An early requirement for FGF signalling in the acquisition of neural cell fate in the chick embryo

Background: In Xenopus embryos, fibroblast growth factors (FGFs) and secret ed inhibitors of bone morphogenetic protein (BMP)-mediated signalling have been implicated in neural induction. The precise roles, if any, that these factors play in neural induction in amniotes remains to be established. Results: To monitor the initial steps of neural induction in the chick embr yo, we developed an in vitro assay of neural differentiation in epiblast ce lls. Using this assay, we found evidence that neural cell fate is specified in utero, before the generation of the primitive streak or Hensen's node. Early epiblast cells expressed both Bmp4 and Bmp7, but the expression of bo th genes was downregulated as cells acquired neural fate. During prestreak and gastrula stages, exposure of epiblast cells to BMP4 activity in vitro w as sufficient to block the acquisition of neural fate and to promote the ge neration of epidermal cells. Fgf3 was also found to be expressed in the ear ly epiblast, and ongoing FGF signalling in epiblast cells was required for acquisition of neural fate and for the suppression of Bmp4 and Bmp7 express ion. Conclusions: The onset of neural differentiation in the chick embryo occurs in utero, before the generation of Hensen's node. Fgf3, Bmp4 and Bmp7 are each expressed in prospective neural cells, and FGF signalling appears to b e required for the repression of Bmp expression and for the acquisition of neural fate. Subsequent exposure of epiblast cells to BMPs, however, can pr event the generation of neural tissue and induce cells of epidermal charact er.