Cell interactions, signals and transcriptional hierarchy governing placode progenitor induction

Cranial placodes contribute to all sense organs and sensory ganglia in the vertebrate head. Despite their diversity they originate from a common pool of Six1/Eya2+ progenitors. In a molecular screen we identify new factors upstream of the Six1/Eya2 cassette and use these to dissect the transcriptional hierarchy that controls progenitor specification. We find that although two different tissues, the lateral head mesoderm and the prechordal mesendoderm, induce placode progenitors, both initiate a common transcriptional state, but over time gradually impart regional character. Thus, as cells acquire placode progenitor identify they pass through successive transcriptional states each identified by a distinct set of factors and controlled by different signalling pathways. Thus, we propose a new model for placode progenitor induction reminiscent of Waddington’s evocation-individuation model for neural induction. Sensory placode progenitors of the pre-placodal region are induced by signals from the lateral head mesoderm. To identify transcripts up- and downregulated during this process, the lateral head mesoderm was dissected from HH6 chick embryos and grafted into the extraembryonic area opaca of HH4- chick hosts. After 12 hours, the mesoderm was removed and the underlying epiblast was collected as was the contralateral non-induced area opaca (control). RNA was prepared and processed for hybridization to Affymetrix arrays