In humans, melanin is the primary determinant of skin color. It is also found in hair, the pigmented tissue underlying the iris of the eye, and the stria vascularis of the inner ear. In the brain, tissues with melanin include the medulla and pigment-bearing neurons within areas of the brainstem, such as the locus coeruleus and the substantia nigra.
It also occurs in the zona reticularis of the adrenal gland. The melanin in the skin is produced by melanocytes, which are found in the basal layer of the epidermis. Although, in general, human beings possess a similar concentration of melanocytes in their skin, the melanocytes in some individuals and ethnic groups more frequently or less frequently express the melanin-producing genes, thereby conferring a greater or lesser concentration of skin melanin.
Some individual animals and humans have very little or no melanin synthesis in their bodies, a condition known as albinism. Because melanin is an aggregate of smaller component molecules, there are many different types of melanin with differing proportions and bonding patterns of these component molecules. Both pheomelanin and eumelanin are found in human skin and hair, but eumelanin is the most abundant melanin in humans, as well as the form most likely to be deficient in albinism.
Melanins have very diverse roles and functions in various organisms. A form of melanin makes up the ink used by many cephalopods (see cephalopod ink) as a defense mechanism against predators. Melanins also protect microorganisms, such as bacteria and fungi, against stresses that involve cell damage such as UV radiation from the sun and reactive oxygen species.
In invertebrates, a major aspect of the innate immune defense system against invading pathogens involves melanin. Within minutes after infection, the microbe is encapsulated within melanin (melanization), and the generation of free radical byproducts during the formation of this capsule is thought to aid in killing them.
Some types of fungi, called radiotrophic fungi, appear to be able to use melanin as a photosynthetic pigment that enables them to capture gamma rays. The black feathers of birds owe their color to melanin; they are much more readily degraded by bacteria than white feathers, or those containing other pigments such as carotenes.
In some mice, melanin is used slightly differently. For instance, in Agouti mice, the hair appears brown because of alternation between black eumelanin production and a yellow variety of pheomelanin. The hairs are actually banded black and yellow, and the net effect is the brown color of most mice. Some genetic irregularities can produce either fully black or fully yellow mice.
Catechol melanins are plant melanins.(wikipedia.org)
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