Two main models have been proposed for neural induction in embryos. In the`default' model, BMP signalling prevents the default, neural differentiation of ectodermal cells. Thus, when BMP signalling is inhibited, no other signal is needed to send these cells down the neural pathway. However, some experiments, particularly in chick embryos, suggest that neural induction also requires FGF signalling. Now, on , Di-Gregorio and colleagues report that in the mouse epiblast, loss of BMP signalling is sufficient for neural induction. By examining mice null for Bmpr1a,the only type I BMP receptor expressed in the epiblast, the researchers show that BMP2/4 signalling inhibits neural differentiation in the epiblast before gastrulation, in part by maintaining Nodal signalling. During gastrulation, BMP7 also helps to maintain the pluripotency of the epiblast. However, inhibition of FGF signalling in post-implantation mouse embryos does not block neural specification. The researchers conclude, therefore, that inhibition of BMP signalling is the critical event required for neural induction in mammals.
