Muscle differentiation after sciatic nerve transection and reinnervation in adult rats

Reinnervation after peripheral nerve transections generally leads to poor f unctional recovery. In order to study whether changes in muscles might be a contributing factor in this phenomenon we studied muscle morphology and fi bre type distributions after sciatic nerve transection in the rat hind limb . Proximally, before the bifurcation in the tibial and common peroneal nerve, a 12 mm segment of the sciatic nerve was resected, reversed and re-implant ed as an autologous nerve graft. After survival periods of 7, 15 and 21 wee ks the lateral gastrocnemius, tibialis anterior and soleus muscles were dis sected, stained with mATP-ase, and fibre type distributions were studied. I n addition, numbers of muscle fibres were counted, and cross sectional area s were calculated. After 7 weeks, cross sectional areas were decreased in all muscles. In the gastrocnemius and tibialis anterior muscles the fibre number remained unalt ered but the hypotrophy had been reversed at later ages. The number of musc le fibres in the soleus muscle remained decreased over the entire period of observation. The percentages of type II fibres in the gastrocnemius and tibialis anterio r muscles were decreased at 7 and 15 weeks but these again approached norma l values at 21 weeks. The type I fibres, however, remained arranged in grou ps. In the soleus muscle a large increase in the percentage of type Il musc le fibres was observed and this remained until 21 weeks. We conclude that a non-selective reinnervation and later readjustments by r egression of polyneural innervation may in part explain the changes in dist ributions of various fibre types.