Last week my three year old scraped up the entire left side of his face. Today, there's barely a trace of the injury. That's the glory of three year old skin, or more precisely, the glory of three year old stem cells.

Researchers at the University of California, Berkeley have found molecular pathways that human muscle stem cells rely on to repair damaged muscle. These pathways are active in younger people but less active in older people, explaining why muscles repair more slowly with age. The group found that younger volunteers had double the number of regenerative muscle stem cells in their thigh muscles compared to older volunteers. After two weeks in a leg cast, both groups began exercise routines to rebuild muscle.

Researchers at UC, Berkeley identified a signaling molecule that interferes with the ability of older skeletal muscle to regenerate. After injury, adult skeletal muscle regenerates by activating muscle stem cells that fuse with the existing muscle cells to repair the damage. This ability to regenerate diminishes with age, not because of a decline in the number of resident stem cells, but because stem cells in the older muscle don't respond when damage occurs. It turns out that older muscles release molecules that actively inhibit the resident stem cells.