Changes in fibre type, number and diameter in developing and ageing skeletal muscle.

Morphometrical and histochemical features of muscle undergo continuous changes with age. Two representative muscles were studied to determine the nature of these changes, the extensor digitorum longus and the soleus. Muscle fibre type ratios were found to change with age so that there were more oxidative types. In the extensor digitorum longus the cross sectional area occupied by fast oxidative glycolytic fibres increased, while in the soleus fast oxidative glycolytic fibres apparently underwent conversion into slow oxidative fibres. Muscle fibre diameter increased dramatically during early growth but later in senile animals there was evidence of both atrophy and splitting. In the extensor digitorum longus the uneven growth of the two dominant fibre types gave rise to a bimodal fibre diameter distribution. The soleus, which is composed of predominantly one fibre type, did not show bimodality. Senile muscles had a characteristic wide distribution of fibre diameters with ill defined peaks. Total fibre number in the extensor digitorum longus decreased in early life while total fibre number in the soleus remained unchanged.