Morphological changes in rat hindlimb muscle fibres during recovery from disuse atrophy

The present study attempted to use HE staining to clarify morphological changes in muscle fibres during recovery from disuse muscle atrophy.

Disuse muscle atrophy was induced by suspending 7-week-old male Wistar rats by their tails for 5 weeks (hindlimb unloading or HU group). Histological changes in the soleus muscle (SOL) during the recovery process were examined and compared with those in control rats who were raised freely without unloading (C group).

Wet muscle mass and muscle cross-sectional area per fibre of SOL in the HU group were 52 +/- 5 and 22 +/- 5% of those in the C group, respectively. Muscle atrophy was largely attributable to decreases in the size of muscle fibres, rather than to muscle fibre damage or loss. Muscle mass in the HU group increased quickly after reloading, but recovery of cross-sectional area per fibre was slow, with mean area in the HU group measuring 69 +/- 10% of that in the C group even after 5 weeks of reloading. After 1, 2 and 5 weeks of reloading, incidences of muscle fibres displaying central nuclei (regenerated muscle fibres) were 7.4 +/- 2.4, 7.2 +/- 6.3 and 19.2 +/- 14.5%, respectively.

These findings suggest that recovery of muscle fibres atrophied by disuse involves not only growth of atrophied muscle fibres, but also regeneration of muscle fibres. Cross-sectional areas recovery of atrophied muscle fibres thus continues after increases of muscle mass.