The mice, which had been engineered to mimic a human disease called progeria, would normally have grown old when they were quite young. But that changed when researchers injected muscle stem cells from healthy young mice into the bellies of the quickly aging mice. Within days, the doddering and frail mice began to act like they were living the storyline of “The Strange Case of Benjamin Button” as they started looking and acting younger.

“It was mind boggling,” said study co-author Johnny Huard, a professor of orthopedic surgery at the University of Pittsburgh School of Medicine. “When I saw them I thought, ‘Oh my God, I must have made a mistake and put the normal mice in the wrong cage.’ But they were indeed the mice we’d injected with the stem cells.”