Fig. 3. Myocardin activates ectopic expression of myocardial markers in the Xenopus embryo. (A-H) 125 pg of myocardin mRNA was injected into one cell of an eight-cell embryo, which was then assayed for cardiac markers by whole-mount in situ hybridization. No expression of the MHC gene is observed in uninjected stage 14 embryos (A), however widespread transcription of MHC is observed in myocardin-injected embryos (B). Similarly, cardiac -actin is observed specifically in the pre-somitic mesoderm at stage 14 control embryos (C), while myocardin injected embryos display widespread expression of cardiac -actin on the side of injection (D). (E) Section through the embryo in D shows ectopic cardiac -actin expression (arrows) in the ectodermal and mesodermal tissue layers. Ectopic cardiac marker expression is not observed in endodermal tissues. (F) MHC expression is heart-specific at stage 28 in un-injected control embryos, but myocardin overexpression, (G), causes MHC transcription in ectopic locations. Arrows indicate normal cardiac expression. (H) Section through the embryo in G shows patches of ectopic MHC expression in the neural tube (nt) and eye. (I) Fluorescence microscopy of a stage 29 Xenopus embryo co-transgenic for NßT-GFP and NßT-myocardin showing GFP expression in neural tissues. (J) In situ hybridization analysis of NßT-GFP/NßT-myocardin co-transgenic embryos using a MHC probe shows ectopic expression of MHC in neural tissues.