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.
See also adrenal gland; albinos; eyes; freckles; hair; pituitary gland; skin; skin colour; sun and the body.
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). 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.
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.
pig·men·ta·tion / ˌpigmənˈtāshən/ • n. the natural coloring of animal or plant tissue. ∎ the coloring of a person's skin, esp. when abnormal or distinctive.