Quantitative studies on the numerical frequency of myonuclei in the muscles of exercised rats: evidence against the occurrence of fibre-splitting.

The histological changes associated with exercise-induced muscular hypertrophy have variously been interpreted as due to fibre-splitting or satellite-cell activity. If due to fibre-splitting, than a marked fall in the number of nuclei per unit volume of muscle must occur. To see if this is the case, we have examined post-mortem specimens of extensor digitorum longus muscles in rats exercised by swimming for up to 30 min twice daily for 35 days. The mean cross-sectional area of muscle fibres was unaltered by exercise but mean length of capillary per unit volume of muscle was increased by 55% and number of nuclei per unit volume of muscle by 30%, both changes being significant. Mean nuclear volume increased by approximately 75%. Thus the muscles did adapt to exercise and new myonuclei were formed, contrary to expectations if fibre-splitting were the essential change. In this study the fibres seemed not to have increased in size and further studies, perhaps using autoradiographic analysis of tritiated thymidine-labelled satellite-cell nuclei, are needed to clarify the precise mechanisms involved.