An ultrastructural study of the temporal progress of neuromuscular junction regeneration after sciatic nerve transection and tubulization repair

ABSTRACT

The aim of this investigation was to study the timecourse of motor endplates in the extensor digitorum logus (EDL) muscle after peripheral nerve transection and tubulization repair. Adult male C57BL/6J mice received sciatic nerve transection at midhigh level and both proximal and distal nerve stumps were stured into a 5-mm long polyethylene tube to bridge a final nerve gap distance of 3mm. At 2 to 40 weeks postoperatively the EDL muscle on the operated side was fixed in situ and processed histochemically for visualization of cholinesterase-rich sites. Groups of muscle fibers containing motor endplates were then processed for electron microscopy (EM). Two weeks after tubulization the EDL muscle was entirely denervated: Schwann cells came into contact with the post-synaptic folds of the muscle fibers; there was an increase in concentration of collagen and fibroblasts at the synaptic sites. Reinnervation began 4 weeks after tube implantation, when the first axon terminals established contact with a small portion of the specialized subneural region, many synaptic folds were still covered by collagen fibers and, in some cases, Schwann cells remained interposed between the folds and the synaptic terminals. Twelve weeks after surgery all the neuromuscular junctions examined were reinnervated and looked normal; motor terminals were alawys located at the primary synaptic clefts, although rarely some subneural folds lacking nerve terminals were seen. At 12 to 40 weeks all motor endplates appeared normally innervated by EM criteria.