The HHMI Bulletin looks at cellular senescence. There's a PDF version for those who don't want to click through six pages to read the whole thing: "cells might not sprout gray hair, get achy joints, or forget where they put their car keys, but they do age. ... researchers are just beginning to learn what happens to cells as they grow old, and they're making connections between those changes and cancer, deficiencies in wound healing, and other problems that increase in likelihood as a person ages. ... In a sense, cancer and senescence are opposite sides of the same coin. To remain robust, tissues rely on dividing cells for replenishment; yet, left unchecked, cell division leads to cancer. Thus, it might seem a Faustian bargain to guard against cancer now at the expense of decrepit tissues later. ... senescent cells produce an enzyme called beta galactosidase. When bathed in a particular sugar compound, cells with this enzyme turn blue, providing a way to spot senescent cells. Since then, researchers have used this method to show that tissues from people, as well as from animals such as rodents and monkeys, carried blue cells. And, 'the older you got the more blue cells you had.'"