"Even though humans do not normally generate new skeletal muscle fibers in adult life, the capacity for doing so is not completely lost. Cells capable of serving as myoblasts are retained as small, flattened, and inactive cells lying in close contact with the mature muscle cell and contained within its sheath of basal lamina. If the muscle is damaged, these satellite cells are activated to proliferate, and their progeny can fuse to repair the damaged muscle. Satellite cells are thus the stem cells of adult skeletal muscle, normally held in reserve in a quiescent state but available when needed as a self-renewing source of terminally differentiated cells. Athletes who specialize in muscular strength often damage their muscle fibers and are thought to depend on this mechanism for muscle repair, resulting in regenerated fibers that are often highly branched.
The process of muscle repair by means of satellite cells is, nevertheless, limited in what it can achieve. In one form of muscular dystrophy, for example, differentiated skeletal muscle cells are damaged because of a genetic defect in the cytoskeletal protein dystrophin. As a result, satellite cells proliferate to repair the damaged muscle fibers. This regenerative response is, however, unable to keep pace with the damage, and the muscle cells are eventually replaced by connective tissue, blocking any further possibility of regeneration. A similar loss of capacity for repair seems to contribute to the weakening of muscle in the elderly."
<I><B>Source</B>: Molecular Biology of the Cell (4th Ed) by Alberts et al.</I><P>
