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Fig. 5. Expression of patterning signals and transcription factors implicated in dorsal midline/paramedial development in E9-9.5 Fgf8^Null/Neo and Fgf8^TelKO embryos. Frontal views of embryos hybridized in whole mount with probes for Bmp4 (A-A''), Msx1 (B-B''), Wnt8b (C-C'') and Sp8 (D-D''). Bmp4 expression is absent in the prosencephalic dorsal midline in both mutants (arrows A-A''). Msx1 expression in prosencephalic dorsal midline is indicated by an arrow (B-B''); this expression is not detectable in the Fgf8^Null/Neo mutant. Msx1 expression is also not detectable in the olfactory placodes. Wnt8b expression is expanded rostrally in the Fgf8^Null/Neo mutant and even more so in the Fgf8^TelKO mutant (C-C'') (note that the embryo in C'' is tilted backward, so the rostral expansion can be more readily observed). Sp8 expression is greatly reduced in the Fgf8^Null/Neo mutant, but is present in the midline of the Fgf8^TelKO mutant (arrow points to midline in D-D''). TUNEL analysis on horizontal sections through the E9.0 forebrain labels apoptotic cells in wild-type, Fgf8^Null/Neo and Fgf8^TelKO embryos (E-E'') (see additional cell death analysis in Fig. 6). In wild-type embryos, apoptotic cells are detected in the telencephalic midline (open arrowhead in E), the optic stalks and hypothalamus. In Fgf8^Null/Neo embryos, the rostral midline has fewer TUNEL⁺ cells, E', although it does exhibit evidence for cell death in the telencephalon (white arrowhead), optic stalk, and particularly in the hypothalamus. Fgf8^TelKO embryos have a higher concentration of TUNEL⁺ cells in the rostral midline (open arrowhead, E'), and have scattered TUNEL⁺ cells in the telencephalon (white arrowhead), optic stalk, and hypothalamus. The number of TUNEL⁺ cells in the rostral midline correlates with the expression of Msx1 at E9.0 (B-B''). Note that the panels showing TUNEL labeling are turned 180° with respect to the panels showing frontal views of in situ hybridization, such that the rostral regions face each other; this was done to facilitate the comparison of Msx1 expression with the number of TUNEL⁺ midline cells. The boxes demarcate the regions in which TUNEL⁺ cells were counted in the rostral midline (box 1) and rostroventral telencephalon (box 2) of embryos (see Table 1). Abbreviations: HT: hypothalamus, OP, olfactory placode; Os, optic stalk; Tel, telencephalon.

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