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Fig. 1 Identification of an enhancer trap line marking progenitors of tenocytes and interstitial cells in zebrafish embryos. (A) Expression of UAS:GFP in Gal4-UAS enhancer trap zebrafish line SAGFF(LF)19A and signals for scxa mRNA transcripts detected by whole-mount in situ hybridization (WISH) at 48 hpf. White arrowheads show segmental boundaries of somites. (B) Whole-mount immunostaining of 60 hpf SAGFF(LF)19A embryos for myogenic (magenta, MF20) and tenogenic (red, tenascin) markers together with UAS:GFP signals at 60 hpf. Asterisks mark muscle bundles. (C) Tenocyte progenitors marked with UAS:GFP and fluorescence in situ hybridization for scxa (red) at 60 hpf. Note that these cells extended long filopodia through the boundaries of skeletal muscle fibers. Arrowheads indicate UAS:GFP+ zScxa:mRNA+ cells. Schematic shows UAS:GFP+ cells in tendon and connective tissues in trunk muscles of SAGFF(LF) 19A line. Scale bars: 100 μm (A,B); 50 μm (C).