Humans can tan easier and faster than previously thought, according to a recent paper from the Department of Molecular Pharmacology, Physiology and Biotechnology. Using the same mechanisms the eye uses to sense light, human skin can "see" ultraviolet light, almost immediately inducing tanning.
When UV light hits human skin, it triggers the accumulation of calcium in the skin cells. This build-up leads to higher levels of melanin, the chemical that causes the pigmentation of the skin, giving skin a darker tint known as a tan.
Scientists previously thought it took several days to produce a tan after exposure to UV rays, and Elena Oancea, assistant professor of medical science and lead author of the report, said her team encountered significant resistance from peer reviewers because the results were so unexpected. The five authors were forced to repeat their results to reduce doubts, Oancea said.
Melanocytes, long-lasting skin cells, have the same proteins as the eye's retina that detect light. But when these proteins in skin cells sense UV, they initiate a very different response from their cousins in the eye. "The tan is actually a self-defense mechanism," Oancea said. UV light elicits the production of melanin in skin cells because it absorbs sunlight and protects the cell's DNA from damage.
Despite the apparent health benefit of tanned skin, Oancea denies that attraction to tanned people might have any evolutionary significance. "Attractiveness is man-made," she said. "If magazines showed fair-skinned models … people would find that type of skin more attractive."
The researchers treated a tissue culture of human skin cells with a fluorescent calcium dye so the sample would glow in the presence of calcium. Oancea and her team, which included Jonathan Ciriello '15, shined UV light onto the tissue culture and were shocked when in just 10 to 20 seconds the cells lit up, Oancea said.
"The results were surprising," she said. "It was a great feeling."
The researchers noticed that only UVA light activated the chain of events that results in tanning. While UVA light makes up the vast majority of UV light, 5 percent is UVB — the carcinogen that causes melanoma. "The presence of UVA helps protect against UVB," Oancea said. Melanocytes can stay in the body for 10 years, so they need to protect themselves. A mutation in a melanocyte can lead to melanoma, a particularly deadly cancer.
Humans are the only species that tan, which makes animal trials difficult. The uniqueness of human skin cells limits researchers' ability to test the implications of these findings, Oancea said. Humans presumably developed this function after losing body hair. Most other animals have body hair, so the sun poses less of a threat to their skin. Researchers at other labs are working on developing mice genetically modified to have human skin, Oancea said.
