Fig. 4. Notochord and Hedgehog signalling are required for normal MRF expression. (A) Notochord was ablated at stage 13 and embryos analysed by in situ hybridisation 2 hours later for XMyf5, XMyoD and actin mRNA. Arrowheads indicate adaxial tissue with high XMyf5 expression that is absent after notochord ablation. (B) Xptc2 expression in stage 28 embryos is ablated by cyclopamine treatment, both in the first wave slow muscle-forming region (arrows) and elsewhere (arrowheads). Solanidine has no effect (inset). (C) Embryos treated with cyclopamine, or ethanol vehicle control, at stage 9 and fixed at stage 28 or 32. XMyf5 in PSM is reduced creating a `gap' in tail expression (arrows). However, dorsal and ventral somite borders retain XMyf5 expression (arrowheads). XMyoD is reduced in tail tip (arrows), but unaffected in somitic stripes anteriorly. The dorsal and ventral somite borders fail to upregulate XMyoD in presence of cyclopamine (white arrows). Note reduced chevron form and dorsoventral extent of anterior XMyoD signal.