The regeneration of neuromuscular junctions during spontaneous re-innervation of the rat diaphragm

Summary Electron microscopic observations have been made on the regeneration of neuromuscular junctions during spontaneous re-innervation of the rat diaphragm, following unilateral transsection of the phrenic nerve. 3 and 4 weeks after denervation motor end plates displayed the pattern of almost complete degeneration, i.e. persisting subneural foldings, deprived of neural contact and covered with collagen fibrils and fibrocytes. From observations at 5, 10 and 24 weeks after denervation the following sequence of events could be established: a few small axon terminals, accompanied by Schwann cells, became apposed to subneural folds, while most foldings were covered initially by Schwann cells or still by collagen fibrils. Gradually an increasing number of subneural folds came into contact with axon terminals. At 24 weeks all junctions displayed the pattern of a mature motor end plate. In the majority of regenerating neuromuscular junctions single dense-cored vesicles of approximately 900–1200 Å were present in axon terminals.It is concluded that under the present conditions restoration of neuromuscular transmission is accomplished by a re-innervation of the preserved subneural apparatuses of former junctions by regenerating axons. The significance of the occurrence of dense-cored vesicles in regenerating motor end plates is discussed.This work was supported by the Deutsche Forschungsgemeinschaft and the Stiftung Volkswagenwerk.