The colored part of the eye that surrounds the pupil is the iris. It is a contractile membrane situated behind the cornea and in front of the lens and which dilates or contracts, opens or closes, depending on the intensity of light in the environment.
These contraction-relaxation movements of the eye are caused by two muscular groups: the iris sphincter muscle, a circular muscle that controls myosis (reducing the size of the pupil) and the iris dilator muscle which controls mydriasis (increasing the pupil´s aperture). The function of the iris is to regulate the amount of light that enters the eye and reaches the retina.
A person´s eye color depends on genetics as well as a general state of well-being.
Color is determined by the first two layers of the iris -the epithelial layer and the stroma- thanks to the distribution of the pigment responsible for color: melanin, found in the cells called melanocytes. The number of melanocytes is the same for all eyes but the amount of melanin of each cell and its distribution within the epithelium and the stroma varies depending on eye color. So, if there is a greater amount of melanin, the eye will be darker and with less, lighter.
This melanin is the same pigment as that in our skin and hair. Therefore part of what we inherit is, to a certain extent, related to color (persons with dark skin have more probability of having dark eyes).
It must be pointed out that melanin is only brown, a very dark brown, which is why we can explain the large variety of colors in human eyes thanks to the distribution of the collagen in the stroma, the melanin in the melanocytes and the nerve cells and blood vessels. Additionally the thickness of the different layers also affects the color. When the light passes through, it disperses and the result is the color we see.
This peculiar distribution of melanin in the iris is caused by a mixed inheritance or polygenics; i.e., several genes intervene at the same time. Dark colors are more predominant than light, but due to the intervention of several genes at the same time, it is not impossible that two parents with dark eyes have a child with light ones.
The EYCL1, EYCL2 and EYCL3 genes directly intervene but there are many other genes with influence the inheritance of eye color. Besides this large genetic mixture, there is also a random component in the shades of color and forms produced around the iris.
The most frequent different eye colors are:
- Brown eyes: the most common color in humans. These have a large amount of melanin distributed more or less evenly, depending on the shade, on the epithelial layer. They are linked to brown or black hair and dark skin.
- Black eyes: these are in reality, very dark brown. Truly black eyes are due to aniridia, an eye disorder (incipient iris or underdeveloped iris). The external appearance of aniridia is a large black central pupil and in many cases with a small colored band.
- Honey colored eyes: a color that is intermediate between light brown and green. It is quite typical in some European countries.
- Green eyes: these have less melanin spread more or less evenly. They are associated with blond, brown or red hair.
- Blue eyes: the quantity of melanin is not excessive. The distribution of this color by countries is quite varied, being most frequent in northern European countries. It is a color that is very predisposed to ocular diseases.
- Gray eyes: the color range that has the least amount of melanin.
Heterchromia: Heterchromia iridium is the condition in which one iris is a different color from the other iris and heterchromia iridis is the condition in which the iris has two distinct colors in the same eye. This is much more frequent in humans.
The causes for these situations is varied although in most cases it is an essential disorder, that is, a person is born with it and as it does not affect vision, it is not important. In this case, the difference in the color of both eyes or within one does not progress through life and ocular function is normal, the condition being merely an anecdote. The basis for this curious anomaly can be hereditary or spontaneous (idiopathic), by a series of alterations during the embryonic stage in cells called melanocytes, which are responsible for eye color as well as in skin color. The amount and distribution of melanin in these cells will be what determines the eye color of each person.
Some acquired factors which can lead to heterchromia of the iris include trauma, the presence of extraneous objects, the use of certain eyedrops, melanoma and Horner syndrome, among others. Nevertheless, if a change in eye color not previously present or abnormal ocular function is noted, it is important to see an ophthalmologist to evaluate the possibility of an illness.
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