Marie Antoinette’s hair turned white overnight, according to folklore, before she was executed by guillotine in 1793 during the French Revolution.
The ill-fated queen embodied an extreme example of the phenomenon of stress-induced greying of the hair.
The biological mechanism behind such greying had long remained a mystery.
But researchers said on Wednesday they have figured out how it happens: It is driven by the body’s “fight or flight” response to danger.
The researchers used mouse experiments to look at how stress affects the stem cells in hair follicles that are responsible for making melanocytes, the pigment-producing cells that give hair its colour – black, brown, blonde, red or somewhere in between.
People generally have around 100,000 hair follicles on their scalp.
The researchers initially suspected that an stress-induced immune attack might be targeting the melanocyte stem cells, but that hypothesis did not pan out.
They then explored whether the hormone cortisol, elevated under stress, might be the culprit, but it also was a dead end.
Instead they found that the body’s sympathetic nervous system, which governs the mammalian “fight or flight” response to danger, played a central role.
It comprises a network of nerves that go everywhere including the skin, in which they are like ribbons wrapping around each hair follicle and are very close to the melanocyte stem cells.
When mice were subjected to short-term pain or placed in stressful laboratory conditions, these nerves released the chemical norepinephrine, which was then taken up by the stem cells in the hair follicle that serve as a finite reservoir of melanocytes.
“Normally, when hair regenerates, some of these stem cells convert into pigment-producing cells that colour the hair,” said Ya-Chieh Hsu, associate professor of stem cell and regenerative biology at Harvard University and a Harvard Stem Cell Institute principal investigator.
“But when they are exposed to norepinephrine from the sympathetic nerve, all of the stem cells are activated and convert into pigment-producing cells.
“That means there are none left.
“In just a few days, the reservoir of pigment-regenerating stem cells is depleted. And once they’re gone, you can’t regenerate pigment any more,” added Professor Hsu, senior author of the research published in the journal Nature.
Hair greying is one of the many ways that stress exacts a toll on the body.
The findings could guide development of treatments for stress-related greying, or potentially other stress-related changes in tissues, though this could take years, she said.