QUESTION: One of my students had a question, and I didn't know the answer so I thought I'd ask you. Why does the sun lighten our hair but darken our skin? And how come sometimes if you get a REALLY bad sunburn, it leaves you with a "permanent tan?" (from Jen, a teacher at Escondido Charter High School, Escondido, CA)
ANSWER: Beats me! I was puzzled, too, so I asked an expert.
Professor Walter Quevedo, a leading expert on the biology of the skin, explained it to me this way:
In the case of sun-induced bleaching of eumelanic (dark) black hair, hydrogen peroxide and superoxides (free radicals) induced by the UV component of sunlight,act to disrupt the melanin "granules" eventually to a degree where the products of disruption are no longer black. That lightens the shade of hair color. Remind your studentthat the hair is dead tissue, so those melanin granules, once bleached, cannot be replaced.
Unlike hair, the skin is alive, The melanocytes of the epidermis are stimulated to produce greater numbers of melanin "granules" (melanosomes that actually are much more complicated than granules). The melanosomes are transferred in increased numbers from the melanocytes to keratinocytes of the epidermis causing the darkening (tanning) of the skin.
The persistent darkness of sunburned ski, results from an inflammatory response (reddening )induced by UV light. It is a complicated process which ultimately results in the destruction of damaged components of the skin and their replacement. Various agents that produce the inflammation as well as growth factors produced by keratinocytes, act on melanocytes,as repair processes go on, to produce more melanin "granules" darkening the repaired epidermis. The elevation of melanin production may keep the skin dark for long periods following "healing" because the stimulating agents remain elevated for long periods.
I hope this helps to answer your student's excellent question!
Ken Miller (10/14/03)
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(A web site developed by Ken Miller and Joe Levine to provide scientific and educational support for teachers and students using our textbooks)