Dictionary:

pigmentation

  (pĭg'mən-tā'shən)

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1. Coloration of tissues by pigment.
2. Deposition of pigment by cells.

A property of biological materials that imparts coloration. Hence, pigmentation determines the quantity and quality of reflected visible light. The characteristics of light returning from living matter are a function of its chemical and physical properties and, therefore, are not only due to pigments proper but can be of structural origin (for example, due to reflection, scattering, or interference) as well.

Pigments are essential constituents of the living world. Their contribution to the evolution and maintenance of life, and its manifold expressions, is most evident in the role of chlorophylls and the associated carotenoids of certain bacteria and most plants. These pigments harvest solar light energy for utilization in the photosynthesis of organic material from inorganic precursors. See also Carotenoid; Chlorophyll; Photosynthesis.

The outermost structures on the animal skin are pigmented for many reasons, for example, to reduce the animal's visibility against a colored background or to provide optical signals to the other sex or to other species. Conspicuously pigmented flowers attract pollinators, and colored fruits are easily found by animals, which eat them and then disperse the undigested seeds.

The role of pigments in communication depends on the ability of organisms to discriminate between different regions of the solar spectrum. In animals with eyes, this is accomplished by differently colored visual pigments contained in specialized receptor cells. Microorganisms, fungi, and plants also have special pigment systems that permit these organisms to move or grow toward, or away from, light (positive and negative phototaxis and phototropism, respectively). See also Plant movements.

Since most organisms are totally dependent on light—at least indirectly—elaborate pigment systems have evolved which tune metabolic and activity patterns to the daily pattern of light and dark, and to the changes in the relative lengths of day and night in the course of a year. The phytochrome of plants and the pigments of the eye or of extraretinal photoreceptor organs of many vertebrates and invertebrates are typical representatives of pigments that correlate biological activity with light-dark cycles (photoperiodism). See also Color vision; Photoperiodism; Photoreception.

In the examples listed above, pigments mediate, in various ways, the beneficial actions of light. Absorbed solar light energy may, however, also have detrimental effects by causing undesirable or even destructive reactions. Pigmentations can provide a light-absorbing shield that protects the tissue below from such potentially damaging radiation of the Sun. See also Skin.

Pigmentation in the body is due to different forms of melanin, the name for a group of closely related substances, of different shades of black and brown, that are synthesized in body cells (melanocytes), mostly from the amino acid tyrosine. The most widespread type of melanin is responsible for skin colour; the melanocytes, deep in the epidermis, make it, and transfer it into the keratin cell layer of the skin. In ‘white’ skin, freckles and moles are areas of concentrated pigmentation; exposure to sunlight results in stimulation of the melanocytes and hence to a ‘tan’. Excessive stimulation can sometimes produce a melanoma, which can have serious consequences if not recognized and treated at an early stage.

Variations in eye and hair colour are due to different types of melanin. As well as the visible sites, there is also melanin pigmentation in the choroid coat of the eye, the vascular layer that invests all but the front part of the eyeball, behind and around the retina; also in the cochlea and vestibule of the inner ear, variably in the adrenal glands, and in the substantia nigra of the brain. It is normal for pigmentation to increase during pregnancy, particularly in the areola of the nipples, and in the ‘stretch marks’ on the abdomen.

Abnormalities of pigmentation are of interest in understanding its normal significance. Vitiligo is a relatively common autoimmune condition, in which melanocytes are patchily destroyed, causing totally white areas in the skin. Provided that sun creams can be used for protection, the problem is only — though disturbingly — a cosmetic one, seriously so for dark-skinned people; small patches of depigmentation can also occur in the iris. In albinos, pigmentation is defective everywhere: not only in the skin, hair, and iris but also behind the retina, with associated visual problems; melanocytes are present, but melanin synthesis is impossible due to congenital lack of the enzyme tyrosinase.

Excessive pigmentation can be caused by oversecretion of adrenocorticotrophic hormones (ACTH) and an associated melanocyte-stimulating hormone (MSH) from the pituitary gland.

In 1855 Thomas Addison, physician at Guy's Hospital, London, reported ‘… a peculiar change of colour in the skin, occurring in connection with a diseased condition of the “supra-renal capsules”.’ It was some considerable time before this link between pigmentation and disease of the adrenal glands was to be explained. We now know that if the adrenal cortex fails to maintain its normal output of hormones (notably cortisol), the low levels in the blood are detected in the hypothalamus, through the normal feedback mechanism, and ‘interpreted’ as requiring a stimulus to action; thus the anterior pituitary is signalled to release more ACTH, which vainly flogs the dying horse. Whenever ACTH synthesis and secretion are increased, there is linked release of the closely associated MSH, and hence an increase in melanin production in the skin. This explanation is confirmed by observation of excessive pigmentation in other conditions in which ACTH secretion is excessive, and absence of such pigmentation when failure of the adrenal cortex is secondary to a failure of ACTH secretion itself. Primary disorder of the adrenal cortex — hypoadrenalism or ‘Addison's disease’ — was formerly most often due to tuberculosis; it is now rare, and usually the result of autoimmune destruction of the hormone-producing cells.

The link between these abnormal situations and the physiological significance of melanin secretion seems at first sight obscure. Whereas ACTH, and the adrenal secretions that it promotes, have now for long been recognized as counteracting the consequences to the body of actual or potential damage or stress, more recently it has been shown that there is a range of associated substances released from the pituitary gland along with ACTH, which may all contribute, one way and another, to protective responses. Melanin in the skin is best known for its protection against UV light; it does this by taking up ‘free radicals’. Since these are released in a variety of other potentially harmful situations, not just in exposure to sunlight, melanin may have additional protective functions.

— Sheila Jennett

See also adrenal gland; albinos; eyes; freckles; hair; pituitary gland; skin; skin colour; sun and the body.

The deposition of coloring matter; the coloration or discoloration of a part by a pigment. See also hyperpigmentation, hypopigmentation.

• bacterial p. — production of pigment is a characteristic of some bacteria which may be useful in identification. Examples are Pseudomonas aeruginosa, Chromobacterium violaceum and Serratia marcescens.
• p. disorders — see hyperpigmentation, leukoderma, leukotrichia, hypopigmentation.
• hematogenous p. — pigmentation produced by accumulation of hemoglobin derivatives, such as hematoidin or hemosiderin.

Dansk (Danish)
n. - pigmentering, farve

Nederlands (Dutch)
natuurlijke kleur van planten/dieren etc., (overmatige) aanwezigheid van kleurstof

Français (French)
n. - pigmentation

Deutsch (German)
n. - Pigmentierung, Pigmentation

Ελληνική (Greek)
n. - φυσικός χρωματισμός (ζώου, φυτού κ.λπ.)

Italiano (Italian)
pigmentazione

Português (Portuguese)
n. - pigmentação (f), cor (f)

Русский (Russian)
пигментация

Español (Spanish)
n. - pigmentación

Svenska (Swedish)
n. - färg, pigmentering

中文（简体） (Chinese (Simplified))
染色, 色素之形成, 着色

中文（繁體） (Chinese (Traditional))
n. - 染色, 色素之形成, 著色

한국어 (Korean)
n. - 염색, 색소 형성

日本語 (Japanese)
n. - 染色, 着色, 色素形成, 色素沈着

العربيه (Arabic)
‏(الاسم) صبغ, خضب, اصطباغ, اختضاب‏

עברית (Hebrew)
n. - ‮צביעה (בפיגמנט)‬

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