Factors influencing the repair and adaptation of muscles in aged individuals: satellite cells and innervation.

From the standpoint of structure, the loss of muscle mass could be attributed to the loss of muscle fibers, the reduction of volume of persisting muscle fibers, or both. This review will concentrate upon factors that could contribute to the presence of thin muscle fibers in old muscles. One mechanism that could account for the presence of a population of thin muscle fibers is the loss of innervation due to the death or the remodeling of motor units in aging muscles. If the denervated muscle fibers fail to become reinnervated, or if they are unable to respond to reinnervation by new nerve terminals, they would undergo a progressive atrophy. If muscle fibers are damaged through exercise, direct trauma or other causes, they would degenerate and regenerate. Early regenerating muscle fibers are thinner than normal. If the regenerating muscle fibers are non-innervated, regeneration will not be complete; instead the regenerating muscle fibers will atrophy. All of the above mechanisms could contribute to an overall reduction in muscle mass. This review ends with an enumeration of outstanding questions concerning muscle atrophy and its reversal in old individuals.