Fig. 6. FGF signals inhibit ventral telencephalic cells from responding to RA signalling. (A) A stage 10 chick embryo (dorsal view, rostral is upwards). Black square indicates ventral telencephalic neuroectoderm explant region. (B) Expression of transcription factors in stage 10 ventral (V) explants cultured alone or with soluble FGFR4 (sFGFR4) for 48-52 hours. Stage 10 V explants cultured alone (n=15 for each) generated NKX2.1⁺ cells (>90%) but no MEIS2⁺, PAX6⁺ or EMX1⁺ cells. In the presence of soluble FGFR4, the generation of NKX2.1⁺ cells was blocked but MEIS2⁺ cells (62±3%) and PAX6⁺ cells (56±5%) were detected in stage 10 V explants. No EMX1⁺ cells were detected. Scale bar: 30 µm. (C) Lateral views of stage 20-22 chick embryos generated in New Culture from stage 10 grafted with control or soluble FGFR4 beads. All control embryos showed normal morphology, while the embryos grafted with soluble FGFR4 had smaller telencephalic vesicles than did controls. (D) Consecutive transversal sections of New Culture embryos, showing the expression of transcription factors in the telencephalon. The broken circle indicates the position of the bead. Embryos (n=10) grafted with control beads in the prospective ventral telencephalon expressed NKX2.1⁺ cells in the ventral region, Meis2⁺ cells in the intermediate region and PAX6⁺ cells in the dorsal domain of the intermediate region and in the dorsal region of the telencephalon. EMX1⁺ cells were located exclusively in the dorsal region. Embryos (n=10) grafted with soluble FGFR4 beads in the prospective ventral telencephalon lacked or showed a severely reduced number of NKX2.1⁺ cells, in four out of 10 embryos ventral cells ectopically expressed Meis2, PAX6 was still expressed in the intermediate and dorsal region of the telencephalon, and the generation of EMX1⁺ cells was blocked.