Senescent cells are often referred to as “zombie cells,” because even though they should be dead, they refuse to die. In the petri dish and in frozen, thinly sliced tissue sections, we can stain zombie cells blue because they make a rare enzyme called beta-galactosidase, and when we do that, they light up clearly. The older the cells, the more blue we see. For example, a sample of white fat looks white when we are in our 20s, pale blue in middle age, and dark royal blue in old age. And that’s scary, because when we have lots of these senescent cells in our bodies, it’s a clear sign that aging
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