Sox3 expression defines a common primordium for the epibranchial placodes in chick

The epibranchial placodes are ectodermal thickenings that generate sensory neurons of the distal ganglia of the branchial nerves. Although significant advances in our understanding of neurogenesis from the placodes have recen tly been made, the events prior to the onset of neurogenesis remain unclear . We found that chick Sox3 (cSox3) shows a highly dynamic pattern of expres sion before becoming confined to the final placodes: one pre-otic (genicula te) and three post-otic (one petrosal and two nodose) placodes. A fate-mapp ing study using lipophilic dyes revealed that all post-otic placodes arise within a single broad cSox3-positive domain, where cSox3 expression and epi thelial thickness will be retained only in much smaller final neurogenic pl acodes. The data presented here suggest that post-otic placodes are remnant s of a common primordium defined as a discrete domain of cSox3 expression. (C) 2001 Academic Press.