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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.
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