With age, the ability of skeletal muscle to adapt to changing environmental needs, and meet continuous maintenance and regeneration needs diminishes, resulting in a loss of muscle mass, reduced regenerative capacity, and decreased functionality. Bradley Olwin, from the University of Colorado (Colorado, USA), and colleagues studied muscle stem cells, which are found within populations of "satellite" cells located between muscle fibers and surrounding connective tissue and are responsible for the repair and maintenance of skeletal muscles.  The researchers transplanted between 10 and 50 stem cells along with attached myofibers -- which are individual skeletal muscle cells -- from donor mice (healthy) into host mice (affected with limb muscle injuries).  Among the young host receiving the muscle stem cells, the stem cells not only repaired the injury within days, they caused the treated muscle to double in mass and sustain itself through the lifetime of the transplanted mice. Writing that: “These near-lifelong, physiological changes suggest an approach for the amelioration of muscle atrophy and diminished function that arise with aging through myofiber-associated satellite cell transplantation.”