Figure 4.
FGF manipulations at E11.5 influence regional identity in deep neocortical layers. A-F, Sagittal sections through electroporated and control brains harvested at P6 and processed with in situ hybridization to reveal regional and laminar transitions in Tbr1 and COUP-TF1 gene expression. A transition in Tbr1 expression appears in the midlayers of anterior cortex (A, arrowhead); this coincides approximately with a transition in COUP-TF1 expression in layer VIa (D, arrowhead). The transition in expression of both genes is shifted posteriorly by excess anterior FGF8 (arrowheads, B, E) and anteriorly by reduced FGF8/17 activity (arrowheads, C, F). Notably, the extent of the shift appears equivalent in both midlayer Tbr1 expression and layer VIa COUP-TF1 expression (B, E and C, F). Regional transitions in expression also shift in upper layers. For example, a region of low Tbr1 expression (arrows in A) is shifted posteriorly by excess FGF8 (arrows in B). In a brain with reduced FGF8/17, anterior and posterior regions of high Tbr1 expression appear to merge (arrow in C). Scale bar (in C): A-F, 1.0 mm.