The vast majority of specialized tissues (organs) in the human body contain reservoirs of adult stem cells or ‘progenitors’ which can become activated in response to cell death, injury, or other biological cues.  We refer to these progenitor cells based on the tissue type which houses them – “neural progenitor cells” exist in the brain, for instance.  In skeletal muscle, we refer to these cells as “myosatellite” or “satellite cells.”  Satellite cells sit along the skeletal muscle fibers [Figure 1] where they await activation in response to injury or stress.  The ability of these satellite cells to self-renew and differentiate decreases with age, which in part may explain why people steadily lose muscle mass as they age, a condition known as sarcopenia.