FGF signalling controls the timing of Pax6 activation in the neural tube

We have recently demonstrated that Pax6 activation occurs in phase with som itogenesis in the spinal cord. Here we show that the presomitic mesoderm ex erts an inhibitory activity on Pax6 expression. This repressive effect is m ediated by the FGF signalling pathway. The presomitic mesoderm displays a d ecreasing caudorostral gradient of FGF8, and grafting FGF8-soaked beads at the level of the neural tube abolishes Pax6 activation. Conversely, when FG F signalling is disrupted, Pax6 is prematurely activated in the neural plat e. We propose that the progression of Pax6 activation in the neural tube is controlled by the caudal regression of the anterior limit of FGF activity. Hence, as part of its posteriorising activity, FGF8 downregulation acts as a switch from early (posterior) to a later (anterior) state of neural epit helial development.