Early Gene Interactions That Discriminate Among The Four Ectodermal Domains In The Embryonic Head

During early vertebrate development, four major domains are established within the ectoderm of the head: neural plate, neural crest (NC), pre-placodal ectoderm (PPE), and epidermis. In this investigation we characterized the relationships between several sets of key transcription factors (TFs) that establish and maintain these domains by mis-expressing TFs in specific precursor blastomeres of the Xenopus embryo and monitoring the consequent changes in gene expression by in situ hybridization in neurula embryos. We find that TFs required for NC formation (Msx1) or PPE formation (Six1) significantly repressed TFs expressed in the early epidermis (FoxI1, Dlx3, Dlx5, Dlx6). Second, TFs that specify the neural plate border zone repress PPE fate. Third, PPE genes, Six1 and one of its co-factors Eya1, significantly reduce NC genes iwhen expressed in combination. Fourth, Six1 and/or Eya1 reduce the expression of placode genes expressed during differentiation. Therefore, cooperativity between Six1 and Eya1 appears to maintain the PPE in a progenitor state and repress genes that are required for later neural differentiation. Supported by NSF and NIH Grant Funding Source: NSF and NIH