Decreased muscle activity alters development of myotomal muscle

The fast muscle fibres that form the bulk of muscle tissue in teleost fish are used in fast escape and attack responses. In adult fish, the fast fibres are arranged in a pseudo-helical pattern that is thought to optimise the work output of the fibres during movement. During development, the muscle fast fibres are initially arranged in parallel with the body axis and the slow muscle fibres are located centrally instead of peripherally. The development of the pseudo-helical pattern and the migration of slow fibres towards the periphery correspond with the onset of movements in the embryo. To test whether embryonic movements are responsible for these architectural changes, we compared wild type embryos with immobile embryos using confocal microscopy. Despite the immobility, distinguished slow and fast muscle fibres developed at the correct location in the axial muscles, helical muscle fibre arrangements were detected and sarcomere architecture was generated. However, in nicb107 mutant embryos the notochord is flatter and the cross-sectional body shape more rounded, also affecting muscle fibre orientation. The stacking of sarcomeres and myofibril arrangement show a less regular pattern in electron microscopy analysis. Finally, judging from real time quantitative PCR, changes in gene expression indicate that muscle growth is not impeded and energy metabolism is not changed by the decrease in muscle activity, but that the composition of muscle is altered. In conclusion, the lack of muscle fibre activity did not prevent the basal muscle components developing, but influenced further organisation and differentiation of these components.