Stress and gray hair have always been closely associated, and now, scientists from Harvard have discovered the physiological mechanism that validates this association. This new study from Harvard’s Department of Stem Cell and Regenerative Biology (HSCRB) was published yesterday in Nature. In a news release from HSCRB regarding the study, senior author Ya-Chieh Hsu said, “we wanted to understand if this connection is true, and if so, how stress leads to changes in diverse tissues. Hair pigmentation is such an accessible and tractable system to start with — and besides, we were genuinely curious to see if stress indeed leads to hair graying.”
The first step of this study was to narrow down which body system connected stress to hair color. Using mice, the team started with an initial hypothesis that stress led to an immune attack on pigment-producing cells. When that proved false, they turned to the hormone cortisol, which is typically elevated under stressful conditions. This also led to a dead end.
The researchers eventually discovered that the sympathetic nervous system—the body’s fight or flight response center—was the responsible party. In a hair follicle, some stem cells are a reservoir of pigment-producing cells responsible for hair color. According to the study, the sympathetic system nerves branch into hair follicles, and stress causes these nerves to release norepinephrine, which is taken up by nearby pigment regenerating stem cells. Excess norepinephrine from stress-induced cues depletes the natural reservoir of pigment-producing cells responsible for hair color. The authors state that once these cells have been depleted, they do not regenerate. Lead author Bing Zhang says that this is a “permanent depletion of stem cells caused by acute stress.”
The research team considers this study an essential exploration of the impact that stress has on body systems, organs, and tissues. In an online article from Time, Hsu stated that we don’t know enough about how stress affects cell biology and tissue turnover. She said that hair color was an easy starting point, “but different stem cells and different organs may respond to the signals of the sympathetic nervous system very differently, and we don’t know exactly how yet.”