Iris (anatomy)

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This article is about the part of the eye. Iris (anatomy)_sentence_0

For other uses, see Iris (disambiguation). Iris (anatomy)_sentence_1

Iris (anatomy)_table_infobox_0

IrisIris (anatomy)_header_cell_0_0_0
DetailsIris (anatomy)_header_cell_0_1_0
PrecursorIris (anatomy)_header_cell_0_2_0 Mesoderm and neural ectodermIris (anatomy)_cell_0_2_1
Part ofIris (anatomy)_header_cell_0_3_0 Front of eyeIris (anatomy)_cell_0_3_1
SystemIris (anatomy)_header_cell_0_4_0 Visual systemIris (anatomy)_cell_0_4_1
ArteryIris (anatomy)_header_cell_0_5_0 long posterior ciliary arteriesIris (anatomy)_cell_0_5_1
NerveIris (anatomy)_header_cell_0_6_0 long ciliary nerves, short ciliary nervesIris (anatomy)_cell_0_6_1
IdentifiersIris (anatomy)_header_cell_0_7_0
LatinIris (anatomy)_header_cell_0_8_0 irisIris (anatomy)_cell_0_8_1
MeSHIris (anatomy)_header_cell_0_9_0 Iris (anatomy)_cell_0_9_1
TA98Iris (anatomy)_header_cell_0_10_0 Iris (anatomy)_cell_0_10_1
TA2Iris (anatomy)_header_cell_0_11_0 Iris (anatomy)_cell_0_11_1
FMAIris (anatomy)_header_cell_0_12_0 Iris (anatomy)_cell_0_12_1

In humans and most mammals and birds, the iris (plural: irides or irises) 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. Iris (anatomy)_sentence_2

Eye color is defined by that of the iris. Iris (anatomy)_sentence_3

In optical terms, the pupil is the eye's aperture, while the iris is the diaphragm. Iris (anatomy)_sentence_4

Structure Iris (anatomy)_section_0

The iris consists of two layers: the front pigmented layer known as a stroma and, beneath the stroma, pigmented epithelial cells. Iris (anatomy)_sentence_5

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. Iris (anatomy)_sentence_6

The circle circumference sphincter constricting muscle is the opposing muscle of the circle-radius dilator muscle. Iris (anatomy)_sentence_7

The iris inner smaller circle-circumference changes size when constricting or dilating. Iris (anatomy)_sentence_8

The iris outer larger circle-circumference does not change size. Iris (anatomy)_sentence_9

The constricting muscle is located on the iris inner smaller circle-circumference. Iris (anatomy)_sentence_10

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. Iris (anatomy)_sentence_11

This anterior surface projects as the dilator muscles. Iris (anatomy)_sentence_12

The high pigment content blocks light from passing through the iris to the retina, restricting it to the pupil. Iris (anatomy)_sentence_13

The outer edge of the iris, known as the root, is attached to the sclera and the anterior ciliary body. Iris (anatomy)_sentence_14

The iris and ciliary body together are known as the anterior uvea. Iris (anatomy)_sentence_15

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. Iris (anatomy)_sentence_16

The iris along with the anterior ciliary body provide a secondary pathway for aqueous humour to drain from the eye. Iris (anatomy)_sentence_17

The iris is divided into two major regions: Iris (anatomy)_sentence_18

Iris (anatomy)_ordered_list_0

  1. The pupillary zone is the inner region whose edge forms the boundary of the pupil.Iris (anatomy)_item_0_0
  2. The ciliary zone is the rest of the iris that extends to its origin at the ciliary body.Iris (anatomy)_item_0_1

The collarette is the thickest region of the iris, separating the pupillary portion from the ciliary portion. Iris (anatomy)_sentence_19

The collarette is a vestige of the coating of the embryonic pupil. Iris (anatomy)_sentence_20

It is typically defined as the region where the sphincter muscle and dilator muscle overlap. Iris (anatomy)_sentence_21

Radial ridges extend from the periphery to the pupillary zone, to supply the iris with blood vessels. Iris (anatomy)_sentence_22

The root of the iris is the thinnest and most peripheral. Iris (anatomy)_sentence_23

The muscle cells of the iris are smooth muscle in mammals and amphibians, but are striated muscle in reptiles (including birds). Iris (anatomy)_sentence_24

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. Iris (anatomy)_sentence_25

Front Iris (anatomy)_section_1

Iris (anatomy)_unordered_list_1

  • 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.Iris (anatomy)_item_1_2
  • 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.Iris (anatomy)_item_1_3
  • 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.Iris (anatomy)_item_1_4

Back Iris (anatomy)_section_2

Iris (anatomy)_unordered_list_2

  • 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.Iris (anatomy)_item_2_5
  • 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.Iris (anatomy)_item_2_6
  • 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.Iris (anatomy)_item_2_7

Microanatomy Iris (anatomy)_section_3

From anterior (front) to posterior (back), the layers of the iris are: Iris (anatomy)_sentence_26

Iris (anatomy)_unordered_list_3

  • Anterior limiting layerIris (anatomy)_item_3_8
  • Stroma of irisIris (anatomy)_item_3_9
  • Iris sphincter muscleIris (anatomy)_item_3_10
  • Iris dilator muscle (myoepithelium)Iris (anatomy)_item_3_11
  • Anterior pigment epitheliumIris (anatomy)_item_3_12
  • Posterior pigment epitheliumIris (anatomy)_item_3_13

Development Iris (anatomy)_section_4

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. Iris (anatomy)_sentence_27

Eye color Iris (anatomy)_section_5

Main article: Eye color Iris (anatomy)_sentence_28

The iris is usually strongly pigmented, with the color typically ranging between brown, hazel, green, gray, and blue. Iris (anatomy)_sentence_29

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. Iris (anatomy)_sentence_30

Despite the wide range of colors, the only pigment that contributes substantially to normal human iris color is the dark pigment melanin. Iris (anatomy)_sentence_31

The quantity of melanin pigment in the iris is one factor in determining the phenotypic eye color of a person. Iris (anatomy)_sentence_32

Structurally, this huge molecule is only slightly different from its equivalent found in skin and hair. Iris (anatomy)_sentence_33

Iris color is due to variable amounts of eumelanin (brown/black melanins) and pheomelanin (red/yellow melanins) produced by melanocytes. Iris (anatomy)_sentence_34

More of the former is found in brown-eyed people and of the latter in blue and green-eyed people. Iris (anatomy)_sentence_35

Genetic and physical factors determining iris color Iris (anatomy)_section_6

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. Iris (anatomy)_sentence_36

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. Iris (anatomy)_sentence_37

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. Iris (anatomy)_sentence_38

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. Iris (anatomy)_sentence_39

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. Iris (anatomy)_sentence_40

Abnormal clumping of melanosomes does occur in disease and may lead to irreversible changes in iris color (see heterochromia, below). Iris (anatomy)_sentence_41

Colors other than brown or black are due to selective reflection and absorption from the other stromal components. Iris (anatomy)_sentence_42

Sometimes lipofuscin, a yellow "wear and tear" pigment, also enters into the visible eye color, especially in aged or diseased green eyes. Iris (anatomy)_sentence_43

The optical mechanisms by which the non-pigmented stromal components influence eye color are complex, and many erroneous statements exist in the literature. Iris (anatomy)_sentence_44

Simple selective absorption and reflection by biological molecules (hemoglobin in the blood vessels, collagen in the vessel and stroma) is the most important element. Iris (anatomy)_sentence_45

Rayleigh scattering and Tyndall scattering, (which also happen in the sky) and diffraction also occur. Iris (anatomy)_sentence_46

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 (iridophores) in many animals. Iris (anatomy)_sentence_47

Interference effects can occur at both molecular and light microscopic scales, and are often associated (in melanin-bearing cells) with quasi-crystalline formations which enhance the optical effects. Iris (anatomy)_sentence_48

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. Iris (anatomy)_sentence_49

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. Iris (anatomy)_sentence_50

If melanin is deposited substantially, there will be brown or black color, if not, they will remain blue or gray. Iris (anatomy)_sentence_51

All the contributing factors towards eye color and its variation are not fully understood. Iris (anatomy)_sentence_52

Autosomal recessive/dominant traits in iris color are inherent in other species but coloration can follow a different pattern. Iris (anatomy)_sentence_53

Amber eyes Iris (anatomy)_section_7

Amber colored eyes are extremely rare in humans. Iris (anatomy)_sentence_54

They consist of a solid orange/gold color that may contain lighter shades of the same pigment within the iris. Iris (anatomy)_sentence_55

This is an unusual occurrence that happens when the yellow pigment called pheomelanin, is dominant within the iris. Iris (anatomy)_sentence_56

Pheomelanin is also found on individuals with green eyes in much smaller amounts. Iris (anatomy)_sentence_57

This is, because green eyes have a strong presence of both melanin and pheomelanin. Iris (anatomy)_sentence_58

Often in poor lighting, one may mistake amber eyes for brown. Iris (anatomy)_sentence_59

This also happens when viewed from far away or in pictures with poor lighting as well. Iris (anatomy)_sentence_60

However, in natural or well lit areas, it is very easy to tell the difference between the two colors. Iris (anatomy)_sentence_61

Another common mistake people make is referring to amber eyes as hazel. Iris (anatomy)_sentence_62

Although similar, hazel eyes have a stronger presence of melanin with two very distinct colors within the iris (usually green/brown) and often contain many speckles or blotches of mixed hues. Iris (anatomy)_sentence_63

Different colors in the two eyes Iris (anatomy)_section_8

Main article: Heterochromia Iris (anatomy)_sentence_64

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). Iris (anatomy)_sentence_65

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. Iris (anatomy)_sentence_66

Sectors or patches of strikingly different colors in the same iris are less common. Iris (anatomy)_sentence_67

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. Iris (anatomy)_sentence_68

In contrast, heterochromia and variegated iris patterns are common in veterinary practice. Iris (anatomy)_sentence_69

Siberian Huskies show heterochromia, possibly analogous to the genetically-determined Waardenburg syndrome of humans. Iris (anatomy)_sentence_70

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. Iris (anatomy)_sentence_71

Striking variation within the same iris is also common in some animals, and is the norm in some species. Iris (anatomy)_sentence_72

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. Iris (anatomy)_sentence_73

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. Iris (anatomy)_sentence_74

One eye with a white or bluish-white iris is also known as a walleye. Iris (anatomy)_sentence_75

Clinical significance Iris (anatomy)_section_9

Iris (anatomy)_unordered_list_4

Society and culture Iris (anatomy)_section_10

Iridology Iris (anatomy)_section_11

Main article: Iridology Iris (anatomy)_sentence_76

Iridology (also known as iridodiagnosis) 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 health. Iris (anatomy)_sentence_77

Practitioners match their observations to iris charts which divide the iris into zones corresponding to specific parts of the human body. Iris (anatomy)_sentence_78

Iridologists see the eyes as "windows" into the body's state of health. Iris (anatomy)_sentence_79

Iridology is not supported by quality research studies and is considered pseudoscience by the majority of medical practitioners and eye care professionals. Iris (anatomy)_sentence_80

Etymology Iris (anatomy)_section_12

The word iris is derived from the Greek goddess of the rainbow, because of the many colours of the iris. Iris (anatomy)_sentence_81

Graphics Iris (anatomy)_section_13

Iris (anatomy)_unordered_list_5

  • Iris (anatomy)_item_5_22
  • Iris (anatomy)_item_5_23
  • Iris (anatomy)_item_5_24

See also Iris (anatomy)_section_14

Iris (anatomy)_unordered_list_6


Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/Iris (anatomy).