I'm looking at a picture of two mice. The one on the right looks healthy. The one on the left has graying fur, a hunched back, and an eye that's been whitened by cataracts. “People ask: What the hell did you do to the mouse on the left?” says Nathaniel David. “We didn't do anything.” Time did that. The left mouse is just old. The one on the right was born at the same time and is genetically identical. It looks spry because scientists have been subjecting it to an unusual treatment: For several months, they cleared retired cells from its body.
Throughout our lives, our cells accumulate damage in their DNA, which could potentially turn them into tumors. Some successfully fix the damage, while others self-destruct. The third option is to retire—to stop growing or dividing, and enter a state called senescence. These senescent cells accumulate as we get older, and they have been implicated in the health problems that accompany the aging process.
