a, In young wild-type animals, healthy MuSCs predominate and are competent to regenerate healthy myofibers in response to injury. b, In aged, wild-type animals, ETC-dysfunctional MuSCs (depicted with red mitochondria) are sensitive to elevated ROS and Scinderin levels, and thereby induced to fuse into neighboring myofibers. In this manner, the remaining MuSC population contains healthy mitochondria, and generates healthy myofibers in response to injury. c, In the absence of Scinderin during aging, MuSC-myofiber fusion is not available to remove ETC-dysfunctional MuSCs. In this setting, injury can trigger the activation, proliferation and fusion of damaged MuSCs, resulting in de novo myofibers with mitochondrial dysfunction.