Parts of the Eye
The eye contains pigment in two areas. Iris pigment epithelium (IPE) is the layer closest to the back of the eye; cells in the IPE contain pigment that impacts eye color. The top layer of the eye, the iris stroma, also contains pigmented cells, called melanocytes, that synthesize pigment. IPE cells tend be orderly, stacking like stones to form a wall, whereas the cells on the stroma are less orderly arranged. Color is defined by the pigment in IPE as well as the pigment and cellular density in the stroma.
Baby Development
Babies' eye color changes as the pigment in their eyes develops. Babies of African or Asian descent are typically born with brown eyes that may become a deeper brown as they mature. Caucasian babies are born with blue or dark blue eyes; their eye color is determined by the genetic background of their parents. Color stabilizes by 9 months of age, though changes can occur until the age of 3. Eyes will not get lighter or more blue. Green eyes appear lighter than blue, but they actually contain more pigment than blue eyes.
Medication and Disease
Eye color usually doesn't change once it has stabilized. A color change indicates either an underlying medical issue or medication side effect. Pigmentary glaucoma causes sight loss and makes the eyes appear cloudy. Fuch's heterochromic iridocyclitis typically affects only one eye and impedes vision when the iris stroma atrophies. Horner's Syndrome is a rare disease that affects the eye and facial nerves. It is diagnosed if there is a variation or lack of color, typically occurring in children under the age of 2 due to genetic issues. Prostaglandin analog medications cause eye color change as they mimic the hormone. The reason for the change in color is not entirely clear; however, a study of rabbits lead by Gui-Lin Zhan and published in the August 1998 issue of Archives of Opthalmology suggests that darkening is a mechanism used by the body to compensate for the lack of sympathetic nervous response in the eye.
Aging also alters eye color; 10 to 15 percent of the Caucasian population with light eyes experiences a change after adolescence. In these cases, eyes become lighter as the person ages.
