Meeting Abstract

21.2  Tuesday, Jan. 4  Molecular and morphological development of the shark head cavity ADACHI, Noritaka*; KURATANI, Shigeru; RIKEN CDB Kobe; RIKEN CDB Kobe adasaurus@cdb.riken.jp

The vertebrate head is considered as an evolutionary novelty. However, the head mesoderm is regarded a derivative of anterior somites in the ancestral chordate. Although, our head mesoderm consists of mesenchymal cells, not epithelial segments like somites, and despite there are lots of reports about different gene expression patterns between the head mesoderm and somites, this hypothesis is widely accepted nowadays. The reason is that the shark head mesoderm contains three pair of epithelial segments called “head cavities”, and there are definite morphological similarities between the head cavity and somites. Therefore, many comparative morphologists thought that the head cavity and somites were serial homologous. Is the head cavity the remainder of the somite that previously existed in the head? In order to address this question, the shark head cavity needs to be observed. Hence, we investigate whether the head cavity follows a somite like developmental process and analyze gene expression patterns, using cloudy dogfish, Scyliorhinus torazame. We found that the premandibular head cavity was not derived from paraxial head mesoderm, but from the prechordal plate, and the formation of this head cavity was later than the other two head cavities. The mandibular head cavity was recognized in the lateral sides of the endoderm, and the hyoid head cavity was seen in the dorsal part of it. On the other hand, all somites are originated from paraxial mesoderm from rostral to caudal. We also analyzed expression patterns of the muscle differentiation marker genes. We detected Pax3 expression in somites, but did not in head cavities, while, Pitx2 expression was seen in head cavities. These results suggest that head cavities are different from somites in terms of the developmental process and gene expression patterns.