Legend has it that Marie Antoinette's hair turned white the night before she was guillotined. Presumably the stress of impending decapitation caused her locks to lose color within hours. Extremely unlikely, scientists say, but stress may play a role in a more gradual graying process.

The first silvery strands usually pop up around age 30 for men and age 35 for women, but graying can begin as early as high school for some and as late as the 50s for others.

Graying begins inside the sunken pits in the scalp called follicles. A typical human head has about 100,000 of these teardrop-shaped cavities, each capable of sprouting several hairs in a lifetime. At the bottom of each follicle is a hair-growing factory where cells work together to assemble colored hair. Keratinocytes (epidermal cells) build the hair from the bottom up, stacking atop one another and eventually dying, leaving behind mostly keratin, a colorless protein that gives hair its texture and strength. (Keratin is also a primary component of nails, the outer layer of skin, animal hooves and claws?even rhinoceros horns.)

As keratinocytes construct hairs, neighboring melanocytes manufacture a pigment called melanin, which is delivered to the keratinocytes in little packages called melanosomes.

Hair melanin comes in two shades—eumelanin (dark brown or black) and pheomelanin (yellow or red)—that combine in different proportions to create a vast array of human hair colors. Hair that has lost most of its melanin is gray; hair that has lost all of this pigment is white.

At any given time, around 80 to 90 percent of the hairs on a person's head are in an active growth phase, which may last anywhere from two to seven years. At the end of this stage, the follicle shrivels, the keratinocytes and melanocytes undergo programmed cell death (apoptosis), and the follicle enters a resting phase, during which the hair falls out.

To begin building a new hair, the follicle factory must be rebuilt. Fresh keratinocytes and melanocytes are recruited from progenitor cells, also called "stem cells," residing at the bottom of the follicle. For unknown reasons, keratinocyte stem cells have a much greater longevity than the melanocyte stem cells, says David Fisher, professor of pediatrics at Harvard Medical School. "It's the gradual depletion of [melanocyte] stem cells that leads to the loss of pigment," he says.

Does stress accelerate this demise of the melanocyte population? "It is not so simple," Fisher says, noting that the process of graying is a multivariable equation. Stress hormones may impact the survival and / or activity of melanocytes, but no clear link has been found between stress and gray hair.

Suspicions—and hypotheses—abound, however. "Graying could be a result of chronic free radical damage," says Ralf Paus, professor of dermatology at the University Hospital Schleswig-Holstein in Lübeck, Germany. Stress hormones produced either systemically or locally (by cells in the follicle) could produce inflammation that drives the production of free radicals—unstable molecules that damage cells—and "it is possible that these free radicals could influence melanin production or induce bleaching of melanin," Paus says.

"There is evidence that local expression of stress hormones mediate the signals instructing melanocytes to deliver melanin to keratinocytes," notes Jennifer Lin, a dermatologist who conducts molecular biology research at the Dana-Farber / Harvard Cancer Center in Boston. "Conceivably, if that signal is disrupted, melanin will not deliver pigment to your hair."

And general practice physicians have observed accelerated graying among patients under stress, says Tyler Cymet, head of family medicine at Sinai Hospital in Baltimore, who conducted a small retrospective study on hair graying among patients at Sinai. "We've seen that people who are stressed two to three years report that they turn gray sooner," he says.

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