Fig. 3. Overexpression of sonic hedgehog induces ectopic slow muscle fibres. Control lacZ RNA, with (right panels) or without (left panels) RNA encoding zebrafish shh, was injected into one side of four-cell Xenopus embryos and the animals allowed to develop for 2 days until stage 35. Embryos were fixed, serially sectioned and stained for slow (red, EB165) and all sarcomeric (green, A4.1025) MyHCs to identify muscle fibre populations. Whereas lacZ-injected embryos never showed alteration in superficial slow muscle fibre number or position, either close to ß-galactosidase activity or elsewhere, Shh-injected embryos frequently contained ectopic slow fibres in regions showing overexpression of ß-galactosidase. Somite is outlined on X-gal panels. Despite injected RNA frequently being highest in anterior regions, ectopic slow muscle was detected posteriorly within embryos. This suggested that induction of ectopic slow fibres was more readily achieved in regions that normally contain slow superficial cells at this stage.