Genetic analysis of zebrafish head skeletal development

The vertebrate skeleton consists of variously shaped cartilages and bones separated by joints. How, during embryonic development, are different skeletal elements programmed to form their specific shapes? How are joints positioned within the skeleton? To address these questions, I focus on the segmental pharyngeal head skeleton and use a genetic approach in the zebrafish, Danio rerio. This genetic analysis revealed that at a surprisingly early stage in the pharyngeal arch primordia, long before cartilage formation, a prepattern is set up, reflected by the expression of developmental regulatory genes. Patterning along each of the three embryonic axes (dorsal/ventral or D/V, anterior/posterior or A/P, and medial/lateral or M/L) is under largely separable genetic control. Along the D/V axis, a localized Sucker/Endothelin-1 (Suc/Et-1) signal specifies joint and ventral fates by positively regulating the expression of two genes, bapx1 and dHAND. bapx1, which encodes a homeobox transcription factor, is expressed in the presumptive first arch joint region. An antisense reduction-of-function method demonstrates that bapx1 is required for the first arch joint and expression of gdf5 and chd in the developing first arch joint. dHAND , which encodes a bHLH transcription factor, is required for ventral cartilage and positions the joint by repressing joint fates, revealing a novel mechanism for the positioning of joints. Along the A/P axis, bimandibular (bmd ) specifies arch 2 identity by positively regulating hoxa2 and hoxb2 expression and negatively regulating bapx1 expression in the second arch. Along the M/L axis, cyclops/nodal signaling specifies the anterior neurocranium and is required for midline patterning in the first arch. Collectively these results elaborate a complex genetic network controlling the development of the vertebrate pharyngeal cartilages and the joints which separate them. This dissertation includes both my previously published and unpublished co-authored materials.