Non-neural and neural expression of myosin heavy chains by regenerated intrafusal fibers of rats

Abstract

Expression of myosin heavy chain (MHC) isoforms was studied in rat soleus (SOL) and extensor digitorum longus (EDL) muscles which regenerated in the presence or absence of innervation. Frozen sections of two 5 day denervated SOL and EDL grafts, two 40 day denervated SOL and EDL grafts, and two reinnervated 40 day SOL and EDL grafts were processed for demonstration of motor endplates, sensory endings, myosin adenosine triphosphatase (mATPase) and for expression of 4 MHCs. No qualitative differences in MHC expression were noted between 5 day or 40 day denervated grafts of the SOL and EDL muscles. All regenerated intrafusal and extrafusal myotubes or myofibers reacted to antibodies against neonatal and fast-twitch MHCs, but not to antibodies against slow-twitch and slow-tonic MHCs in these grafts. These data indicate that MHCs expressed by regenerated intrafusal myotubes do not parallel those expressed by myotubes which give rise to the three types of intrafusal fibers during development and that MHC expression by regenerated intrafusal myotubes parallels that of regenerated extrafusal myotubes prior to innervation. However, some regenerated intrafusal fibers in 40 day nerve-intact grafts bound antibodies to slow-twitch and slow-tonic MHCs, thus expressions of these two MHCs are nerve-dependent in regenerated muscle spindles.