Melanocytes are specialized cells that contain the pigment found in our hair skin, eyes, and other area of the body. The eye has melanocytes in the iris, retina, and the choroidal layer under the retina that supplies it with blood. Underneath the retina is a layer of cells referred to as the retinal pigmented epithelium. These cells interact with the photoreceptor cells, the rods and cones that register light you see and turn it into electrical impulses. Melanin is the pigment and it comes in two different forms. In the retina, melanin acts as an antioxidant to help protect the tissue from free radicals that can damage the cellular DNA.
Various detrimental influences can reduce this protective function and may even cause it to accelerate damage from free radicals. Ultraviolet exposure, blue visible light, and high levels of oxidative stress can cause cell damage and increase the rate of cell death. An article by Meyskens FL, Farmer P, Fruehauf JP suggested that this may be a contributory factor in macular degeneration and choroidal melanomas.
Melanocytes also populate our skin where they protect the underlying levels from ultraviolet damage. UV-B exposure is an essential step in producing the active form of vitamin D. Darker skinned intervals have more UV screening and subsequently are more at risk for low levels of vitamin D. When you wear highly protective sunscreen the same effect occurs, lower levels of UV are absorbed by the skin and the proactive form of Vitamin D does not get its needed UV-B exposure to form. It is a very narrow band of UV-B that is needed, not the entire spectrum.
Vitamin D degeneration is related to multiple forms of cancer and other health problems. Macular degeneration may have an association with lowered vitamin D. One possibility is the overall reduction in UVB on the skin from sunscreen, living indoors, and poor dietary habits results in systemic changes in vitamin D levels that precipitates cellular changes in the retina unrelated to the melanin in the eye. Another possibility is lowered systemic levels of vitamin D resulting in decreased melanin production in the retina. This could lead to an overload to the pigmented epithelial cells from oxidative stress and UV exposure inside the eye, degrading the functional capacity of the melanin to protect the retina.
The irony is UV exposure is a risk factor for cataracts and macular degeneration and we are always encouraging eye protection against UV (and rightfully so given today’s evidence). With the known association of skin cancer and UV exposure it is not prudent to drop recommendation for sunscreen and limiting sun exposure.
At some point there will be better answers. For now, some mild daily exposure to UV with eyewear that includes UV protection is something you should discuss with your eye doctor and dermatologist or family physician. Vitamin D supplementation during winter months and in geographic locations that get limited sunlight should also be considered with your healthcare providers. Perhaps sunscreen lotions will be developed that allow the narrow band of UV-B needed to pass through in the future. Many people have vision insurance coverage like Vision Service Plan that provides coverage for eyeglasses. Even if your prescription is minor, having protection against UV and visible blue light is good preventative medicine if you spend time outdoors. And if you spend six months on a submarine or live underground you should examine your options-and maybe see a psychologist!
Reference:
Meyskens FL, Farmer P, Fruehauf JP (June 2001). “Redox regulation in human melanocytes and melanoma”. Pigment cell research / sponsored by the European Society for Pigment Cell Research and the International Pigment Cell Society 14 (3): 148-54. doi:10.1034/j.1600-0749.2001.140303.x. PMID 11434561.