Fig. 7. FGF/MEK/ERK activity is required for row I versus row II fates. (A) Activation of ERK1/2 visualised by dpERK1/2 antibody staining. The schematics indicate the neural plate cells positive for ERK1/2 activation, with the rows of cells indicated. The cell outlines were estimated using the ERK staining image and a Hoechst-stained image of the same embryo (data not shown). The embryo on the left contains four rows of cells in the neural plate. At this stage, weak ERK1/2 activation can also be seen in the row III/IV precursors, which are not visible in the panel because of the orientation of the embryo. (B) Expression of row I markers following UO126 treatment from the early gastrula stage. Ci-Cdx is expressed in A9.31, A9.29 and A9.15 in control embryos (Imai et al., 2006). (C) Expression of row II markers following UO126 treatment from the early gastrula stage. Hoechst staining, shown on the right for Ci-COE and Ci-FGF8/17/18, confirms that ectopic expression in row I was seen in column 4 and column 3, respectively. (D) Injection of dnFGFRc mRNA into the A5.2 blastomere on the right-hand side. Descendants of A5.2 include those circled in red on the neural plate schematic. (Right) Numbers below the figures show the percentage of embryos that the panels represent and the total number of embryos analysed. Cont., control.