells that after migrating from the neural tube gives rise to structure and cell types: the jaw, part of the peripheral ganglia, and melanocytes. Although much is known about neural crest development in jawed vertebrates, a clear picture of trunk neural crest development for elasmobranchs is yet to be developed. Here we present a detailed study of trunk neural crest development in the bamboo shark, Chiloscyllium punctatum. Vital labeling with dioctadecyl tetramethylindocarbocyanine perchlorate (DiI) and in situ hybridization using cloned Sox8 and Sox9 probes demonstrated that trunk neural crest cells follow a pattern similar to the migratory paths already described in zebrafish and amphibians. We found shark trunk neural crest along the rostral side of the somites, the ventromedial pathway, the branchial arches, the gut, the sensory ganglia, and the nerves. Interestingly, C. punctatum Sox8 and Sox9 sequences aligned with vertebrate SoxE genes, but appeared to be more ancient than the corresponding vertebrate paralogs. The expression of these two SoxE genes in trunk neural crest cells, especially Sox9, matched the Sox10 migratory patterns observed in teleosts. Also of interest, we observed DiI cells and Sox9 labeling along the lateral line, suggesting that in C. punctatum, glial cells in the lateral line are likely of neural crest origin. Although this has been observed in other vertebrates, we are the first to show that the pattern is present in cartilaginous fishes. These findings demonstrate that trunk neural crest cell development in C. punctatum follows the same highly conserved migratory pattern observed in jawed vertebrates.

Embryonic development of glial cells and myelin in the shark, Chiloscyllium punctatum.

Glial cells are responsible for a wide range of functions in the nervous system of vertebrates. The myelinated nervous systems of extant elasmobranchs have the longest independent history of all gnathostomes. Much is known about the development of glia in other jawed vertebrates, but research in elasmobranchs is just beginning to reveal the mechanisms guiding neurodevelopment. This study examines the development of glial cells in the bamboo shark, Chiloscyllium punctatum, by identifying the expression pattern of several classic glial and myelin proteins. We show for the first time that glial development in the bamboo shark (C. punctamum) embryo follows closely the one observed in other vertebrates and that neural development seems to proceed at a faster rate in the PNS than in the CNS. In addition, we observed more myelinated tracts in the PNS than in the CNS, and as early as stage 32, suggesting that the ontogeny of myelin in sharks is closer to osteichthyans than agnathans.