URAIQAT , C.A.*; BIRD, N.C.; HERNANDEZ, L.P.; George Washington University; George Washington University; George Washington University: Pharyngeal arch development in midline and Hedgehog signaling mutants: Implications for morphogenesis of the visceral skeleton

Zebrafish have recently become an important model system in developmental biology. Although there are several important reasons behind using zebrafish, the availability of zebrafish mutants is a major factor. By examining the morphology of fish with mutations in known gene products we can investigate the role these genes play in development. Zebrafish mutants in the midline group display a number of abnormal phenotypes such as downward-curled tails, partial cyclopia, hanging lower jaws, fused braincases, and partial or irregular development of the floorplate. While abnormalities within the neurocranium of these mutants have been described, the pharyngeal arches have largely been ignored. To set a baseline for comparison we first examined growth within wild type (WT) zebrafish, assessing growth rates for all pharyngeal cartilages from 3-6 days post-fertilization (dpf). We then examined embryos with mutations in the sonic you (syu, encoding sonic hedgehog), you too (yot, encoding Gli2), detour (dtr, encoding Gli1) or slow muscle omitted (smu, encoding Smoothened) genes. We also examined iguana and chameleon, two other midline mutants that may encode genes within the Hedgehog pathway. We found slower development of pharyngeal arch cartilages when compared to WT larvae. Moreover, while many mutants fully recovered a full complement of pharyngeal cartilages by 6dpf, smu larvae never developed branchial cartilages and both the hyoid and mandibular cartilages were greatly reduced in size. In addition some mutants never recovered basihyal and basibranchial cartilages. These findings strongly suggest that the Hedgehog pathway plays an important role in growth and chondrogenesis within pharyngeal arches.