Iris (eye)
   HOME

TheInfoList



OR:

The iris (: irides or irises) is a thin, annular structure in the eye in most mammals and birds that is responsible for controlling the diameter and size of the pupil, and thus the amount of light reaching the
retina The retina (; or retinas) is the innermost, photosensitivity, light-sensitive layer of tissue (biology), tissue of the eye of most vertebrates and some Mollusca, molluscs. The optics of the eye create a focus (optics), focused two-dimensional ...
. In optical terms, the pupil is the eye's aperture, while the iris is the diaphragm.
Eye color Eye color is a polygene, polygenic phenotypic trait determined by two factors: the pigmentation of the eye's Iris (anatomy), iris and the frequency-dependence of the scattering of light by the Turbidity, turbid medium in the Stroma of iris, str ...
is defined by the iris.


Etymology

The word "iris" is derived from the Greek word for "
rainbow A rainbow is an optical phenomenon caused by refraction, internal reflection and dispersion of light in water droplets resulting in a continuous spectrum of light appearing in the sky. The rainbow takes the form of a multicoloured circular ...
", also its goddess plus messenger of the gods in the ''
Iliad The ''Iliad'' (; , ; ) is one of two major Ancient Greek epic poems attributed to Homer. It is one of the oldest extant works of literature still widely read by modern audiences. As with the ''Odyssey'', the poem is divided into 24 books and ...
'', because of the many colours of this eye part.


Structure

The iris consists of two layers: the front
pigment A pigment is a powder used to add or alter color or change visual appearance. Pigments are completely or nearly solubility, insoluble and reactivity (chemistry), chemically unreactive in water or another medium; in contrast, dyes are colored sub ...
ed fibrovascular layer known as a stroma and, behind the stroma, pigmented epithelial cells. The stroma is connected to a sphincter muscle ( sphincter pupillae), which contracts the pupil in a circular motion, and a set of dilator muscles ( dilator pupillae), which pull the iris radially to enlarge the pupil, pulling it in folds. The sphincter pupillae is the opposing muscle of the dilator pupillae. The pupil's diameter, and thus the inner border of the iris, changes size when constricting or dilating. The outer border of the iris does not change size. The constricting muscle is located on the inner border. The back surface is covered by a heavily pigmented epithelial layer that is two cells thick (the iris pigment epithelium), 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."eye, human." Encyclopædia Britannica from Encyclopædia Britannica 2006 Ultimate Reference Suite DVD The outer edge of the iris, known as the root, is attached to the
sclera The sclera, also known as the white of the eye or, in older literature, as the tunica albuginea oculi, is the opaque, fibrous, protective outer layer of the eye containing mainly collagen and some crucial elastic fiber. In the development of t ...
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: # The pupillary zone is the inner region whose edge forms the boundary of the pupil. # 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.Gold, Daniel H; Lewis, Richard; "Clinical Eye Atlas," pp. 396–397 The muscle cells of the iris are
smooth muscle Smooth muscle is one of the three major types of vertebrate muscle tissue, the others being skeletal and cardiac muscle. It can also be found in invertebrates and is controlled by the autonomic nervous system. It is non- striated, so-called bec ...
in mammals and amphibians, but are striated muscle in reptiles (including birds). Many fish have neither, and, as a result, their irises are unable to dilate and contract, so that the pupil always remains of a fixed size.


Front

*The crypts of Fuchs are a series of openings located on either side of the collarette that allow the stroma and deeper iris tissues to be bathed in aqueous humor. Collagen trabeculae that surround the border of the crypts can be seen in blue irises. *The midway between the collarette and the origin of the iris: These folds result from changes in the surface of the iris as it dilates. *Crypts on the base of the iris are additional openings that can be observed close to the outermost part of the ciliary portion of the iris.


Back

*The radial contraction folds of Schwalbe are a series of very fine radial folds in the pupillary portion of the iris extending from the pupillary margin to the collarette. They are associated with the scalloped appearance of the pupillary ruff. *The structural folds of Schwalbe are radial folds extending from the border of the ciliary and pupillary zones that are much broader and more widely spaced, continuous with the "valleys" between the ciliary processes. *Some of the circular contraction folds are a fine series of ridges that run near the pupillary margin and vary in thickness of the iris pigment epithelium; others are in ciliary portion of iris.


Microanatomy

From anterior (front) to posterior (back), the layers of the iris are: *Anterior limiting layer * Stroma of iris * Iris sphincter muscle * Iris dilator muscle (myoepithelium) *Anterior pigment epithelium *Posterior pigment epithelium


Development

The stroma and the anterior border layer of the iris are derived from the
neural crest The neural crest is a ridge-like structure that is formed transiently between the epidermal ectoderm and neural plate during vertebrate development. Neural crest cells originate from this structure through the epithelial-mesenchymal transition, ...
, 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.


Function

The iris controls the size of the pupil by means of contracting the iris sphincter muscle and/or the iris dilator muscle. The size of the pupils is dependent on many factors (including light, emotional state, cognitive load, arousal, stimulation), and can range from less than 2 mm in diameter, to as large as 9 mm in diameter. However, there is considerable variation in maximal pupil diameter by individual humans, and decreases with age. The irises also contract the pupils when accommodation is initiated, to increase the depth of field. Very few humans possess the ability to exert direct voluntary control over their iris muscles, which grants them the ability to dilate and constrict their pupils on command. However, there is no clear purpose or advantage to this.


Eye color

The iris is usually strongly
pigment A pigment is a powder used to add or alter color or change visual appearance. Pigments are completely or nearly solubility, insoluble and reactivity (chemistry), chemically unreactive in water or another medium; in contrast, dyes are colored sub ...
ed, with the
color Color (or colour in English in the Commonwealth of Nations, Commonwealth English; American and British English spelling differences#-our, -or, see spelling differences) is the visual perception based on the electromagnetic spectrum. Though co ...
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 oculocutaneous
albinism Albinism is the congenital absence of melanin in an animal or plant resulting in white hair, feathers, scales and skin and reddish pink or blue eyes. Individuals with the condition are referred to as albinos. Varied use and interpretation of ...
, 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 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 an organism. Structurally, this huge molecule is only slightly different from its equivalent found in
skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other animal coverings, such as the arthropod exoskeleton, have different ...
and
hair Hair is a protein filament that grows from follicles found in the dermis. Hair is one of the defining characteristics of mammals. The human body, apart from areas of glabrous skin, is covered in follicles which produce thick terminal and ...
. Iris color is due to variable amounts of eumelanin (brown/black melanins) and pheomelanin (red/yellow melanins) produced by melanocytes. More of the former is found in brown-eyed people and of the latter in blue- and green-eyed people. The limbal ring appears as a dark ring encircling the iris on some individuals, but is a result of the optical properties of the region between the
cornea The cornea is the transparency (optics), transparent front part of the eyeball which covers the Iris (anatomy), iris, pupil, and Anterior chamber of eyeball, anterior chamber. Along with the anterior chamber and Lens (anatomy), lens, the cornea ...
and
sclera The sclera, also known as the white of the eye or, in older literature, as the tunica albuginea oculi, is the opaque, fibrous, protective outer layer of the eye containing mainly collagen and some crucial elastic fiber. In the development of t ...
, not of pigments in the iris.


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. An organism's "eye color" is actually the color of one's iris, the
cornea The cornea is the transparency (optics), transparent front part of the eyeball which covers the Iris (anatomy), iris, pupil, and Anterior chamber of eyeball, anterior chamber. Along with the anterior chamber and Lens (anatomy), lens, the cornea ...
being transparent and the white
sclera The sclera, also known as the white of the eye or, in older literature, as the tunica albuginea oculi, is the opaque, fibrous, protective outer layer of the eye containing mainly collagen and some crucial elastic fiber. In the development of t ...
entirely outside the area of interest. Melanin is yellowish to dark hazel 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 (see heterochromia, below). 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 nonpigmented stromal components influence eye color are complex, and many erroneous statements exist in the literature. Simple selective absorption and reflection by biological molecules (
hemoglobin Hemoglobin (haemoglobin, Hb or Hgb) is a protein containing iron that facilitates the transportation of oxygen in red blood cells. Almost all vertebrates contain hemoglobin, with the sole exception of the fish family Channichthyidae. Hemoglobin ...
in the blood vessels,
collagen Collagen () is the main structural protein in the extracellular matrix of the connective tissues of many animals. It is the most abundant protein in mammals, making up 25% to 35% of protein content. Amino acids are bound together to form a trip ...
in the vessel and stroma) is the most important element.
Rayleigh scattering Rayleigh scattering ( ) is the scattering or deflection of light, or other electromagnetic radiation, by particles with a size much smaller than the wavelength of the radiation. For light frequencies well below the resonance frequency of the scat ...
and Tyndall scattering, (which also happen in the sky) and
diffraction Diffraction is the deviation of waves from straight-line propagation without any change in their energy due to an obstacle or through an aperture. The diffracting object or aperture effectively becomes a secondary source of the Wave propagation ...
also occur. Raman scattering, and constructive interference, as in the feathers of birds, do not contribute to the color of the eye, but interference phenomena are important in the brilliantly colored iris pigment cells ( iridophores) in many animals. Interference effects can occur at both molecular and light-microscopic scales, and are often associated (in melanin-bearing cells) with quasicrystalline 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 Butterflies are winged insects from the lepidopteran superfamily Papilionoidea, characterized by large, often brightly coloured wings that often fold together when at rest, and a conspicuous, fluttering flight. The oldest butterfly fossi ...
wing A wing is a type of fin that produces both Lift (force), lift and drag while moving through air. Wings are defined by two shape characteristics, an airfoil section and a planform (aeronautics), planform. Wing efficiency is expressed as lift-to-d ...
s, although the chemical components remain the same. White babies are usually born blue-eyed since no pigment is in the stroma, and their eyes appear blue due to scattering and selective absorption from the posterior epithelium. If melanin is deposited substantially, brown or black color is seen; if not, they will remain blue or gray."Sensory Reception: Human Vision: Structure and function of the Human Eye" vol. 27, p. 175 Encyclopædia Britannica, 1987 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.


Different colors in the two eyes

Heterochromia (also known as a heterochromia iridis or heterochromia iridum) is an ocular condition in which one iris is a different color from the other iris (complete heterochromia), or where the part of one iris is a different color from the remainder (partial heterochromia or sectoral heterochromia). 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'' (having two irises) 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 Husky dogs show heterochromia, possibly analogous to the genetically determined
Waardenburg syndrome Waardenburg syndrome is a group of rare genetic conditions characterised by at least some degree of congenital hearing loss and pigmentation deficiencies, which can include bright blue eyes (or Heterochromia iridum, one blue eye and one brown ey ...
of humans. Some white cat fancies (e.g., white Turkish Angora or white Turkish Van cats) 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 (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".


Clinical significance

* Angle closure glaucoma * Aniridia *
Anisocoria Anisocoria is a condition characterized by an unequal size of the eyes' pupils. Affecting up to 20% of the population, anisocoria is often entirely harmless, but can be a sign of more serious medical problems. Causes Anisocoria is a common cond ...
* Horner's syndrome * Iridocyclitis * Iridoplegia * Iritis *
Miosis Miosis, or myosis (), is excessive constriction of the pupil.Farlex medical dictionary
citing: ...
/
Mydriasis Mydriasis is the Pupillary dilation, dilation of the pupil, usually having a non-physiological cause, or sometimes a physiological pupillary response. Non-physiological causes of mydriasis include disease, Physical trauma, trauma, or the use of c ...
* Synechia * Third nerve palsy


Alternative medicine


Iridology

Iridology (also known as iridodiagnosis) is an
alternative medicine Alternative medicine refers to practices that aim to achieve the healing effects of conventional medicine, but that typically lack biological plausibility, testability, repeatability, or supporting evidence of effectiveness. Such practices are ...
technique whose proponents believe that patterns, colors, and other characteristics of the iris can be examined to determine information about a patient's 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.


Graphics

Image:Gray878.png, Iris, front view File:Fluorescein angiograpy of iris reveals radial layout of blood vessels.jpg, Fluorescein angiograpy of the iris reveals a radial layout of blood vessels.


See also

* Blood–ocular barrier * Coloboma * Gonioscopy *
Human eye The human eye is a sensory organ in the visual system that reacts to light, visible light allowing eyesight. Other functions include maintaining the circadian rhythm, and Balance (ability), keeping balance. The eye can be considered as a living ...
*
Iris recognition Iris recognition is an automated method of biometrics, biometric identification that uses mathematical pattern-recognition techniques on video images of one or both of the iris (anatomy), irises of an individual's Human eye, eyes, whose complex ...


References


External links


Detailed photographs of human irides
* * – "Sagittal Section Through the Eyeball" {{Authority control Eye color