Ultrastructure of the 2-months denervated rat leg muscle

Ultrastructure of slow (soleus, SOL) and fast (extensor digitorum longus, EDL) rat leg muscles were examined two months after denervation when progress of atrophy slow down, and it enters in some more stable stage. In the experimental muscles the mass decreased to about 25% of the control value. Muscle fibres were decreased in size considerably, but not uniformly. The most muscle fibres were preserved showing features of denervation atrophy. However about 1.4% of muscle fibres, both in SOL and EDL, were seriously damaged, that is, they underwent severe atrophy, necrosis and/or programmed cell death. They showed generally increased electron density, disorganised contractile structure with unrecognisable Z-line, damaged or not recognisable mitochondria and large vacuoles. Such fibres were grouped or situated individually close to preserved ones or to myotubes. Myotubes constituted about 2% of muscle fibres. Some of them were degenerating or dying. Large accumulation of collagen fibrils was common among muscle fibres, surrounding often blood vessels or myotubes. Thus, in addition to “simple” muscle atrophy, as early as 2 months after denervation, both slow and fast muscles show “degenerating and dying/regenerating myofibers.