Step into the sun and it's not just your eyes that sense the light. Your skin contains photoreceptors like those in the retina, allowing it to mount an immediate defence against damaging ultraviolet radiation.
The skin tans by producing melanin, a pigment that protects DNA from dangerous UV rays. We already know that one component of the UV rays – called UVB – ramps up melanin production a few days after sun exposure in response to DNA damage in the skin. But another component of the radiation – UVA – encourages pigment production within minutes.
Elena Oancea and colleagues at Brown University in Providence, Rhode Island, think they know why. They analysed the genes expressed in melanocytes, the cells responsible for melanin production, and discovered that the cells also make rhodopsin, a light-sensitive chemical found in the retina.
When the team shone UVA light on melanocytes, they saw melanin production rise. Within 24 hours there was a five-fold increase in the production of the pigment. Knocking out the gene responsible for making rhodopsin in melanocytes blocked this immediate response to UVA.
According to Oancea, the immediate tanning, although modest, may help to protect the skin against early DNA damage.
Rick Sturm at the University of Queensland in Brisbane, Australia, isn't convinced that "seeing" the light offers the skin much protection against damage, though. "Immediate tanning does not protect against UV-induced sunburn or DNA damage," he says.
