Iris (anatomy)

The human iris
The iris is the grey-brown area. The other structures visible are the transparent pupil (showing as black), the white outer sclera, of which the central part, the cornea, is transparent.</center>

In anatomy, the iris (plural irises or irides) is the most visible part of the eye of vertebrates, including humans. The following covers the iris of vertebrates, not the separately evolving iris found in some cephalopods. The word comes from Greek mythology, in which Iris is the personification of the rainbow.

The iris is an annulus (or flattened ring) consisting of pigmented fibrovascular tissue known as a stroma. The stroma connects a sphincter muscle, which contracts the pupil, and a set of dialator muscles which open it. The back surface is covered by a two-cell thick epithelial layer, the iris pigment epithelium, but the front surface has no epithelium. 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 through which the aqueous humour constantly drains out of the eye, with the result that diseases of the iris often have important effects on intra-ocular pressure, and indirectly on vision.

Colour

See also: eye color

The iris is usually strongly pigmented, with colours ranging from brown to green, blue, grey, and hazel. Occasionally its colour is due to 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 colours, there is only one pigment that contributes substantially to normal human iris colour, the dark pigment called melanin. Structurally, this huge molecule is only slightly different from its equivalent found in skin and hair.

Genetic and physical factors determining iris colour

Despite periodical reports that specific eye colour genes had been "discovered" (but not so far independently confirmed), iris colour remains a highly complex phenomenon due to the combined effects of texture, pigmentation, fibrous tissue and blood vessels within the iris stroma, which together make up an individual's epigenetic constitution. A person's "eye colour" is actually the colour of one's iris, since the cornea is transparent and the white sclera lies entirely outside the area of interest. It is a common misconception that the iris colour is all due to its melanin pigment, but this varies only from brown to black.

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 irides also show a condensation of the brownish stromal melanin in the thin anterior border layer, which by its position has an undue influence on the overall colour. The degree of dispersion of the melanin, which is in subcellular bundles called melanosomes, has some influence on the observed colour, but melanosomes in the iris of man and other vertebrates are not mobile, and we cannot reversibly change the degree of pigment dispersion. Abnormal clumping of melanosomes does occur in disease and may lead to irreversible changes in iris colour (see heterochromia, below). Colours 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 colour, especially in aged or diseased green eyes (but not in healthy green human eyes).

The optical mechanisms by which the non-pigmented stromal components influence eye colour are complex, and many erroneous statements exist in the literature. Simple selective absorption and reflection by biological molecules (h歯globin in the blood vessels, collagen in the vessel walls and stroma) is the most important element. Rayleigh scattering (which happens in the sky), Tyndall scattering, and diffraction also occur. Raman scattering, and constructive interference, as in the feathers of birds, do not contribute to the colour of the human eye, but interference phenomena are important in the brilliantly coloured iris pigment cells (iridophores) in many animals. Interference effects can occur at both molecular and light microscopic scales, and are often associated (in the melanin-bearing cells) with quasi-crystalline formations which enhance the optical effects. Interference is recognised by characteristic dependence of colour on the angle of view, as seen in eye spots of some butterfly wings, although the chemical components remain the same.

Blue is one of the possible eye colours in humans. The "blue" allele, existing in the Bey2 and Gey genes of Chromosome 15, is recessive. This means that both genes must have both blue alleles i.e. "blue-blue", in a person with blue eyes. If one of the alleles were not "blue" ("green" for Gey or "brown" for Bey2) then the person would have those coloured eyes respectively. As either allele (though not both) can be passed on to offspring it is perfectly possible for someone who does not have blue eyes to have blue-eyed children. Because of its recessive nature, this is a certainty if both parents have blue eyes. Though this explanation gives an idea of eye colour delineation, it is incomplete, and all the contributing factors towards eye colour and its variation are not fully understood.

Faking the iris colour

Certain eye colours are sometimes seen as being especially attractive and motif-expressing contact lenses can be worn to mask one's natural eye colour with another. They are rarely needed and almost never recommended by serious medical doctors, unless the patient's retina needs extra protection, as in aniridia. Since the introduction of machines which can automatically analyse iris patterns, and their use at some airports as a security measure, it is reported that some people have resorted to coloured contact lenses, or deliberate iris injury with lasers, to prevent personal identification.

Iris colour as paternity test

As stated above, although there has been much fuss about finding the genes for eye colour, there is no simple genetic determinism for such a complex trait, as there is more to iris colour than pigmentation. Overall, there is no simple Mendelian inheritance in iris colour. Consequently no serious test of paternity can be based on observations or even measurements of iris colour, except to note that blue eyes are normally phenotypically recessive, so that a brown-eyed child of two blue-eyed parents may create some doubt about paternity.

Different colours in the two eyes

The occurrence of two irides with different colours or heterochromia iridis is a rare situation in humans. This is often an indicator of ocular disease, such as chronic iritis or diffuse iris melanoma, but may also occur as a normal variant. Alexander the Great and Anastasios the First were dubbed δικορος (dikoros, "with two pupils") for their patent heterochromias. In their case, this was not a true dicoria (two pupils in the same iris). Real polycoria can be due to disease but is most often due to previous trauma or surgery. Sectors or patches of strikingly different colours in the same iris are less rare and are sometimes described as heterochromia iridium, but the term has little currency.

In contrast, heterochromia and variegated iris patterns are common in veterinary practice. Vets tend to use the term heterochromia indiscriminately for these conditions. Siberian Huskies show heterochromia due to interbreeding, possibly analogous to the genetically-determined Waardenburg syndrome of humans. Some white cat fancies (e.g., white Persians) may show striking heterochromia, with the commonest pattern being one uniformly blue, the other green. Striking variegation 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 (such as Australian Shepherds and Border Collies) may show well-defined blue areas within a brown iris as well as separate blue and darker eyes. Some horses (usually within the white, spotted, palomino or cremello groups of breeds) 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.

"Red eye"

When photographed with a flash, the iris only reacts to protect the retina, and not fast enough to avoid the red eye effect. This represents reflection of light from the back of the eye, and is closely related to the term red reflex, used by ophthalmologists in describing appearances on fundal examination.
When used as a descriptive term in medicine, the meaning of "red eye" is quite different, and indicates that the bulbar conjunctiva is reddened due to dilatation of superficial blood vessels. Leaving aside rarities, it indicates surface infection (conjunctivitis), intraocular inflammation (e.g., iridocyclitis) or high intraocular pressure (acute glaucoma or occasionally severe, untreated chronic glaucoma). This use of "red eye" implies disease. The term is therefore not used in medicine for ocular albinism, in which the eye is otherwise healthy despite an obviously red pupil and a translucent pinkish iris due to reflected light from the fundus. "Red eye" is used more loosely in veterinary practice, where investigation of eye diseases can be difficult, but even so albinotic breeds are easily recognised and are usually described as having "pink eye" rather than "red eye".

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See also

• eye contact
• iridocyclitis
• visual system
• iris scan
• iridology
• Jane Elliott's "brown eyes, blue eyes exercise"