The decline in muscle mass and strength that occurs with aging, known as sarcopenia, is thought to correlate with a loss of motor neurons, theorized to be an important cause of the process. Researchers here point instead to loss of stem cells as the primary cause of age-related muscle decline. Stem cell activity is well known to fade with age, an evolutionary adaptation to increasing levels of tissue damage that may serve to reduce cancer risk. Progress in the stem cell research field to date, such as the development of therapies based on spurring more youthful levels of stem cell activity in the old, suggest that there is considerable room for greater regeneration without higher rates of cancer, however.
Researchers have discovered that loss of muscle stem cells is the main driving force behind muscle decline in old age in mice. Their finding challenges the current prevailing theory that age-related muscle decline is primarily caused by loss of motor neurons. Study authors hope to develop a drug or therapy that can slow muscle stem cell loss and muscle decline in the future. As early as your mid 30s, the size and strength of your muscles begins to decline. The changes are subtle to start – activities that once came easily are not so easy now – but by your 70s or 80s, this decline can leave you frail and reliant on others even for simple daily tasks. While the speed of decline varies from person to person and may be slowed by diet and exercise, virtually no one completely escapes the decline.
All adults have a pool of stem cells that reside in muscle tissue that respond to exercise or injury – pumping out new muscle cells to repair or grow your muscles. While it was already known that muscle stem cells die off as you age, the study is the first to suggest that this is the main driving factor behind muscle loss. To better understand the role of stem cells in age-related muscle decline, researchers depleted muscle stem cells in mice without disrupting motor neurons, nerve cells that control muscle. The loss of stem cells sped up muscle decline in the mice, starting in middle, rather than old age. Mice that were genetically altered to prevent muscle stem cell loss maintained healthier muscles at older ages than age-matched control mice.
At the same time, researchers did not find evidence to support motor neuron loss in aging mice. Very few muscle fibers had completely lost connection with their corresponding motor neurons, which questions long-held and popular theory. According to the theory, age-related muscle decline is primarily driven by motor neurons dying or losing connection with the muscle, which then causes the muscle cells to atrophy and die. “I think we’ve shown a formal demonstration that even for aging sedentary individuals, your stem cells are doing something. They do play a role in the normal maintenance of your muscle throughout life.”