A fascinating experiment with genetically identical mice of different ages demonstrates that an influsion of young blood helps old muscles to heal as though they too were young. Wired reports that researchers "must pinpoint the youth-inducing molecules in the blood that are responsible for switching on the stem-cell capabilities in old muscles. Rando said that grueling process will be his lab's next project. ... Rando and his colleagues studied muscle stem cells called satellite cells, which in young mice and humans induce repair when injury strikes. Rando found in previous work that satellite cells exist in older muscle, but they don't respond to a muscle's cry for help after injury. In the new study, the presence of younger blood helped the satellite cells work more like they do in young mice."