Fig. 1. sap mutants develop lesions in skeletal muscle where fibres detach from myosepta and retract. Fibres in wild type (WT) span the entire somite between myosepta (arrowheads in A; lateral views, 48 hours post-fertilisation), whereas lesions within sap somites are evident as cell-free spaces (arrowheads in B). Toluidine blue histology reveals detached, retracted fibres in association with lesions in sap (arrowhead in D; parasagittal sections, 72 hours post-fertilisation) but not in WT (C). Reconstruction of somites in 3D using confocal microscopy of fluorescence from the Tg(acta:GFP) transgene reveals extensive fibre loss in sap (arrowhead in F) but not WT (E). Anti-MyHC (green) reveals that whereas differentiation is normal in both WT (G) and sap (H), lesions in sap mutant somites lack contractile apparatus (arrowhead). Examination of head musculature using fluorescence from the Tg(acta:GFP) transgene shows that these muscles are unaffected in sap (J) compared with WT (I).

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