Many of our patients at Georgia Eye Physicians and Surgeons wonder why different people have different color eyes, and if those eye colors have any underlying medical significance, particularly as they may relate to the development of certain vision affecting conditions, like cataracts. While the underlying genetic basis that determines eye color can be extremely complex, allowing for the whole range of eye colors to manifest even within the same nuclear family, the presence of color in the iris is essentially the result of melanocytes (cells found within the iris of human eyes, as well as skin and hair follicles). These melanocytes slowly begin to produce melanin, the dark brown pigment responsible for the tanning of skin that helps protect against harmful ultraviolet light. The amount of melanin produced, along with the presence of other pigments in varying concentrations, determines the color of the eyes.
In simple terms, think of the iris as having three layers, a thin layer of pigmented cells at the front and the back, and a layer of clear liquid in the middle. Brown eyes (which occur in more than 55% of the world’s population) have a relatively high concentration of melanin in the stroma of the iris, which causes light of both shorter and longer wavelengths to be absorbed and makes the eyes appear darker shaded. Blue eyes, on the other hand, have little to no melanin in the stroma at the front of the iris, but a dark epithelium at the back. This means that the longer wavelengths of light (the red to yellow end of the spectrum) tend to be absorbed by the dark underlying epithelium, while shorter wavelengths (blues and greens) are reflected. As the reflected light passes through the fluid-filled middle layer, it scatters and diffuses, making the entire iris appear blue.
Other colors in the eyes are the result of varying concentrations of pigments. Amber eyes, for example, have a strong concentration of the yellow pigment in the iris, making them appear golden. Green eye color, however, is a result of a mild amount of pigmentation in the eye with a golden tint which, when combined with the natural blue scattering of the eye, gives the eye a green appearance. Even in those with the lightest blue eyes there is normally a thick, dark colored layer of melanin on the back of the iris, to prevent light from scattering around inside the eye. However, those with severe forms of albinism have no pigment on the back of the iris, and light from inside the eye can pass through the iris to the front freely. In these cases, the only color visible is the red from the blood vessels at the back of the retina, giving the clear iris a pink or reddish tint.
Because the melanin in the eyes, just like the melanin in the skin, helps protect against the damage that can be caused by ultraviolet light, those with lighter colored eyes should be particularly careful to keep their eyes protected from the sun. Those with lighter iris color have been found to have a higher prevalence of age-related macular degeneration (ARMD) than those with darker iris color. However, a study conducted in 2000 suggests that people with dark brown eyes are at increased risk of developing cataracts and so should protect their eyes from direct exposure to sunlight as well.
If you have questions about the function or diseases of the eye, or would like to schedule an appointment for an eye exam, please contact Georgia Eye Physicians and Surgeons today to make an appointment. Be sure to follow us on Facebook, Twitter, and Google+ for more tips for healthy eyes.