Scanning electron microscopic study of denervated and reinnervated neuromuscular junction.

Morphological changes of the subneural apparatus (SNA) during denervation and reinnervation are demonstrated using the hamster peroneus longus muscle. One week after nerve section, synaptic grooves (SGs) were more shallow, with lowered sarcoplasmic ridges. After 4 weeks, the whole SNA area was elevated above the sarcolemma as a flat plate, which persisted as a fusiform bulge for more than 8 weeks. There was a rapid and progressive decrease in the number of junctional folds (JFs) and slit-to-pit transformation of their openings. There was no trace of the SNA after 16 weeks. By 4 weeks after nerve suture, SGs were flattened but did not form the plate-like elevation. Many shallow pit-like JFs still persisted. At 10 weeks, deep SGs started to be restored with an increased number of deep slit-like JFs. The SNA recovered its normal structural organization by the week 20. However, a few pit-like vestigial JFs were present outside the groove.