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Fig. 3. FGFR signalling is required for neural induction. (A) Four-cell embryos were treated until fixation with various concentrations of SU5402, as indicated. Phenotypic classes were defined according to the severity of axial deficiencies. Class V embryos lacked muscle, NCAM-positive neural tissue and neural crests [as revealed by 12.101 (B) and 4d (C) immunostaining at tailbud stage, or by Slug RNA hybridisation at neurula stage (D)]. Class V embryos at gastrula stages lacked prospective posterior and axial mesoderm (XBra, Xcad3, ADMP; E-G), prospective haematopoietic mesoderm (Xnr2, H), but contained prospective endodermal tissue (sox17α, I). (J-M) Sox2 (J,K) and opl (L,M) expression are shown at early (J,L) or late (K,M) gastrula stages in control and class V embryos. No neural precursors are present in class V embryos. (N,O) Thirty-two-cell embryos were injected with 250 pg lacZ RNA in one A1 blastomere (animal-most, dorsal-most). In untreated controls, injected cells populate mostly the eye and the brain and some head epidermis (N). In the presence of 120 μM SU5402, embryos were class V, and the injected cells were now found entirely in the epidermis below the cement gland (O). These cells expressed the marker K81, indicating that prospective neural cells were converted into epidermal progenitors in absence of FGF activity. SU5402 is not toxic to ectoderm cells as they remain alive, and expressβ -galactosidase and K81. (A-C) Lateral view, anterior towards the left. (N) Lateral view, anterior towards the right. (D,G,J-M) Dorsal view, anterior towards the top. In M, the embryo is slightly tilted upwards. (E,F,H) Vegetal view, dorsal towards the top. (I) Hemisectioned embryo, dorsal towards the right. O, frontal view. |
