A comparison of the morphology of denervated with aneurally regenerated soleus muscle of rat.

Soleus muscles of rats were denervated, treated with bupivacaine and autografted in order to induce aneural regeneration or only denervated. After 2-70 days, muscles were fixed and embedded in epoxy resin for light and electron microscopy. Regenerating muscles after 2 and 5 days consisted of a necrotic core and a rim of new fibres; the necrotic core had disappeared by day 10. The fibre size (mean cross-sectional area) was 139 microns2 after 10 days; it decreased to 22 microns2 after 70 days. Denervated muscle fibres measured 26 microns2 after 70 days. The sarcomere pattern was regular in 10-day regenerates but later degenerated. There were signs of fibre breakdown; immature regenerating fibres were still found after 70 days. Denervated muscles already contained a few new myotubes at 10 days; at 40 and 70 days fibre loss and regeneration were distinct. The percentage of satellite cells relative to myonuclei (normal 8%) doubled within the first 10 days after necrosis and then fell to less than one fourth of normal. Denervation for 70 days increased threefold the percentage of satellite cells in some muscles and in others caused a decrease to half of normal. The number of fibre cross-sections in regenerated muscles was about 3000 after 10-70 days; this was roughly the same number as in normal soleus muscles. The true number of fibres, however, may have been smaller because regenerated fibres tend to branch. It is concluded that aneurally regenerating muscle fibres reach a high degree of structural differentiation, but then atrophy, degenerate and vanish; they are replaced by new fibres, which do not reach the size of the first fibres, presumably because the satellite cell pool without innervation eventually becomes exhausted. The slow and non-synchronized breakdown of denervated soleus muscle fibres induces proliferation of satellite cells as in necrotized muscles; those muscles which had fewer satellite cells than normal after 70 days might already be showing exhaustion.