Mice bred to age rapidly lived longer when injected with stem cells from younger, healthier mice, a new study published in the journal Nature Communications has found.

“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,” said study co-author Johnny Huard, professor of orthopedic surgery at the University of Pittsburgh School of Medicine.

The mice were engineered to have progeria syndrome, a condition that causes accelerated aging. Mice with progeria normally die when they are about 21 days old. Once injected with muscle stem cells from healthy young mice, the quickly aging mice started looking and acting young. Some lived over 66 days.

Researchers theorize that as stem cells age, they lose the ability to self-repair and these ‘tired stem cells’ divide slowly.

The brains of the injected mice revealed new blood vessel growth, indicating the stem cells must be spurring growth of healthy cells, Huard said.