pigmentation


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pigmentation,

name for the coloring matter found in certain plant and animal cells and for the color produced thereby. Pigmentation occurs in nearly all living organisms. Almost all plants synthesize their own pigments; animals either derive pigments from plant foods or synthesize them themselves.

In plants the major pigments are the carotenes (reddish orange to yellow), the anthocyanins (red, blue, and violet), and the chlorophylls (green). The red and yellow colors of autumn foliage are due to the exposure of the anthocyanins after the green chlorophyll pigments, which usually mask them, have decomposed and faded. The major animal pigments are the hemes (red) of blood hemoglobin, the carotenes, the melanins (black and brown), and guanine (white and iridescent). The latter three produce the surface coloration of most animals.

Pigments not only provide external coloration but also function in some important physiological processes. In the retina of the eye the pigment cells (rods and cones) adjust or regulate the entering light (see visionvision,
physiological sense of sight by which the form, color, size, movements, and distance of objects are perceived. Vision in Humans

The human eye functions somewhat like a camera; that is, it receives and focuses light upon a photosensitive receiver, the retina.
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). Among its other functions, carotene operates in the synthesis of vitamins and of chlorophyll. Chlorophyll is essential for plant photosynthesisphotosynthesis
, process in which green plants, algae, and cyanobacteria utilize the energy of sunlight to manufacture carbohydrates from carbon dioxide and water in the presence of chlorophyll. Some of the plants that lack chlorophyll, e.g.
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. Hemoglobin in the blood carries oxygen for respiration. Chlorophyll and hemoglobin are structurally quite similar, both belonging to the pyrrole group of pigments.

Human Pigmentation

In humans the degree of darkness of the skin, hair, and iris of the eye depends primarily on the amount of melanin present. The presence of hemoglobin and carotene in the bloodblood,
fluid pumped by the heart that circulates throughout the body via the arteries, veins, and capillaries (see circulatory system; heart). An adult male of average size normally has about 6 quarts (5.6 liters) of blood.
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 contributes to skin color. Moles and freckles are caused by high local concentrations of melanin; albinism by a lack of melanin; and some birthmarks, e.g., "strawberry marks," by an unusual local proliferation of blood vessels (and hence of hemoglobin) near the skin surface. Tanning of human skin results from an increase of melanin production under the stimulation of ultraviolet light.

Pigment and Refraction in Coloration

The coloration of an organism may be caused by deposits of organic pigments in the tissues (as in human skin or in plant leaves), by optical effects of the refraction of light rays (as in mollusk shells and in some butterfly wings and bird feathers), or by a combination of both (see colorcolor,
effect produced on the eye and its associated nerves by light waves of different wavelength or frequency. Light transmitted from an object to the eye stimulates the different color cones of the retina, thus making possible perception of various colors in the object.
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). The different modes are illustrated in the baboon and the mandrill: the predominantly brown coloring is due to melanin, but the red and blue markings are also caused by melanin, in the latter case by the refraction of light due to specific spatial arrangements of the pigment granules in the skin areas involved.

Pigmentation Adaptation in Animals

The pigmentation of many animals is adapted to their environment and aids in their survival (see mimicrymimicry,
in biology, the advantageous resemblance of one species to another, often unrelated, species or to a feature of its own environment. (When the latter results from pigmentation it is classed as protective coloration.
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; protective colorationprotective coloration,
coloration or color pattern of an animal that affords it protection from observation either by its predators or by its prey. The most widespread form of protective coloration is called cryptic resemblance, in which various effects that supplement the
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). In some animals the pigment is changeable; the flounder and the squid, for example, are capable of adapting themselves to the color of their background and thus often of escaping detection by their enemies. The exact mechanism of such changeability is not clearly understood, but in most cases it is due primarily to visual stimulation. In the squid the chromatophores (containing melanin granules) are controlled by muscles and can expand from an almost invisible pinpoint to 60 times their original size, giving the whole animal a dark appearance. Pigmentation changes are also at least partially controlled by hormones—as, in part, is pigmentation synthesis itself.

Pigmentation

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 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 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 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 Skin

Pigmentation

 

the coloration of tissues and organs as a result of the formation and deposition of pigments. The pigmentation of skin, hair, and the iris, which depends on the quantity and distribution of the pigment melanin, is one of the main racial features that are considered in anthropology. Melanin is found in tissues in the form of granules or a solution in the cellular protoplasm. Skin color results from the presence of melanin in the epidermis and the translucence of capillaries in the derma. Melanin is formed by melanocytes, which are specialized cells in the basal layer of the epidermis. In light-skinned peoples the basal layer contains only a few melanin granules, while in dark-skinned peoples the layer is packed with granules.

Skin color is not uniform throughout the body. The extensor surfaces of the extremities are darker than the flexor surfaces, and the back is more pigmented than the chest or abdomen. Coloration is most intense around the nipples. Even in dark-skinned peoples, the palms and soles are light. The lips of light-skinned peoples are covered by a nonpigmented mucous membrane, and the red color of such lips is due to the translucence of labial capillaries. On the other hand, melanin in the mucous membrane imparts a bluish color to the lips of dark-skinned peoples. Exposure to the sun darkens the skin because solar rays intensify the formation of melanin, which is capable of absorbing the ultraviolet light that harms tissues. In this manner, dark skin protects an organism against sunlight. Skin color varies from pinkish, in light-skinned peoples of Europe, to chocolate, mostly in peoples that inhabit the tropics, for example, African Negroes, Papuans, Melanesians, and Australians. It is one of the most important inherited racial characteristics.

Hair color is also determined by the content of melanin, chiefly in the cortex of the hair. Melanin granules are formed in melanocytes, which are present in the epithelium of the hair follicle. Dark hair is rich in pigment granules, which can penetrate into the medullary column, or pith, of the hair; light hair has fewer and smaller granules. Melanin in solution imparts a reddish tint to hair. Hair color changes with age, light hair becoming darker. Graying is caused by the cessation of melanin biosynthesis. People with light or reddish hair are predominantly found only in northwestern Europe; elsewhere, dark hair is prevalent, but the intensity and shade vary even among dark-pigmented populations.

The color of the iris depends on the quantity of melanin and the depth at which the melanin is deposited. The pigment is found in the pigmented layer and the posterior limiting layer of the iris. If the anterior layers are nonpigmented, the melanin that shows through them imparts a dark or light blue color to the iris. If the pigment is also present in the anterior layers (the simple squamous epithelium and the stroma), the iris appears yellow or brown. An uneven distribution of pigment in the anterior layers produces green eyes, gray eyes, and eyes that appear to contain a mixture of yellow, brown, and light blue. The presence of large quantities of pigment in the anterior layers accounts for eyes that are black or brown. In some populations, women’s eyes are somewhat darker than men’s. Dark eyes lighten with age, while light eyes darken. Eye color more or less corresponds to hair color, although in moderately pigmented populations light eyes are found more often than light hair. The absence of normal pigmentation in the skin, hair, and iris, a condition called albinism, is due to a hereditary disturbance in melanin biosynthesis.

REFERENCES

Roginskii, Ia. Ia., and M. G. Levin. Antropologiia, 2nd ed. Moscow, 1963.
Biologiia cheloveka. Moscow, 1968. (Translated from English.)

T. D. GLADKOVA

pigmentation

[‚pig·mən′tā·shən]
(physiology)
The normal color of the body and its organs, resulting from a summation of the natural color of the tissue, the pigments deposited therein, and the pigments carried through the blood bathing the tissue.

pigmentation

1. coloration in plants, animals, or man caused by the presence of pigments
2. the deposition of pigment in animals, plants, or man
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