Picture a barracks where the soldiers have taken off their uniforms but haven't left. Researchers used to think that muscle stem cells disappear with age, because they couldn't find them by the usual markers. But when the cells were tagged with a glowing dye for life, it turned out they were still there: they had simply stopped wearing their ID tags.
These "quiet" cells were named GERI-MuSCs. They don't turn into muscle fibers, but when transplanted they help restore damaged muscle, though they do it worse than young ones. Aging genes switch on in them, yet they aren't completely decrepit. The male hormone dihydrotestosterone brings back their lost markers and improves muscle repair. So rejuvenation may work not by replacing cells, but by waking up the body's own dormant reserves.
Mice at the age of 28 months correspond to humans of about 90 years. Yet even in such deeply old animals, stem cells retain the ability to repair muscle if they are woken up by a hormone.
In the novel "Elysium," aging people gain access to medical pods that rejuvenate tissues. The discovery of GERI-MuSCs hints that rejuvenation may work not by replacing cells but by awakening the body's own dormant reserves.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.