Compensatory hypertrophy was induced in the rat soleus muscle by sectioning the tendon of the ipsilateral gastrocnemius and plantaris muscle. Seven days after tenotomy of synergistic muscles, when soleus hypertrophy attains about 40, the number of satellite cells (expressed as percentage of all muscle nuclei found in the same cross-sections) as revealed by electron microscopy, was increased from 5.8±0.06 in the normal soleus muscle to 16.6±1.26. After four days' denervation of the soleus muscle the percentage of satellite cells was increased to 7.2±0.62. In experiments where hypertrophy of the soleus muscle was combined with denervation three days after tenotomy of synergists, and examined after another four days (during which time it loses, as has previously been shown, over 40 of its predenervation weight), the number of satellite cells was greatly increased to 29.9±3.42. This increase is apparently due to two independent processes which take place during the first postoperative period: a) mitotic division of satellite cells during the early stages of compensatory hypertrophy and b) pinching off of muscle nuclei from rapidly atrophying muscle fibres due to subsequent denervation. Activation of satellite cells was mainly manifested by expansion of smooth and especially of rough endoplasmic reticulum, a rich Golgi complex, high pinocytotic activity, increased number of ribosomes and by nuclear changes. Concomitantly with the increased number of satellite cells, proliferation of fibroblasts, macrophages and mast cells could be obser
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