Development of a mammalian series-fibered muscle.

This study examines the processes by which multiply innervated, serially fibered mammalian muscles are constructed during development. We previously reported that primary myotubes of such a muscle, the guinea pig sternomastoid muscle, span from tendon to tendon and are innervated at each of the muscle's four innervation zones. Secondary myotubes form later, in association with each point of innervation (Duxson and Sheard, Dev. Dyn., 1995; 204:391-405). We now describe the further growth and development of the muscle. Secondary myotubes initially insert onto and grow along the primary myotube. However, as they reach a critical length, they encounter other secondary myotubes growing from serially adjacent innervation zones and may transfer their attachment(s) to these serially positioned secondary myotubes. Other secondary myotubes maintain attachment at one or both ends to their primary myotube. Thus, an interconnected network of primary and secondary myotubes is formed. Patterns of reactivity for cell adhesion molecules suggest that early attachment points between myotubes are the embryonic precursors of adult myomyonal junctions, characterized by the expression of alpha7Bbeta1 integrin. Finally, the results show that secondary myotubes positioned near a tendon are generally longer than those lying in the mid belly of the muscle, and we suggest that the environment surrounding the tendinous zone may somehow stimulate myotube growth.