Fig. 3. Msxb expression is increased in Bmp/Smad mutant zebrafish embryos. Wild-type and mutant zebrafish embryos were analyzed by whole-mount in situ hybridization for the expression of Msxb at the five-somite stage. Lateral views, anterior is upwards. (A) Wild-type embryos show the characteristic dorsal expression in the embryo. (B) A swr mutant shows an expansion of the Msxb territory in anterior regions. (C) A sbn mutant embryo shows a dramatic ventral expansion in Msxb expression. (D) A snh mutant embryo shows a moderate, lateral expansion in the expression of Msxb, where the two domains of expression can be seen. (E) Flat-mount of a wild-type embryo analyzed for the expression of Krox20 (arrows) and Myod (bracket). (F) Swr mutant showing the ventral expansion of Krox20 (arrows) and Myod (bracket). Note that the Myod expression is disorganized but can be found in dorsal and ventral sides. (G) swr mutant injected with ntl/sptl morpholinos. A complete absence of Myod expression but an expansion in Krox20 (arrows) was observed. (H-J) swr mutant embryos were injected with chordin mRNA and the expression of Msxb was analyzed. Anterior is towards the left. (H) Control uninjected embryos show the characteristic expansion of Msxb expression. (I) Embryos injected with 50 pg of chordin mRNA; note a reduction in Msxb expression. (J) Embryos injected with 200 pg of chordin mRNA exhibit a total inhibition in the expression of Msxb. Each experiment was repeated at least twice, with similar results. Reducing Bmp signaling with Bmpr treatment yielded similar results to those shown here for chordin.