FORMATION OF NEW MYOTUBES OCCURS EXCLUSIVELY AT THE MULTIPLE INNERVATION ZONES OF AN EMBRYONIC LARGE MUSCLE

This work examines the general principle of whether production of embr yonic muscle fibres is invariably linked to sites of innervation, as w e have previously reported in small rodent muscles (Duxson et al. [198 9] Development 107:743-750). The experimental strategy has been to mak e a detailed electron microscopic analysis of the formation of new myo tubes in a large muscle having multiple, discrete innervation zones. T he particular model system is the guinea pig sternomastoid muscle, a s trap-like, parallel-fibred muscle with four distinct endplate bands, b oth in the embryo and the adult. Primary myotubes in the developing mu scle extended from tendon to tendon of the muscle and were innervated at each of the multiple endplate zones. Each point of innervation of t he primary myotubes was a focus around which many new secondary myotub es formed, and each secondary myotube was approximately centred on one of the innervation sites of its supporting primary myotube. This conf irms our previous report, in rat IVth lumbrical muscle, of an invariab le association between sites of formation of new secondary myotubes an d sites of innervation. We suggest that, in vivo, nerve terminals eith er directly induce the initial myoblast fusions which give rise to new secondary myotubes, or induce some precondition for fusion. An altern ative hypothesis is that a common patterning influence in the muscle l ocalizes both innervation and secondary myotube formation to the same zone. The pattern of secondary myotube production in the embryo has im portant implications for the size and final architecture of muscles in larger animals, and some of these are discussed. (C) 1995 Wiley-Liss, Inc.