Aging changes in satellite cells and their functions.

Vertebrate skeletal muscle fibers have two traits that make them unique: the fibers are multinucleated and their nuclei are post-mitotic. The activity and mass of the muscles in the body make them susceptible to constant injury. When this occurs, myonuclei can be increased or replaced by the adult stem cells of muscle, satellite cells (SCs). These SCs are vital for normal growth, repair and regeneration. This review collates recent studies to determine the size of the nuclear domains and its change with activity. The relationship between the percent change in myonuclear number, cross-sectional area, and myonuclear domain indicates that the nucleus generally maintains a highly regulated domain size in spite of large variations in fiber size. The SC divides to add nuclei for growth and repair, and the SC identification and number are discussed. It is concluded that SC number does not reflect a change in regenerative ability by the muscle. However, the SC number increases with changes in muscular activity, and any reduced number of satellite cells in the elderly does not appear to reflect a decline in reparative or regenerative ability. The effects of aging on SC function are reviewed, and the significance of the SC's connective tissue environment is emphasized as being a major factor in the decrement of the SC's ability to repair and regenerate the aging muscle. Therefore growth factors and cytokines in the connective tissue around the SC are major influences in the decline of SC function with age.