The prdm1:gfp transgenic reporter marks primary and secondary slow fibres. (A) Weak prdm1 in situ hybridisation in presumptive secondary superficial slow fibre cells (arrows) at 35 hpf in smo mutants. (B) prdm1:gfp fluorescence in adaxial cells at the six-somite stage is almost eliminated in smo mutants. (C) At 24 hpf, Prox1-expressing primary slow fibres are marked with prdm1:gfp and are absent in smo mutants. (D) Lateral views of posterior trunk of 48 hpf smo mutants showing prdm1:gfp colocalised with smyhc1-CDS in situ hybridisation and slow troponin C (stnnC) in situ hybridisation in secondary superficial slow fibres at the dorsal and ventral edges of the somites. (E) In smo mutants, prdm1:gfp colocalises with Prox1 antigen in secondary fibres (arrows) in somite 13 at 36 hpf. Note the intense Prox1 staining in isolated cells that are external to the myotome (arrowheads). (F) At 48 hpf, in smo mutant posterior trunk, slow MyHC S58 antigen and prdm1:gfp mark secondary superficial slow fibres that do not colocalise with fast muscle F310 antigen. (G) Dorsolateral view of the iob and dorsal view of the sca at 96 hpf showing colocalisation of prdm1:gfp with slow MyHC S58 antigen but not with fast muscle F310 antigen (see Fig. 1 for gross location of these muscles). Scale bars: 25 μm. Abbreviations: iob, inferior obliquus; sca, supracarinalis anterior.
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