pigmentation (redirected from pigmentation disorders)

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 vision

Vision in Humans

The human eye functions somewhat like a camera; that is, it receives and focuses light upon a photosensitive
..... ). Among its other functions, carotene operates in the synthesis of vitamins and of chlorophyll. Chlorophyll is essential for plant photosynthesis. 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 blood 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 color). 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 mimicry; protective coloration). 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.

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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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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