Scientists have long puzzled over why old people and animals heal slower than young ones do. Now, researchers report that a pivotal factor behind this phenomenon circulates in blood.
Previous work by Thomas Rando and his colleagues at Stanford University showed that both young and old mice have muscle stem cells that proliferate into new tissue whenever muscle is injured. In young mice, these cells respond immediately to injury, but in old mice, the cells are sluggish. Rando’s team suspected that a chemical signal either triggers stem cells into action in young mice or inhibits the cells from working in old ones.
To determine whether such a signal is in blood, Rando’s group connected the circulatory systems of young and old mice. The scientists removed skin along the flanks of young mice, ages 2 to 3 months, and old mice, ages 19 to 26 months. They then sutured pairs of the mice together. The mice quickly adjusted to life as partners, cooperatively eating and roaming their cage. Within several weeks, each pair’s blood systems merged.
After giving each mouse a leg injury, the researchers found that among the young-old pairs, young mice healed more slowly than did young mice that hadn’t been joined and old mice showed significantly faster healing rates than old, solo mice did. Mice paired with a partner of similar age responded much as their unpaired counterparts did.
Rando says that these results, published in the Feb. 17 Nature, suggest that blood carries a signal that activates or inhibits stem cells. However, it’s still unclear whether old mice make a factor that retards healing or whether young mice make one that spurs healing.