Change in fibre size, not number, in ageing skeletal muscle

Skeletal muscle fibre loss and other changes were evaluated in 25 female rats aged 6-24 months. Soleus (SOL) and extensor digitorum longus (EDL) muscles were removed bilaterally. Left SOL and EDL muscles were digested in nitric acid, then, using a dissecting microscope, total fibre numbers for each muscle were obtained. Right muscles were frozen, sectioned and stained with ATPase and alkaline phosphatase to assess fibre-type distribution and capillary number. No significant change in total fibre number was seen with age in either muscle. Atrophy was common after age 12 months but was not found consistently. No reduction in capillary number was observed, even among atrophic fibres. Results suggest that atrophy and not fibre loss, per se, occurs with ageing and that atrophy is not due to capillary loss.