Fig. 5. Neocortical progenitor cell expression of Mest is directly regulated by Fgf8 signalling. (A) Explants of the middle third of the E11.5 neocortex were cultured on polycarbonate membranes in defined, serum-free medium, as shown. Two explants were used for each single treatment. (B) Four hours of Fgf8 exposure strongly induced the expression of Mest, Etv5 and Pax6 and strongly repressed expression of COUP-TF1. No significant changes in the expression of Hey1 and Emx2 were seen. Histograms of the expression levels of all six genes relative to the average expression level of each gene from four control, non-Fgf8-treated experiments are shown. Results from three independent experiments are shown, with each experiment containing two neocortical explants. Expression levels within each sample were normalised to that of the abundant transcript encoding the ribosomal protein rpS17, whose expression does not change in this system.