Fig. 5. Overexpression of FGF8 in somites promotes myogenic differentiation and FGFR4 activation. (A-C) Protocol used for electroporation of somites in vivo. FGF8 plasmid cDNA was injected into the somitocoel of the newly formed somites of (Hamburger-Hamilton) stage 15 chick embryos (A). These somites correspond to the prospective interlimb level (somites 22-27). By placing the positive electrode on the right-hand side of the embryo, we electroporated the lateral side of somites (B,C). Three days later, embryos were analyzed by in situ hybridization for MyoD (D) and FGFR4 (E) expression. Massive overexpression was observed with both probes (green arrowheads) along the entire mediolateral axis of somites (i.e. prospective epaxial and hypaxial domains). Red arrowheads in E indicate an overgrowth that, in some embryos, developed into a partial ectopic limb. HL, hindlimb; FL, forelimb.