Early stages of trunk muscles differentiation in small-spotted catshark (Scyliorhinus canicula): Ultrastructural studies.

We report here a study of early trunk muscle differentiation in the catshark S. canicula, a representative of Chondrichthyes. Light and TEM investigations revealed that during S. canicula somitogenesis, the metameric cell segments (somites) are formed and their cell arrangement resembles the shape of a rosette. The somite structure shares similarities with those observed in lungfish and urodeles. As embryogenesis proceeds, somites differentiate into three compartments: the dermomyotome (with dorsomedial and ventrolateral lips), myotome, and sclerotome. In the developing myotome two, morphologically different classes of cells are distinguished: elongated cells with homogenous nuclei and bottle-shaped cells with condensed nuclei. Based on morphological studies, we hypothesize that in S. canicula, the second class of cells observed in the early myotome can be classified as muscle progenitor cells. Myogenesis in small-spotted catshark starts in the ventral part of the somite, as observed for the development of trunk muscles in teleosts. In summary, the structure of somites in S. canicula shares similarities with lungfish and urodeles, which could reflect plesiomorphic features of small-spotted catshark myogenesis. On the other hand, the myogenesis appear to be apomorphic feature. Many issues related to myogenesis in sharks, cartilaginous fish representatives, remain unresolved. These data provide new insights into myogenesis in Chondrichthyes, the closest outgroup to Osteichthyes, and contribute to a better understanding of ancestral features of this process and its diversification across vertebrates.