Fig. 8. Diagram depicting sites of fgf3 and fgf8 expression relative to their proposed roles in forebrain patterning in the zebrafish embryo. (A) Inhibition of Fgf signalling by SU5402 treatment prior to the onset of fgf8 expression in the presumptive forebrain at tailbud stage results in forebrain patterning defects that correlate with those observed following injection of Fgf3mo. Based on these and other results it is proposed that fgf3 expression during gastrulation is required for subsequent correct regional specification of the pallial telencephalon (tbr1), the subpallial telencephalon (dlx2), the ventral thalamus (dlx2) and the ZLI (shh), and for gene expression in precursor and differentiated neurons throughout the forebrain (zash1a, lim1 and isl1). (B) fgf3 and fgf8 expression overlaps in the dorsal telencephalon during early somitogenesis, and these Fgfs are proposed to have combined roles in patterning the subpallial telencephalon (restriction of emx1 expression) and the anterior ventral thalamus (positive regulation of twhh and negative regulation of pax2.1 expression). In addition, Fgf8 is proposed to function independently of Fgf3 in some aspects of telencephalic patterning (e.g. repression of tbr1 and fgf3 expression). h, hypothalamus; tel, telencephalon; vt, ventral thalamus. Asterisks indicate the ZLI. In the diagrams, the early domain of Fgf3 expression is indicated in turquoise in A; the domain of Fgf3 and Fgf8 co-expression is indicated in dark blue in B. In both figures, forebrain regions with defects are indicated in pink and yolk is depicted in yellow.