Differential satellite cell density of type I and II fibres with lifelong endurance running in old men

Aim To investigate the influence of lifelong endurance running on the satellite cell pool of type I and type II fibres in healthy human skeletal muscle. Methods Muscle biopsies were collected from 15 healthy old trained men (O‐Tr) who had been running 43 ± 16 (mean ± SD) kilometres a week for 28 ± 9 years. Twelve age‐matched untrained men (O‐Un) and a group of young trained and young untrained men were recruited for comparison. Frozen sections were immunohistochemically stained for Pax7, type I myosin and laminin, from which fibre area, the number of satellite cells, and the relationship between these variables were determined. Results In O‐Un and O‐Tr, type II fibres were smaller and contained fewer satellite cells than type I fibres. However, when expressed relative to fibre area, the difference in satellite cell content between fibre types was eliminated in O‐Tr, but not O‐Un. A strong positive relationship between fibre size and satellite cell content was detected in trained individuals. In line with a history of myofibre repair, a greater number of fibres with centrally located myonuclei were detected in O‐Tr. Conclusion Lifelong endurance training (i) does not deplete the satellite cell pool and (ii) is associated with a similar density of satellite cells in type I and II fibres despite a failure to preserve the equal fibre type distribution of satellite cells observed in young individuals. Taken together, these data reveal a differential regulation of satellite cell content between fibre types, in young and old healthy men with dramatically different training histories.