The study, led by Thomas Rando, MD, PhD, associate professor of neurology and neurological sciences at the Stanford University School of Medicine, built on previous work showing that old muscles have the capacity to repair themselves but fail to do so. Rando and his group studied specialized cells called satellite cells, the muscle stem cells, that dot muscle tissue. These normally lie dormant but come to the rescue in response to damaged muscle-at least they do in young mice and humans.
In older mice the satellite cells hold the same position, but are deaf to the muscle's cry for help. In the Nature study, Rando and his group first attached old mice to their younger lab-mates in a way that caused the two mice to share a blood supply. They then induced muscle damage only in the older mice. Bathed in the presence of younger blood, the old muscles healed normally. In contrast, when old mice were connected to other old mice they healed slowly.
In similar work, the group examined the livers of older mice connected to younger lab-mates. The cells that help liver tissue regenerate are less active in older animals, but again the cells responded more robustly when the livers in older mice were bathed in the younger blood. Clearly, something in the youthful blood revived the regenerative cells in muscle and liver.
"We need to consider the possibility that the niche in which stem cells sit is as important in terms of stem cell aging as the cells themselves," said Rando, who is also an investigator at the Veterans Affairs Palo Alto Health Care System. It could be the chemical soup surrounding the cells, not the cells themselves, that's at fault in aging.