Iris (anatomy)


In humans and most mammals and birds, the iris is a thin, annular structure in the eye, responsible for controlling the diameter and size of the pupil and thus the amount of light reaching the retina. Eye color is defined by that of the iris. In optical terms, the pupil is the eye's aperture, while the iris is the diaphragm.

Structure

The iris consists of two layers: the front pigmented layer known as a stroma and, beneath the stroma, pigmented epithelial cells.
The stroma is connected to a sphincter muscle, which contracts the pupil in a circular motion, and a set of dilator muscles which pull the iris radially to enlarge the pupil, pulling it in folds.
The circle circumference sphincter constricting muscle is the opposing muscle of the circle-radius dilator muscle. The iris inner smaller circle-circumference changes size when constricting or dilating. The iris outer larger circle-circumference does not change size. The constricting muscle is located on the iris inner smaller circle-circumference.
The back surface is covered by a heavily pigmented epithelial layer that is two cells thick, but the front surface has no epithelium. This anterior surface projects as the dilator muscles. The high pigment content blocks light from passing through the iris to the retina, restricting it to the pupil. The outer edge of the iris, known as the root, is attached to the sclera and the anterior ciliary body. The iris and ciliary body together are known as the anterior uvea. Just in front of the root of the iris is the region referred to as the trabecular meshwork, through which the aqueous humour constantly drains out of the eye, with the result that diseases of the iris often have important effects on intraocular pressure and indirectly on vision. The iris along with the anterior ciliary body provide a secondary pathway for aqueous humour to drain from the eye.
The iris is divided into two major regions:
  1. The pupillary zone is the inner region whose edge forms the boundary of the pupil.
  2. The ciliary zone is the rest of the iris that extends to its origin at the ciliary body.
The collarette is the thickest region of the iris, separating the pupillary portion from the ciliary portion. The collarette is a vestige of the coating of the embryonic pupil. It is typically defined as the region where the sphincter muscle and dilator muscle overlap. Radial ridges extend from the periphery to the pupillary zone, to supply the iris with blood vessels. The root of the iris is the thinnest and most peripheral.
The muscle cells of the iris are smooth muscle in mammals and amphibians, but are striated muscle in reptiles. Many fish have neither, and, as a result, their irides are unable to dilate and contract, so that the pupil always remains of a fixed size.

Front

From anterior to posterior, the layers of the iris are:
The stroma and the anterior border layer of the iris are derived from the neural crest, and behind the stroma of the iris, the sphincter pupillae and dilator pupillae muscles as well as the iris epithelium develop from optic cup neuroectoderm.

Eye color

The iris is usually strongly pigmented, with the color typically ranging between brown, hazel, green, gray, and blue. Occasionally, the color of the iris is due to a lack of pigmentation, as in the pinkish-white of oculo-cutaneous albinism, or to obscuration of its pigment by blood vessels, as in the red of an abnormally vascularised iris. Despite the wide range of colors, the only pigment that contributes substantially to normal human iris color is the dark pigment melanin. The quantity of melanin pigment in the iris is one factor in determining the phenotypic eye color of a person. Structurally, this huge molecule is only slightly different from its equivalent found in skin and hair. Iris color is due to variable amounts of eumelanin and pheomelanin produced by melanocytes. More of the former is found in brown-eyed people and of the latter in blue and green-eyed people.

Genetic and physical factors determining iris color

Iris' color is a highly complex phenomenon consisting of the combined effects of texture, pigmentation, fibrous tissue and blood vessels within the iris stroma, which together make up an individual's epigenetic constitution in this context. A person's "eye color" is actually the color of one's iris, the cornea being transparent and the white sclera entirely outside the area of interest.
Melanin is yellowish-brown to dark brown in the stromal pigment cells, and black in the iris pigment epithelium, which lies in a thin but very opaque layer across the back of the iris. Most human irises also show a condensation of the brownish stromal melanin in the thin anterior border layer, which by its position has an overt influence on the overall color. The degree of dispersion of the melanin, which is in subcellular bundles called melanosomes, has some influence on the observed color, but melanosomes in the iris of humans and other vertebrates are not mobile, and the degree of pigment dispersion cannot be reversed. Abnormal clumping of melanosomes does occur in disease and may lead to irreversible changes in iris color. Colors other than brown or black are due to selective reflection and absorption from the other stromal components. Sometimes lipofuscin, a yellow "wear and tear" pigment, also enters into the visible eye color, especially in aged or diseased green eyes.
The optical mechanisms by which the non-pigmented stromal components influence eye color are complex, and many erroneous statements exist in the literature. Simple selective absorption and reflection by biological molecules is the most important element. Rayleigh scattering and Tyndall scattering, and diffraction also occur. Raman scattering, and constructive interference, as in the feathers of birds, do not contribute to the color of the human eye, but interference phenomena are important in the brilliantly colored iris pigment cells in many animals. Interference effects can occur at both molecular and light microscopic scales, and are often associated with quasi-crystalline formations which enhance the optical effects. Interference is recognised by characteristic dependence of color on the angle of view, as seen in eyespots of some butterfly wings, although the chemical components remain the same.
White babies are usually born blue-eyed since there is no pigment in the stroma, and their eyes appear blue due to scattering and selective absorption from the posterior epithelium. If melanin is deposited substantially, there will be brown or black color, if not, they will remain blue or gray.
All the contributing factors towards eye color and its variation are not fully understood. Autosomal recessive/dominant traits in iris color are inherent in other species but coloration can follow a different pattern.

Amber eyes

Amber colored eyes are extremely rare in humans. They consist of a solid orange/gold color that may contain lighter shades of the same pigment within the iris. This is an unusual occurrence that happens when the yellow pigment called pheomelanin, is dominant within the iris. Pheomelanin is also found on individuals with green eyes in much smaller amounts. This is, because green eyes have a strong presence of both melanin and pheomelanin. Often in poor lighting, one may mistake amber eyes for brown. This also happens when viewed from far away or in pictures with poor lighting as well. However, in natural or well lit areas, it is very easy to tell the difference between the two colors. Another common mistake people make is referring to amber eyes as hazel. Although similar, hazel eyes have a stronger presence of melanin with two very distinct colors within the iris and often contain many speckles or blotches of mixed hues.

Different colors in the two eyes

Heterochromia is an ocular condition in which one iris is a different color from the other iris, or where the part of one iris is a different color from the remainder. Uncommon in humans, it is often an indicator of ocular disease, such as chronic iritis or diffuse iris melanoma, but may also occur as a normal variant. Sectors or patches of strikingly different colors in the same iris are less common. Anastasius the First was dubbed dikoros for his patent heterochromia since his right iris had a darker color than the left one.
In contrast, heterochromia and variegated iris patterns are common in veterinary practice. Siberian Huskies show heterochromia, possibly analogous to the genetically-determined Waardenburg syndrome of humans. Some white cat fancies may show striking heterochromia, with the most common pattern being one uniformly blue, the other copper, orange, yellow or green. Striking variation within the same iris is also common in some animals, and is the norm in some species. Several herding breeds, particularly those with a blue merle coat color may show well-defined blue areas within a brown iris as well as separate blue and darker eyes. Some horses may show amber, brown, white and blue all within the same eye, without any sign of eye disease.
One eye with a white or bluish-white iris is also known as a walleye.

Clinical significance

Iridology

Iridology is an alternative medicine technique whose proponents believe that patterns, colors and other characteristics of the iris can be examined to determine information about a patient's :wikt:systemic|systemic health. Practitioners match their observations to iris charts which divide the iris into zones corresponding to specific parts of the human body. Iridologists see the eyes as "windows" into the body's state of health.
Iridology is not supported by quality research studies and is considered pseudoscience by the majority of medical practitioners and eye care professionals.

Etymology

The word iris is derived from the Greek goddess of the rainbow, because of the many colours of the iris.

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