In vertebrates, cranial sensory ganglia, including trigeminal, epibranchial (EB), and lateral line (LL) ganglia, derive from two sources: neural crest and ectodermal placodes. The proper development of cranial nerves is essential for the formation of cranial sensory systems such as vision, smell, hearing, somatosensation, and taste. Despite their importance, very little is known about molecular mechanisms that govern the development of specific cranial placodes. Our preliminary data indicate that fibroblast growth factor (Fgf) signaling is required for the formation of EB and LL placodes. We will take advantage of advanced genetic tools in zebrafish to define roles for Fgf signaling during formation of EB and LL placodes. First, we will establish temporal and special requirements for Fgf signaling using pharmacologic Fgf-receptor inhibitor and inducible, dominant-negative Fgf-receptor transgenic zebrafish. Second, we will determine whether Fgf signaling is required during early steps of cranial placodes formation (placode induction and cell migration) or during later steps (proliferation and survival of neurons). Finally, we will identify specific Fgf ligands that are required for the formation of EB and LL line placodes.
