Stem cell exhaustion is not depletion—it is niche signaling failure

We think old tissues lack stem cells because they died out. But stem cells persist in aging tissues—they just don't activate.

The niche (local environment) loses the signals that trigger stem cell division. Not enough growth factors, too much inflammation, stiffened ECM.

It's not that stem cells are gone. It's that they can't hear the call to divide. The phone works, but the line is busy.