Melanin is a polymer of either or both of two monomer molecules: indolequinone, and dihydroxyindole carboxylic acid. Melanin exists in the plant, animal and protista kingdoms, where, among other functions, it serves as a pigment. The presence of melanin in the archaea and bacteria kingdoms is an issue of ongoing scholarly disagreement.
Melanin in humans
In humans melanin is found in skin, hair, the coat of the retina, the medulla and zona reticularis of the adrenal gland, the inner ear, and the substantia nigra of the brain. Melanin is produced by melanocytes, which in the skin are found in the stratum basale of the epidermis. Although human beings generally possess a similar concentration of melanocytes in their skin, the melanocytes in some individuals and races more frequently express the melanin-producing genes, thereby conferring a greater concentration of skin melanin. Some individual animals and humans have no or very little melanin in their bodies, which is a condition known as albinism.
Because melanin is an aggregate of smaller component molecules, there are a number of different types of melanin with differing proportions and bonding patterns of these component molecules. Eumelanin is found in skin and hair, and is the most abundant melanin in humans as well as the form most likely to be deficient in albinism.
Eumelanin polymers have long been thought to comprise numerous cross-linked dihydroxyindole polymers; recent research into the electrical properties of eumelanin, however, has indicated that it may consist of more basic oligomers adhering to one another by some other mechanism. Thus, the precise nature of eumelanin's molecular structure is once again the object of study.
Pheomelanin is found in human skin and hair, though it is considerably less abundant in humans than eumelanin. Pheomelanin imparts a reddish hue, and is found in particularly large quantities in red hair.
Neuromelanin is a byproduct of dopamine metabolism, and is found in the substantia nigra of the human brain.
Melanin and human adaptation
Melanocytes insert granules of melanin called melanosomes into the other skin cells of the human epidermis. The melanosomes in each recipient cell accumulate atop the cellular nucleus, where they protect the nuclear DNA from mutations caused by the sun's ionizing radiation. People whose ancestors lived for long periods in the regions of the globe near the Equator generally have more active melanocytes, and therefore larger quantities of melanin in their skins. This makes their skins dark brown or black and protects them against high levels of exposure to the sun. In areas of the globe closer to the poles, people have far less need for protection from ionizing radiation, so their skin is usually lighter in colour. This allows sunlight to stimulate vitamin D production by the liver. The darker one's skin is, however, the more likely one is to suffer from vitamin D deficiency.
The most recent scientific evidence indicates that all humanity originated in Africa. As a result, all original people had relatively large numbers of active melanocytes and, accordingly, darker skin. As some of these original tribes of people migrated and settled in areas of Asia and Europe, their melanin production gradually decreased to allow acclimation to climates where radiation from the sun was less intense. It is estimated that this color evolution in an immigrated populace takes approximately 10,000 years to complete.
As with peoples that migrated northward, those with light skin that migrated southward had to acclimate to the much stronger solar radiation. Those people, who produced more skin-protecting melanin, were less likely to suffer from skin cancer, as well as other health problems related to exposure to strong solar radiation, including the photodegradation of certain vitamins such as riboflavins, carotenoids, tocopherol, and folate.
Most people's skin darkens when exposed to UV light, giving them more protection when it is needed. The closer one lives to the Equator, where the sun's radiation is strongest, the greater the advantage of having high melanin levels in the skin. Dark skin affords far greater protection than fair skin against sunburn and the development of melanoma, a potentially extremely deadly form of skin cancer.
Higher melanin levels also can be a disadvantage, however, beyond a higher disposition in darker-skinned individuals toward vitamin D deficiency. Dark skin is a complicating factor in the laser removal of port-wine stains. Effective in treating fair skin, lasers generally are less successful in removing port-wine stains in Asians and blacks. Higher concentrations of melanin in darker-skinned individuals simply diffuse and absorb the laser radiation, inhibiting light absorption by the targeted tissue. Melanin similarly can complicate laser treatment of other dermatological conditions in people of color.
Freckles and moles are formed where there is a greater concentration of melanin in the skin.
Albinism is a condition in which an animal (such as a human or a mouse) is incapable of producing melanin, and hence appears white with pink eyes.
Physical properties and technological applications
Melanin is a biopolymer and a neuropeptide. In the early 1970's, researchers found melanin to be an organic semiconductor. Studies revealed that melanin acted as an electrical threshold switch, emitting a flash of light— electroluminescence— when it switched. This knowledge was considered lost for a time, but in 2000, the Nobel Prize in Chemistry was awarded to three scientists for their work in the discovery and development of conductive polymers. The polymers utilized in the research included melanins.
Melanin influences neural activity and mediates the conduction of radiation, light, heat and kinetic energy. As such, it is the subject of intense interest in biotech research and development, most notably in organic electronics (sometimes called "plastic electronics") and nanotechnology.
Melanin-based bias in human societies
When skin pigmentation as a characteristic of race becomes significant in some way, this phenomenon is known as racialism. Many people and societies overlay racialism with racist perceptions and systems which arbitrarily assign to groups of people a status of inherent superiority or inferiority, privilege or disadvantage based on skin color or racial classification. Apartheid-era South Africa is an example of a white supremacist society based on a system of stratification of power and privilege by skin color, as well as racial admixture. Similar examples can be found in India's caste system; Brazil's highly socially color-stratified society; and, in the U.S., segregation and institutional racism on the part of white-controlled institutions, and internal "color consciousness" on the part some ethnic minorities. Because of the pervasive influence of white supremacist values worldwide, prejudice against people with more highly pigmented skin is the most pervasive form of color bias. Conversely, black supremacy is a far less pervasive phenomenon. Many other societies remain informally divided on the basis of skin color and, often, related ethnicity. See also colonialism, Nazism and institutional racism.
Illogical presumptions about people with regard to hair color are far less common than skin-color bias, have far fewer and less serious real-word implications, and are more often applied to women than to men. Common stereotypes in the West are dumb blondes, hot-tempered redheads and vixen brunettes.
See also
Last updated: 05-13-2005 07:56:04
