[A scanning electron microscopic study of denervated neuromuscular junctions]

The morphological changes of the neuromuscular junctions (NMJs) caused by nerve transection have been examined by scanning electron microscopy in the peroneus longus muscle of the Chinese hamster. The synaptic grooves in the normal NMJs are deep labyrinthine depressions partitioned by ridges and contain numerous slit-like subsynaptic folds. After denervation, the grooves become shallower with lower sarcoplasmic ridges, and the subsynaptic area of the muscle fibers gradually flattens as a whole. The subsynaptic area shows a plate-like sarcoplasmic elevation by 4 weeks and persists as a fusiform focal bulge on the atrophied muscle fiber after 4 weeks. Concurrently the subsynaptic folds decrease in number and transform into shallow pit-like invaginations. Any subsynaptic specialization has not been discernible after 16 weeks. Changes in acetylcholinesterase stainability at the NMJs have also been observed by light microscopy. No remarkable changes were noted until 4 weeks after denervation, thereafter, however, acetylcholine positive area became smaller showing more diffuse staining pattern. Atrophied muscle fibers often exhibit longitudinal splitting and the satellite cells tend to detach from the muscle surface. These evidences suggest a regenerative process which may take place during muscle degeneration.