The quest for eternal youth might be in vein. A new study indicates that blood from young mice helps older ones heal injured muscle. The findings may enable researchers to identify blood-borne molecules that kindle regeneration and develop new therapies for aging tissues.

Overdoing it in the weight room or slipping on a patch of ice can tear muscles. To repair the damage, young people quickly turn progenitor muscle cells into mature cells and weave them into healthy fibers. But the elderly lose these restorative powers and fill in rips with scar tissue. As a result, their muscles weaken, decreasing mobility and making falls more likely. Previous research has shown that muscles from old mice regain the ability to doctor themselves when transplanted into young mice. So Thomas Rando of Stanford University in Palo Alto, California, reasoned that rodents might harbor regenerative molecules in their bloodstream.

To test that idea, Rando and colleagues paired young and old rodents by stitching their skin together, causing the blood vessels of both animals to intertwine. Pairs of only old or only young mice served as controls. The team then froze a small section of each animal's shin muscle, damaging the tissue. Five days later, the young mice had fixed the injured tissue, regardless of whom they were paired with. Old mice attached to other old mice did not, but old mice coupled to young ones mended their muscles nearly as well as young ones did.

The new tissue arose from the old animals' own progenitor muscle cells, not from young cells that insinuated themselves into old muscle, the researchers report in the 17 February Nature. Further experiments revealed that young blood encourages old progenitor cells to produce Delta protein, which ignites a regeneration signal.

"It's really an enormous advance," says muscle researcher Charles Emerson of the Boston Biomedical Research Institute in Watertown, Massachusetts. The existence of blood components that influence regeneration "is a new idea," he says. Once identified, those factors might lead to novel treatments for muscle wasting, he adds, which is encouraging news for those who hope to regain the muscles of a teenager.

Related site

The Rando Lab