The relationship of nerve to myoblasts and newly-formed secondary myotubes in the fourth lumbrical muscle of the rat foetus

The formation of normal numbers of skeletal muscle fibres depends on functional innervation of the muscle before and during the period of secondary myotube formation, but little has been known about the physical relationship between nerve terminals and the myoblasts and secondary myotubes over the critical period. This paper reports the results of a serial-section electron microscopic study of the IVth lumbrical muscle of the rat hindlimb, studied on embryonic day 20 (E20), a time when all secondary myotubes are less than 24 h old, and new ones are rapidly forming. Most myoblasts lying within the endplate region of the muscle received some direct neural contact; in almost all cases, the contact originated from an extension of a differentiated nerve terminal present at the endplate of an adjacent primary myotube. At six of 15 neural contact sites on myoblasts, primitive synaptic specialization was present. The newly-formed secondary myotubes were also directly, although sparsely, innervated in nine of ten instances. One secondary myotube was never seen to be innervated, despite extensive serial tracing. Nerve terminals passing to secondary myotubes were also principally derived from the innervation zone of the earlier-formed primary myotubes. Primary myotubes were profusely innervated by multiple axons. The results suggest that most nerve terminals are initially accommodated on the primary generation of myotubes, but progressively transfer to pre-fusion myoblasts or to secondary myotubes as these appear. In general, very young secondary myotubes are innervated by only a single axon, rather than being polyneuronally innervated. The existence of some secondary myotubes which lack any direct innervation suggests that intimate nerve contact may not be obligatory for formation of new secondary myotubes.