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Fig. 3. Foxg1 expression and proliferation are reduced in the forebrain of Fgf8^Null/Neo and Fgf8^TelKO mutant embryos. Whole-mount in situ hybridization on E9 embryos showing Six3 (A-B''), Foxg1 (C-D'') and Emx2 (E9.5 embryos; F-G'') expression. For each probe, the top panel shows lateral views of the embryos, and the lower one frontal views. The Six3 expression domain in the rostroventral telencephalon appears smaller (arrowheads in B-B'') and Emx2 expression in the caudodorsal telencephalon expands rostrally in Fgf8^Null/Neo and Fgf8^TelKO mutants (arrowheads in F-F''). By contrast, the Foxg1 expression domain in the telencephalon is severely reduced in Fgf8^Null/Neoembryos and almost absent in Fgf8^TelKO embryos (solid arrowheads in C-D''). By contrast, Foxg1 expression in the olfactory placodes is still detected in both mutants (open arrowheads). Anti-phosphohistone3 (PH3) immunofluorescence on horizontal sections through the forebrain labels the nuclei of mitotic cells in wild-type (E), Fgf8^Null/Neo (E') and Fgf8^TelKO (E'') embryos (additional sections are in Fig. 4). The reduction in PH3⁺-labeled cells correlates with the reduction in Foxg1 expression (D,D',D''). Foxg1 is required for proliferation in the telencephalon (Xuan et al., 1995). Note, the panels showing PH3 labeling are turned 180° with respect to the panels showing frontal views of in situ hybridizations, such that the rostral regions face each other; this was done to facilitate comparison of Foxg1 expression and the number of PH3⁺ cells. The boxes indicate the regions in which the numbers of PH3⁺ cells were counted in the rostral midline (box 1) and in the rostroventral telencephalon (box 2) (see Table 1). Os, optic stalk; Tel, telencephalon.

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