Fgf/Ets signalling in Xenopus ectoderm initiates neural induction and patterning in an autonomous and paracrine manners

Fibroblast growth factor (Fgf) and anti-bone morphogenetic protein (Bmp) signals are derived from the organiser of mesoderm origin and cooperate to promote Xenopus neural development from the gastrula ectoderm. Using antisense oligos to Fgf2 and Fgf8 and dominant-negative Ets transcription factors, we showed that the expression of Fgf2, Fgf8, and Ets in ectoderm cells is essential to initiate neural induction both in vivo and in vitro. Our findings show that neural induction is initiated primarily by autonomous signalling in ectoderm cells, rather than by paracrine signalling from organiser cells. The signalling in ectoderm cells is transduced via the Fgf/Ras/Mapk/Ets pathway, independent of Bmp signal inhibition via the Fgf/Ras/Mapk/Smad1 route, as indicated by earlier studies. Through the same pathway, Fgfs activated position-specific neural genes dose-dependently along the anteroposterior axis in cultured ectoderm cells. The expression of these genes coincides with the establishment of the activated Ets gradient within the gastrula ectoderm. Organiser cells, being located posteriorly to the ectoderm, secrete Fgfs as gastrulation proceeds, which among several candidate molecules initially promote neural patterning of the induced neuroectoderm as morphogens.