Epibranchial placodes are local thickenings of the surface ectoderm, which give rise to sensory neurons of the distal cranial ganglia. The development of these placodes has remained unclear due to the lack of any definitive marker for these structures. We show here that the chick transcription factor, cSox3, is expressed in four lateral patches at the rostral edge of the epibranchial arches and that these mark the epibranchial placodes. These patches of cSox3 expression arise by gradual thinning from broader areas of cSox3 expression with concomitant loss of cSox3 in nonplacodal regions. Cells leaving the epithelial placodes as they initiate neurogenesis, lose cSox3 expression and sequentially express Ngn1, NeuroD, NeuroM, and Phox2a, but do not express Ngn2. This is in contrast to studies in the mouse where it is Ngn2, rather than Ngn1, that is predominantly expressed in epibranchial-derived neuroblasts. Overexpression of cSox3 interferes with normal neuroblast migration and results in changes in ectodermal morphology. Thus, cSox3 provides a useful tool for the study of placode formation, and loss of cSox3 expression appears to be a necessary event in normal neurogenesis from the epibranchial placodes. (C) 2001 Academic Press
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