HERNANDEZ, L.P.*; DEVOTO, S.H.: Distribution and development of cranial slow muscle fibers in zebrafish

The distribution of muscle fiber types within cranial musculature has great physiological implications for both feeding and breathing in fishes. While the distribution and development of slow muscle fibers in the trunk of zebrafish have been well described, cranial muscles have received much less attention. Moreover, ontogenetic changes in fiber type distribution within the head, which may be significantly affected by patterns of muscle firing, have not been investigated in zebrafish. First, we established a normal baseline by describing the relative proportion of fast and slow muscle fibers in the cranial muscles of three stages (72h, 96h, 120h) of wild type zebrafish. We then investigated the genetic mechanisms responsible for fiber type specification by examining a number of zebrafish mutants. Sonic hedgehog is necessary for the specification of embryonic slow fibers in zebrafish trunk muscles. To test whether hedgehog signaling from midline tissues also plays a significant role in cranial muscle fiber type specification, we have examined head development in midline and/or Shh-signaling mutants. We compared fiber type distribution in wild type to that of larvae mutant in syu (sonic hedgehog), yot (Gli2), and smu (smoothened). These mutants showed varying degrees of musculoskeletal anomalies with smu mutants showing the most pronounced aberrations. Moreover, a phenotype shared by these mutants was reduction or loss of pharyngeal arches, a phenotype also present in chicks deficient in Hh signaling.