Fig. 7. bts1 expression at the MHD during gastrulation requires Fgf8 and is activated by Wnt signalling. (A,B) Whole-mount dorsal views of bts1 expression at the tail bud stage, anterior to the top, without (A) or after (B) treatment with the inhibitor of Fgf signalling SU5402 between the stages dome and tail bud. Note the strong reduction in expression at the mid-hindbrain in B (arrowhead), while expression at the blastoderm margin is not affected (white arrow). (C,D) Flat-mounted views of bts1 expression in the mid-hindbrain area at the 90% epiboly stage, anterior towards the top, in wild-type (+/+) (C) versus ace homozygous mutants (D), as indicated. Mid-hindbrain expression of bts1 is strongly reduced and maintained only laterally (arrowheads); it remains unperturbed at the blastoderm margin (white arrow). (E,F) bts1 expression in embryos grafted with wnt1-expressing cells within the anterior neural plate. Endogenous bts1 expression at the MHD is indicated by the arrowhead. Grafted cells were co-injected with nls-lacZ RNA and are visualized by anti-ß-galactosidase immunocytochemistry (brown nuclei). (F) A high magnification of the grafted area (boxed in E). bts1 expression is induced around wnt1-expressing cells.