While the New York prosecutor's case against Dominique Strauss-Kahn is near collapse, a second accuser intends to press attempted rape charges against him in France. As lawyers argue over DSK's guilt or innocence, the Explainer is wondering about something far less consequential: How come he has such shock-white hair on his head, while his eyebrows are bushy black?
Dopachrome tautomerase, of course. The graying of hair arises from the fact that human cells are constantly at war with oxidizing compounds. These eventually rob a hair follicle of its ability to grow new pigment cells and turn a black strand white. To stave this off, follicles produce protective enzymes that take the edge off of the oxidants. The varying pace of graying across the body isn't well-understood, but a theory has begun to emerge. Researchers have noted that eyebrows have high levels of a chemical called dopachrome tautomerase, a protective enzyme that is undetectable in scalp follicles. Over time, this enzyme and others like it may have insulated DSK's eyebrow follicles from the weathering effects of stress, while his scalp was unprotected. This remains an active area of research, however, and the protective enzyme theory has not been proved. Scientists are still identifying enzymes, and there may be unique combinations present in hair on the scalp, genitals, chest, and chin. In addition, while different types of hair gray at different rates, it's entirely possible that Strauss-Kahn is coloring either his head hair or eyebrow hair to maintain his look, much like Mexican singer Vicente Fernandez, who is famous for his jet black mustache and eyebrows.
Variations in follicular chemistry across the body aren't particularly surprising. Hair on the scalp clearly reacts differently to androgens, the hormones involved in balding. That's why many people, like Fed Chairman Ben Bernanke, are afflicted with male pattern baldness but can grow distinguished beards. (Fed watchers will also note that Bernanke's remaining scalp hair has retained more pigment than his white beard, demonstrating the endless complexity of graying patterns.) There may even be differences in follicles across the scalp, as many men gray in the temples before the top or back of the head.
While researchers have been observing these quirks for decades, they've just begun to identify some molecular explanations. Hair follicles go through cycles of growth and rest. At the end of each cycle, the hair shaft falls out and certain parts of the follicle die, only to be replaced by new parts derived from stem cells. At a certain point, however, a person loses his or her reserve of melanocyte stem cells—the ones that turn into the pigment-producing parts of a follicle. Dermatologists aren't entirely sure what causes this to happen (and why other types of follicular stem cells are more resilient). We do know that the process varies across racial groups: Caucasians typically start noticing gray scalp hairs in their mid-30s, Asians in their late 30s, and Africans not until their mid-40s.
Some cancer patients experience a sort of reverse Strauss-Kahn as a result of treatment. Certain cancer drugs help the immune system attack cancerous skin cells, but also tend to go after the hair pigmentation machinery. As a result, the patients often develop gray hair, especially in the eyebrows.
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Explainer thanks David Fisher of Harvard Medical School and Massachusetts General Hospital.