Old age is a massacre,” Philip Roth wrote. But in truth it is a maceration—the steady grind of injury upon injury, the unstoppable decline of function into dysfunction, and the inexorable loss of resilience. Humans counter this decline by two overlapping processes—repair and rejuvenation. By “repair,” I am referring to the cellular cascade that begins upon injury. It is typically marked by inflammation, followed by the growth of cells to seal the damage. “Rejuvenation,” on the other hand, refers to the constant replenishment of cells, typically from a reservoir of stem or progenitor cells, in
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