Pigmented Rings On The Iris
The iris is the colored part which surrounds the pupil of the eye. Similar to those of finger prints, the patterns of the iris are very complex and uniquely identify each person. Examples of this the characteristics of the iris include the crypts which are the oval-shaped areas inside of the iris; pigmented rings which surround the iris; furrow contractions, which are radial and concentric bands around the pupil; and pigmented spots on the iris, also called nevi. The heritability of various types of iris patterns ranges between 70-95%, meaning that genetics contribute strongly to these traits.
A person´s iris (colored part of the eye) can say a lot about him/her. Like a fingerprints, iris patterns are highly complex and unique. For this reason, iris recognition is used in some countries for the national identification systems and to automate border crossings.
The human eye
The lens is supported by very fine conjunctive fibers which in turn are joined to the constrictor muscle of the ciliary body. The lens is formed during the third or fourth week of gestation. In children, the lens is bland and elastic but hardens and increases in size as we get older. In a 70 year old person, it is almost three times larger than in a baby.
The vitreous humor, found behind the lens, is a whitish-clear, gelatinous substance filling the central cavity of the eye. This mass is surrounded by the retina, the nerve layer lining the back of the eye. The retina senses light and creates electrical impulses that are sent through the optic nerve to the brain.
The zone that surrounds the optic nerve is the optic disc, an area that does not contain sensory nerve cells and makes up what is known as the blind spot. On the surface of the retina is a small yellowish area lying slightly lateral to the center of the retina that constitutes the region of maximum visual acuity —called also yellow spot.
The iris is the colored part of the eye and can be blue, green, brown, etc., defining the eye color. The pupil, black, is in the center of the iris and the white area around the iris is the sclera. The iris is located behind the cornea, between the anterior chamber and the lens, which it covers in greater or lesser manner depending on its dilation. This anatomical part of the anterior chamber of the eye is in constant motion, permitting the pupil to dilate (mydriasis) or contract (myosis) depending on the intensity of light it receives and regulating the amount of light which reaches the retina.
Each person has a different eye color, with particular nuances and unique shadings. Eye color is a genetic trait determined by the distribution of melanin in the iris by three elements: the melanin of the iris epithelium, the melanin of the anterior part of the iris and the density of the iris stroma. What is your eye color?
The function of the iris
The iris regulates the light that comes into the eye much like the diaphragm of a camera. The iris is comprised of muscular tissue and pigmented cells. If the sphincter muscle of the iris contract, the pupil becomes smaller and allows less light to enter the eye. If these muscles relax or expand, more light comes into the retina. The pigmented cells determine the eye´s color. The more pigment in the iris, the darker the color of the eyes will be. The color of the iris depends on the stromas transparency and the amount of pigmentation that it has. When there is less pigmentation, the eye color will be lighter, blue; when there is more pigmentation, the eye color will be darker and greenish or have brown nuances. This pigmentation is formed during the first months of a baby´s life which explains why all newborn babies have grayish blue eyes and the definite eye color does not occur until the second or third month.
Black colors of the iris do not exist as such as there is always a slight difference between the iris and the pupil. However, Aniridia is an eye disorder that makes the iris seem to disappear, that there is only pupil, giving the appearance of a completely black eye. People with this disorder have reduced vision and increased sensitivity to light (photophobia). They can also develop eye problems such as glaucoma, cataracts, macular degeneration, etc.
Approximately 60% of the people in the world have brown eyes (clearer or darker, they are all grouped in the same category). This color is due to the large amounts of melanin in the anterior part of the iris. The majority of Asians and Africans have this eye color and according to some studies, because it is a dominant gene, it will extend even more among the world population.
Brownish gold eyes
These can vary between yellowish to golden reddish or coppery color, or even greenish. Lipochrome or yellowish pigment is what makes this color exist. It is also found in green eyes. Although it is not very common, there is no genetic explanation for this color.
Amber eyes are of a yellowish gold color and result from a predominance of lipochrome in the iris. Like brownish gold eyes, there is no genetic explanation for this color. The difference between the two is that while an amber color is uniform in all of the iris, the brownish gold tone shows small clear color rings around the pupil. Although it is not very common (only 2% of the people in the world have this color eyes), it can be found in European countries.
Hazel color eyes falls between brown and green and may contain specks of amber, gold or green. They are more common than amber and usually tend to comprise many other colors, including green, brown and orange. Also, hazel eyes may appear to shift in color and consist of flecks and ripples, making it a very original and variable color.
Green eyes, despite their fame, are found only in about 4% of the population. This color is due to the moderate amounts of melanin in the iris and is an intermediate color between brown and blue. Even though eye color is determined by genetics, it is possible for a person to have light-colored eyes (blue or green) even if the parents have brown eyes. It is infrequent but not a rare color.
Although it is well known that there are no two iris the same, very little else is known of the underlying genetics for these differences. Researchers are studying this subject to learn more about the disorders related to the iris and to provide insights about the brain since the development of the iris and the brain seem to be connected. It is known that on the genetic level, for example, that a mutation tied to a loss in the iris is also associated with defects in the frontal lobe of the brain. Additionally, some characteristics of the iris are correlated to neurologic illnesses such a Down Syndrome.
Gene or region studied