Iris (anatomy)

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Iris
Human eye close up, anterior view.jpg
The iris in humans is the colored (typically brown, blue, or green) area, with the pupil (the circular black spot) in its center, and surrounded by the white sclera.
Schematic diagram of the human eye en.svg
Schematic diagram of the human eye (iris labeled at upper right)
Details
Precursor Mesoderm and neural ectoderm
Part ofFront of eye
System Visual system
Artery long posterior ciliary arteries
Nerve long ciliary nerves, short ciliary nerves
Identifiers
Latin iris
MeSH D007498
TA98 A15.2.03.020
TA2 6753
FMA 58235
Anatomical terminology

The iris (pl.: irides or irises) is a thin, annular structure in the eye in most mammals and birds, responsible for controlling the diameter and size of the pupil, and thus the amount of light reaching the retina. In optical terms, the pupil is the eye's aperture, while the iris is the diaphragm. Eye color is defined by the iris.

Contents

Etymology

The word "iris" is derived from the Greek word for "rainbow", also its goddess plus messenger of the gods in the Iliad , [1] because of the many colours of this eye part. [2]

Structure

The iris consists of two layers: the front pigmented fibrovascular layer known as a stroma and, beneath 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 iris (brown coloured portion of the eye) controls the size of the pupil by contracting the sphincter pupillae and dilator pupillae muscles Pupillary light reflex.jpg
The iris (brown coloured portion of the eye) controls the size of the pupil by contracting the sphincter pupillae and dilator pupillae muscles

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. [3] 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.

Iris regions.jpg

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. [3] 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. [4]

The muscle cells of the iris are smooth muscle 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. [5]

Front

Back

Microanatomy

Light micrograph of the iris near to the pupil. M. sph. sphincter muscle, L lens Auge Iris.jpg
Light micrograph of the iris near to the pupil. M. sph. sphincter muscle, L lens
Constriction of the pupil (miosis) observed by laser Doppler imaging reveals radial vessels of the iris. Doppler holography of Olivier Martinache's myosis.gif
Constriction of the pupil (miosis) observed by laser Doppler imaging reveals radial vessels of the iris.
A human eye demonstrating its owner's rare ability to voluntarily dilate and constrict his pupil on command, via voluntary control of his iris muscles. Voluntary pupil dilation.gif
A human eye demonstrating its owner's rare ability to voluntarily dilate and constrict his pupil on command, via voluntary control of his iris muscles.
Anterior chamber cross-section imaged by an SD-OCT. SD OCT - Anterior Chamber Angle Cross-Section (with viewfinder).png
Anterior chamber cross-section imaged by an SD-OCT.

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

Development

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.

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. [6] [7] 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. [8] However, there is no clear purpose or advantage to this.

Eye color

Among human phenotypes, blue-green-gray eyes are a relatively rare eye color and the exact color is often perceived to vary according to its surroundings. Ageev iris.jpg
Among human phenotypes, blue-green-gray eyes are a relatively rare eye color and the exact color is often perceived to vary according to its surroundings.

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 oculocutaneous albinism, [3] 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 and hair. 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 and sclera, not of pigments in the iris.

Genetic and physical factors determining iris color

A light brown iris with prominent limbal ring. Light brown irises contain pheomelanin Human eye with limbal ring, anterior view.jpg
A light brown iris with prominent limbal ring. Light brown irises contain pheomelanin

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. [4] An organism'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 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. [4] 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.[ citation needed ]

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 in the blood vessels, collagen in the vessel and stroma) is the most important element. Rayleigh scattering and Tyndall scattering, (which also happen in the sky) and diffraction 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 wings, 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. [9]

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

Example of heterochromia - one eye of the subject is brown, the other hazel. Heterochromia.jpg
Example of heterochromia - one eye of the subject is brown, the other hazel.

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. [10] [11]

In contrast, heterochromia and variegated iris patterns are common in veterinary practice. Siberian Husky dogs show heterochromia, [12] [ better source needed ] possibly analogous to the genetically determined Waardenburg syndrome 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. [12] 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.[ citation needed ] 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.[ citation needed ]

One eye with a white or bluish-white iris is also known as a "walleye". [13]

Clinical significance

Alternative medicine

Iridology

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 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. [14]

Iridology is not supported by quality research studies, [15] and is considered pseudoscience. [16]

Graphics

See also

Related Research Articles

<span class="mw-page-title-main">Red-eye effect</span> Photography appearance

The red-eye effect in photography is the common appearance of red pupils in color photographs of the eyes of humans and several other animals. It occurs when using a photographic flash that is very close to the camera lens in ambient low light.

<span class="mw-page-title-main">Pupil</span> Part of an eye

The pupil is a hole located in the center of the iris of the eye that allows light to strike the retina. It appears black because light rays entering the pupil are either absorbed by the tissues inside the eye directly, or absorbed after diffuse reflections within the eye that mostly miss exiting the narrow pupil. The size of the pupil is controlled by the iris, and varies depending on many factors, the most significant being the amount of light in the environment. The term "pupil" was coined by Gerard of Cremona.

<span class="mw-page-title-main">Mydriasis</span> Excessive dilation of the pupil

Mydriasis is the dilation of the pupil, usually having a non-physiological cause, or sometimes a physiological pupillary response. Non-physiological causes of mydriasis include disease, trauma, or the use of certain types of drug. It may also be of unknown cause.

<span class="mw-page-title-main">Oculomotor nerve</span> Cranial nerve III, for eye movements

The oculomotor nerve, also known as the third cranial nerve, cranial nerve III, or simply CN III, is a cranial nerve that enters the orbit through the superior orbital fissure and innervates extraocular muscles that enable most movements of the eye and that raise the eyelid. The nerve also contains fibers that innervate the intrinsic eye muscles that enable pupillary constriction and accommodation. The oculomotor nerve is derived from the basal plate of the embryonic midbrain. Cranial nerves IV and VI also participate in control of eye movement.

<span class="mw-page-title-main">Heterochromia iridum</span> Difference in coloration, usually of the iris but also of hair or skin

Heterochromia is a variation in coloration most often used to describe color differences of the iris, but can also be applied to color variation of hair or skin. Heterochromia is determined by the production, delivery, and concentration of melanin. It may be inherited, or caused by genetic mosaicism, chimerism, disease, or injury. It occurs in humans and certain breeds of domesticated animals.

<span class="mw-page-title-main">Pupillary light reflex</span> Eye reflex which alters the pupils size in response to light intensity

The pupillary light reflex (PLR) or photopupillary reflex is a reflex that controls the diameter of the pupil, in response to the intensity (luminance) of light that falls on the retinal ganglion cells of the retina in the back of the eye, thereby assisting in adaptation of vision to various levels of lightness/darkness. A greater intensity of light causes the pupil to constrict, whereas a lower intensity of light causes the pupil to dilate. Thus, the pupillary light reflex regulates the intensity of light entering the eye. Light shone into one eye will cause both pupils to constrict.

<span class="mw-page-title-main">Eye color</span> Polygenic phenotypic characteristic

Eye color is a polygenic phenotypic trait determined by two factors: the pigmentation of the eye's iris and the frequency-dependence of the scattering of light by the turbid medium in the stroma of the iris.

The iris pigment epithelium (IPE) is a one cell thick layer of cuboidal cells lying behind the iris. The epithelial cells are highly pigmented due to the numerous large melanosomes which pack the cytoplasm of each cell. Towards the central axis, the IPE terminates at the pupillary margin. Peripherally, the IPE is continuous with the inner, non-pigmented layer of the ciliary epithelium. The iris dilator muscle is strictly attached to the anterior side of the iris pigmented epithelium and represents the anterior continuation of the pigmented ciliary epithelium. The ciliary epithelia represent the anterior continuation of the multilayered retina, whose retinal pigmented epithelium (RPE) corresponds to the pigmented ciliary epithelium, while the multilayered sensory retina fades into the non-pigmented ciliary epithelium. Despite their very different functions and histological appearances, these regions have a common origin from the two layers of the embryological optic cup. The melanosomes of the IPE are distinctive, being larger, blacker and rounder than those in the ciliary epithelium or RPE.

<span class="mw-page-title-main">Ciliary body</span> Part of the eye

The ciliary body is a part of the eye that includes the ciliary muscle, which controls the shape of the lens, and the ciliary epithelium, which produces the aqueous humor. The aqueous humor is produced in the non-pigmented portion of the ciliary body. The ciliary body is part of the uvea, the layer of tissue that delivers oxygen and nutrients to the eye tissues. The ciliary body joins the ora serrata of the choroid to the root of the iris.

<span class="mw-page-title-main">Horner's syndrome</span> Facial disorder due to damage of the sympathetic nerves

Horner's syndrome, also known as oculosympathetic paresis, is a combination of symptoms that arises when a group of nerves known as the sympathetic trunk is damaged. The signs and symptoms occur on the same side (ipsilateral) as it is a lesion of the sympathetic trunk. It is characterized by miosis, partial ptosis, apparent anhidrosis, with apparent enophthalmos.

<span class="mw-page-title-main">Ciliary ganglion</span> Bundle of nerves, parasympathetic ganglion

The ciliary ganglion is a parasympathetic ganglion located just behind the eye in the posterior orbit. It is 1–2 mm in diameter and in humans contains approximately 2,500 neurons. The ganglion contains postganglionic parasympathetic neurons. These neurons supply the pupillary sphincter muscle, which constricts the pupil, and the ciliary muscle which contracts to make the lens more convex. Both of these muscles are involuntary since they are controlled by the parasympathetic division of the autonomic nervous system.

<span class="mw-page-title-main">Iris dilator muscle</span> Smooth muscle of the eye

The iris dilator muscle, is a smooth muscle of the eye, running radially in the iris and therefore fit as a dilator. The pupillary dilator consists of a spokelike arrangement of modified contractile cells called myoepithelial cells. These cells are stimulated by the sympathetic nervous system. When stimulated, the cells contract, widening the pupil and allowing more light to enter the eye.

<span class="mw-page-title-main">Iris sphincter muscle</span> Muscle in the eye which constricts the pupil

The iris sphincter muscle is a muscle in the part of the eye called the iris. It encircles the pupil of the iris, appropriate to its function as a constrictor of the pupil.

<span class="mw-page-title-main">Zonule of Zinn</span> Part of the eye

The zonule of Zinn is a ring of fibrous strands forming a zonule that connects the ciliary body with the crystalline lens of the eye. These fibers are sometimes collectively referred to as the suspensory ligaments of the lens, as they act like suspensory ligaments.

<span class="mw-page-title-main">Iris cyst</span>

Iris cysts are hollow cavities in the eye filled with secretion. They come in various sizes, numbers, shapes, pigments and can be free-floating, attached to the pupillary margin or within the posterior chamber. Most frequently iris cysts don't cause any issues, but they can cause problems like: "fly biting" behavior, corneal endothelial pigment, lens capsular pigmentation, altered iris movement, decreased aqueous outflow with subsequent glaucoma or block the vision when grown too big. They can be acquired or innate. Possible causes are inflammation, drug-induced, uveitis, a trauma, tumor-induced, parasitic or implantation. Most frequently iris cysts are benign and need no treatment. Sometimes iris cysts are causing problems and need to be deflated. Iris cysts can be treated with trans corneal diode laser treatment, fine-needle aspiration or surgical excision. For the treatment of iris cysts is a conservative approach favored.

<span class="mw-page-title-main">Stroma of iris</span> Connective and vascular tissue of the iris

The stroma of the iris is a fibrovascular layer of tissue. It is the upper layer of two in the iris.

Polycoria is a pathological condition of the eye characterized by more than one pupillary opening in the iris. It may be congenital or result from a disease affecting the iris. It results in decreased function of the iris and pupil, affecting the physical eye and visualization.

<span class="mw-page-title-main">Pupillary response</span> Physiological response that varies the size of the pupil

Pupillary response is a physiological response that varies the size of the pupil, via the optic and oculomotor cranial nerve.

<span class="mw-page-title-main">Amelanism</span> Pigmentation abnormality

Amelanism is a pigmentation abnormality characterized by the lack of pigments called melanins, commonly associated with a genetic loss of tyrosinase function. Amelanism can affect fish, amphibians, reptiles, birds, and mammals including humans. The appearance of an amelanistic animal depends on the remaining non-melanin pigments. The opposite of amelanism is melanism, a higher percentage of melanin.

<span class="mw-page-title-main">Roots of the ciliary ganglion</span>

The ciliary ganglion is a parasympathetic ganglion located just behind the eye in the posterior orbit. Three types of axons enter the ciliary ganglion but only the preganglionic parasympathetic axons synapse there. The entering axons are arranged into three roots of the ciliary ganglion, which join enter the posterior surface of the ganglion.

References

  1. ↑ Liddell, Henry George; Scott, Robert (1940). "ἶρις". A Greek-English Lexicon. Perseus Digital Library.
  2. ↑ "iris" . Oxford English Dictionary (Online ed.). Oxford University Press.(Subscription or participating institution membership required.)
  3. 1 2 3 "eye, human." Encyclopædia Britannica from Encyclopædia Britannica 2006 Ultimate Reference Suite DVD
  4. 1 2 3 4 5 Gold, Daniel H; Lewis, Richard; "Clinical Eye Atlas," pp. 396–397
  5. ↑ Romer, Alfred Sherwood; Parsons, Thomas S. (1977). The Vertebrate Body. Philadelphia, PA: Holt-Saunders International. p. 462. ISBN   0-03-910284-X.
  6. ↑ "Aging Eyes and Pupil Size". Amateurastronomy.org. Archived from the original on 2013-10-23. Retrieved 2013-08-28.
  7. ↑ Winn, B.; Whitaker, D.; Elliott, D. B.; Phillips, N. J. (March 1994). "Factors Affecting Light-Adapted Pupil Size in Normal Human Subjects" (PDF). Investigative Ophthalmology & Visual Science. 35 (3): 1132–1137. PMID   8125724 . Retrieved 2013-08-28.
  8. ↑ Eberhardt, Lisa V.; Grön, Georg; Ulrich, Martin; Huckauf, Anke; Strauch, Christoph (2021-10-01). "Direct voluntary control of pupil constriction and dilation: Exploratory evidence from pupillometry, optometry, skin conductance, perception, and functional MRI". International Journal of Psychophysiology. 168: 33–42. doi:10.1016/j.ijpsycho.2021.08.001. ISSN   0167-8760.
  9. ↑ "Sensory Reception: Human Vision: Structure and function of the Human Eye" vol. 27, p. 175 Encyclopædia Britannica, 1987
  10. ↑ Baldwin, Barry (1981). "Physical Descriptions of Byzantine Emperors". Byzantion. 51 (1): 8–21. ISSN   0378-2506. JSTOR   44170668.
  11. ↑ Fronimopoulos, John; Lascaratos, John (1992-03-01). "Some Byzantine chroniclers and historians on ophthalmological topics". Documenta Ophthalmologica. 81 (1): 121–132. doi:10.1007/BF00155022. ISSN   1573-2622. PMID   1473460. S2CID   26240821.
  12. 1 2 Fabricius, Karl. "Heterochromia in Animals". Environmental Graffiti. Archived from the original on 2010-09-23. Retrieved 2010-10-27.
  13. ↑ "walleye", def. 1a, Merriam-Webster Dictionary
  14. ↑ Novella, Steven. "Iridology". Science-Based Medicine. Archived from the original on 1 July 2017. Retrieved 20 August 2017.
  15. ↑ Ernst E (January 2000). "Iridology: not useful and potentially harmful". Arch. Ophthalmol. 118 (1): 120–1. doi:10.1001/archopht.118.1.120. PMID   10636425.
  16. ↑ Stephen Barrett (9 November 2015). "Iridology Is Nonsense". Quackwatch . Retrieved 6 August 2023.
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