Neural determination of muscle fibre numbers in embryonic rat lumbrical muscles.

The generation and development of muscle cells in the IVth hindlimb lumbrical muscle of the rat was studied following total or partial denervation. Denervation was carried out by injection of beta-bungarotoxin (beta-BTX), a neurotoxin which binds to and destroys peripheral nerves. Primary myotubes were generated in denervated muscles and reached their normal stable number on embryonic day 17 (E17). This number was not maintained and denervated muscles examined on E19 or E21 contained many degenerating primary myotubes. Embryos injected with beta-bungarotoxin (beta-BTX) on E12 or E13 suffered a partial loss of motoneurones, resulting in a reduced number of axons in the L4 ventral root (the IVth lumbrical muscle is supplied by axons in L4, L5 and L6 ventral roots) and reduced numbers of nerve terminals in the intrinsic muscles of the hindfoot. Twitch tension measurements showed that all myotubes in partly innervated muscles examined on E21 contracted in response to nerve stimulation. Primary myotubes were formed and maintained at normal numbers in muscles with innervation reduced throughout development, but a diminished number of secondary myotubes formed by E21. The latter was correlated with a reduction in number of mononucleate cells within the muscles. If beta-BTX was injected on E18 to denervate muscles after primary myotube formation was complete, E21 embryo muscles contained degenerating primary myotubes. After injection to denervate muscles on E19, the day secondary myotubes begin to form, E21 muscles possessed normal numbers of primary myotubes. In both cases, secondary myotube formation had stopped about 1 day after the injection and the number of mononucleate cells was greatly reduced, indicating that cessation of secondary myotube generation was most probably due to exhaustion of the supply of competent myoblasts. We conclude that nerve terminals regulate the number of secondary myotubes by stimulating mitosis in a nerve-dependent population of myoblasts and that activation of these myoblasts requires the physical presence of nerve terminals as well as activation of contraction in primary myotubes.