Early events in myofibrillogenesis and innervation of skeletal, sound‐generating muscle in a teleost fish

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Abstract
The plainfin midshipman, Porichthys notatus, generates acoustic communication signals through the rapid contraction of a pair of vocal (sonic) muscles attached to the walls of the swimbladder. Light and electron microscopic methods were used to study two aspects of sonic muscle ontogeny: (1) the development and transformation of myotubes into muscle fibers and (2) innervation, including the formation of sonic neuromuscular junctions and the myelination of sonic motor axons. Sonic motor axons are associated with sonic mesenchyme during its initial migration away from occipital somites. However, myofibrillogenesis, the formation of neuromuscular junctions, and axon myelination do not occur until sonic mesenchyme reaches its final destination (i.e., the swimbladder). A continuum of developing myotubes is present rather than two temporally distinct populations of primary and secondary myotubes as observed for skeletal muscles in mammalian and avian species. Potential reasons for the lack of primary and secondary myotubes are considered, including the functional homogeneity of the sonic motor system and the sonic muscle's unique architecture, namely its direct attachment to the wall of the swim-bladder.