When stress triggers a chemical called norepinephrine, it can permanently wipe out the stem cells that give hair its color, so once they're gone, your hair can't naturally regain its color.

Stress can turn your hair gray by messing with nerve signals in your hair follicles, which damages the cells that give hair its color — and once they're gone, the graying is permanent.

As we get older, the stem cells that keep our hair colored might stop working properly because of problems in their internal repair system, and once they're gone, our hair turns gray for good.

Your hair color depends on special stem cells in your hair roots, and long-term stress from inflammation or oxidation can mess them up, making them disappear or mature too soon.

As we age, the stem cells that give color to our hair can start acting up — they might turn into pigment cells too soon or in the wrong place, which could be why our hair turns gray.

In mouse hair follicles, pigment cells in the hair germ area actively make new pigment cells and stem cells, while those in the bulge stay quiet and only make more stem cells.