Enlarge Image Young at heart. Cross-sections of mouse ventricles show the visible change in size when old hearts are immersed in young blood. Credit: Francesco Loffredo It's time to turn back the clock on an aging ticker. Drawing on an odd experimental technique invented more than a century ago but rarely done now, researchers have found that a blood-borne protein makes old mouse hearts appear young and healthy again. It's not clear yet whether humans would react the same way, but scientists are hopeful that this discovery may help treat one of the heart's most frustrating ailments. "This is probably...