When hair turns grey with age, it is because the body has started to lose the pigments that give hair its colour. What was unclear so far, however, was why hair sometimes turn grey prematurely, and suddenly. This is called the Marie Antoinette Syndrome; the French queen’s hair is said to have turned grey overnight at age 38, before her execution in 1793. Many people also have personal anecdotes of stress causing the sudden greying of hair.
Now, science may have finally found out the reason why. In a new study in the journal Nature (shorturl.at/fFL69), Harvard University researchers describe the mechanism that causes greying under stress. In short: stress activates certain nerves, which permanently damages stem cells that generate pigments in hair follicles.
Experimenting with mice, the researchers looked at and eliminated various possible reasons. One hypothesis was that stress causes an immune attack on pigment-producing cells, but the researchers found that mice without immune cells, too, showed hair greying. Experiments with the hormone cortisol, too, reached a dead end.
The researchers eventually found the answer in the nerves that cause the “fight-or-flight response”. This refers to the way the body reacts to a terrifying event or a threat. It is marked by the release of hormones that prepares the body to either face the situation or run away.
The nerves that cause this response are called the sympathetic nerve system.
So, what happens?
The researchers found that stress causes sympathetic nerves to release a chemical called norepinephrine. This turned out to be the culprit.
In the hair follicle, certain stem cells act as reservoirs of pigment-producing cells. When hair regenerates, some of the stem cells convert into pigment-producing cells.
When stress causes sympathetic nerves to release norepinephrine, the researchers found that the chemical, in turn, causes the stem cells to activate excessively. All the stem cells then convert into pigment-producing cells, and this depletes the reservoir.
“After just a few days, all of the pigment-regenerating stem cells were lost. Once they’re gone, you can’t regenerate pigments anymore. The damage is permanent,” said senior author Ya-Chieh Hsu, stem cell and regenerative biologist at Harvard.
Mice and humans
There are several reasons why scientists use mice for laboratory experiments. Mice are easy to house, easy to handle, and are often considered ideal for research with implications for humans, because they resemble humans closely in genetic and biological characteristics, and in behaviour. But how sure can one be that the human body will replicate the stress response mice showed leading to the greying of hair?
“This is an area that needs more research,” Hsu said by email, when asked this question. “While we are hopeful the relationship will be close, we do not have definitive evidence at this point. The reason we’re hopeful the mechanisms are related is that both of these systems (pigment-producing stem cells and sympathetic nerve) are very similar in mice and humans,” she said.
The bigger picture
The researchers are hopeful that their findings can have wider implications.
“Our study shows the impact of stress on stem cells— and we wonder if other stem cells in the body may be affected, too. This research is critical to helping scientists understand how stress affects stem cells in the body, and how stress affects different tissues. We hope our work will help other researchers look into this important but understudied area. Understanding how stress affects stem cells is essential to developing future interventions that are both safe and effective,” Hsu said.
“To be clear, this is a fundamental discovery about the biology of stress and stem cells, not treatments at this moment.”
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