youth transfusion

Harvard Magazine:

Using mice that have been surgically joined so that their bloodstreams become shared, Amy Wagers investigated whether the blood of a young animal might awaken the muscle stem cells in an old one and enhance muscle repair.

…when a young and an old mouse were joined, and a leg of the old mouse was injured, the healing was rapid: new muscle formed almost as well as in a young animal.

Something from the young mouse—an unknown factor circulating in the blood—was reaching muscle stem cells in the old mouse and turning on the biological machinery of repair.