It's no surprise that chameleons can change colors to pink, blue, orange, red, and black. These color changes are partly mediated by stress. For example, stress increases dark gray pigmentation of tawny dragon lizards. That same stress and color change relationship also applies to human hair. Hair graying has long been associated with increased stress and aging. But little actual evidence as proven science behind this observation. 

A team of scientists from Harvard University have now explained this mystery. 

For hair follicles to grow, they go through three main phases: growth, degeneration and inactivity. The growth phase activates two population of stem cells, one called hair follicle stem cells, or HFSCs, and another called melanocyte stem cells, MeSCs. Activation of HFSCs produces hair follicles. Activation of MeSCs produces fully formed melanocytes, which migrate to the base of the hair follice and make the melanin that colors hair. Melanocytes die and degenerate, and the cycle repeats with a new cohort of melanin-producing cells. 

But where in this cycle does stress play a role?

When researchers caused pain-induced stress in rats, it triggered a fight or flight response, which in turn increased production of noradrenaline. Noradrenaline, generally functions to increase action and attention in the mind and body and binds to the surface of MeSCs. This stress effect accelerated the MeSCs growth cycle, pushing the cells to become melanocytes and possibly causing them to migrate elsewhere.

After the end of a cycle of stressed hair follicle growth , remaining MeSCs were also reduced. Interestingly, decreased MeSCs also occur with age. That effect on MeSCs resulted in less mature melanocytes and thus less pigmentation in subsequent hair follicle growth cycles. 

After years of empirical evidence, it is exciting to understand the role MeSCs may play in stress and if it contributes to the accelerated ageing process.  

Ed.: Originally this article was published with an image of Claire Saffitz as an illustration of someone with gray hair. It was pointed out to me that this is an unfair and hurtful use of a person's appearance. I apologize for the error and have changed the photo to a stock image. -DS