Regeneration and reinnervation of the dystrophic mouse soleus muscle. A light- and electron-microscopic study.

The regeneration and reinnervation of the dystrophic mouse soleus muscle was investigated in response to a double crush-lesion, which causes degeneration of muscle fibres leaving the innervation intact. In normal and dystrophic muscles, injury produced degeneration of muscle fibres, proliferation and fusion of muscle satellite cells, and growth and reinnervation of regenerating fibres. Four, 6 and 21 days after injury, regenerating dystrophic fibres were 50% smaller in cross-sectional area than regenerating normal fibres and showed several pathological changes. Nerve terminal morphology was initially unaffected by the crush, and nerve terminals were associated with degenerating muscle fibres 2 days after injury and with regenerating muscle fibres 6-28 days after crushing. In intact muscles dystrophic endplates were longer and showed increased ultraterminal sprouting compared to normal endplates. At 28 days after crushing normal nerve terminal sprouting was significantly increased compared to the contralateral control. The extent of nerve terminal sprouting and endplate length in dystrophic muscles was not affected by the degeneration and subsequent regeneration of the muscle fibres. We conclude that a proportion of dystrophic mouse soleus muscle fibres can regenerate after a crush when the innervation is left intact.