Melanin: Difference between revisions

Melanin
Identifiers
CAS Number	8049-97-6;
ChemSpider	4884931 ;
PubChem CID	6325610;
Properties
Chemical formula	C18H10N2O4
Molar mass	318.288 g·mol−1
Density	1.6 to 1.8 g/cm3
Melting point	< −20 °C (−4 °F; 253 K)
Boiling point	450 to 550 °C (842 to 1,022 °F; 723 to 823 K)
	Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

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Melanin (/ˈmɛlənɪn/ ^ⓘ; from Greek: μέλας melas, "black, dark") is a broad term for a group of natural pigments found in most organisms. Melanin is produced through a multistage chemical process known as melanogenesis, where the oxidation of the amino acid tyrosine is followed by polymerization. The melanin pigments are produced in a specialized group of cells known as melanocytes.

There are three basic types of melanin: eumelanin, pheomelanin, and neuromelanin. The most common type is eumelanin, of which there are two types— brown eumelanin and black eumelanin. Pheomelanin is a cysteine-derivative that contains polybenzothiazine portions that are largely responsible for the color of red hair, among other pigmentation. Neuromelanin is found in the brain. Research has been undertaken to investigate its efficacy in treating neurodegenerative disorders such as Parkinson's.^[1]

In the human skin, melanogenesis is initiated by exposure to UV radiation, causing the skin to darken. Melanin is an effective absorbent of light; the pigment is able to dissipate over 99.9% of absorbed UV radiation.^[2] Because of this property, melanin is thought to protect skin cells from UVB radiation damage, reducing the risk of folate depletion and dermal degradation, and it is considered that exposure to UV radiation is associated with increased risk of malignant melanoma, a cancer of melanocytes (melanin cells). Studies have shown a lower incidence for skin cancer in individuals with more concentrated melanin, i.e. darker skin tone. However, the relationship between skin pigmentation and photoprotection is still uncertain.^[3]

Humans

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 . It also occurs in the zona reticularis of the adrenal gland.^[4]

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 produce variable amounts of melanin. Some humans have very little or no melanin synthesis in their bodies, a condition known as albinism.^[5]

Because melanin is an aggregate of smaller component molecules, there are many different types of melanin with different 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.^[6]

Eumelanin

Eumelanin polymers have long been thought to comprise numerous cross-linked 5,6-dihydroxyindole (DHI) and 5,6-dihydroxyindole-2-carboxylic acid (DHICA) polymers.^[7]

There are two types of eumelanin, which are brown eumelanin and black eumelanin. Those two types of eumelanin chemically differ from each other in their pattern of polymeric bonds. A small amount of black eumelanin in the absence of other pigments causes grey hair. A small amount of brown eumelanin in the absence of other pigments causes yellow (blond) hair. As the body ages, it continues to produce black eumelanin but stops producing brown eumelanin, resulting in the grey hair that is common in elderly people.^[8]

Pheomelanin

Pheomelanins (or phaeomelanins) impart a range of yellowish to reddish colors.^[9] Pheomelanins are particularly concentrated in the lips, nipples, glans of the penis, and vagina.^[10] When a small amount of brown eumelanin in hair, which would otherwise cause blond hair, is mixed with red pheomelanin, the result is orange hair, which is typically called "red" or "ginger" hair. Pheomelanin is also present in the skin, and redheads consequently often have a more pinkish hue to their skin as well.

In chemical terms, pheomelanins differ from eumelanins in that the oligomer structure incorporates benzothiazine and benzothiazole units that are produced,^[11] instead of DHI and DHICA, when the amino acid L-cysteine is present.

Trichochromes

Trichochromes (formerly called trichosiderins) are pigments produced from the same metabolic pathway as the eumelanins and pheomelanins, but unlike those molecules they have low molecular weight. They occur in some red human hair.^[12]

Neuromelanin

Neuromelanin (NM) is a dark insoluble polymer pigment produced in specific populations of catecholaminergic neurons in the brain. Humans have the largest amount of NM, which is present in lesser amounts in other primates, and totally absent in many other species.^[13] The biological function remains unknown, although human NM has been shown to efficiently bind transition metals such as iron, as well as other potentially toxic molecules. Therefore, it may play crucial roles in apoptosis and the related Parkinson's disease.^[14]

Other organisms

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. Melanin also protects against damage from high temperatures, chemical stresses (such as heavy metals and oxidizing agents), and biochemical threats (such as host defenses against invading microbes).^[15] Therefore, in many pathogenic microbes (for example, in Cryptococcus neoformans, a fungus) melanins appear to play important roles in virulence and pathogenicity by protecting the microbe against immune responses of its host. 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.^[16] 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^[17] and harness this energy for growth.^[18]

The darker feathers of birds owe their color to melanin and are less readily degraded by bacteria than unpigmented ones or those containing carotenoid pigments.^[19] Feathers that contain melanin are also 39% more resistant to abrasion than those that do not because melanin granules help fill the space between the keratin strands that form feathers.^[20]^[21] Pheomelanin synthesis in birds implies the consumption of cysteine, a semi‐essential amino acid that is necessary for the synthesis of the antioxidant glutathione (GSH) but that may be toxic if in excess in the diet. Indeed, carnivorous birds, which have a high protein content in their diet, exhibit pheomelanin‐based coloration.^[22]

Melanin is also important in mammalian pigmentation.^[23] The coat pattern of mammals is determined by the agouti gene which regulates the distribution of melanin.^[24]^[25] The mechanisms of the gene have been extensively studied in mice to provide an insight into the diversity of mammalian coat patterns.^[26]

Melanin in arthropods has been observed to be deposited in layers thus producing a Bragg reflector of alternating refractive index. When the scale of this pattern matches the wavelength of visible light, structural coloration arises: giving a number of species an iridescent color.^[27]

Arachnids are one of the few groups in which melanin has not been easily detected, though researchers found data suggesting spiders do in fact produce melanin.^[28]

Some moth species, including the wood tiger moth, convert resources to melanin in order to enhance their thermoregulation. As the wood tiger moth has populations over a large range of latitudes, it has been observed that more northern populations showed higher rates of melanization. In both yellow and white male phenotypes of the wood tiger moth, individuals with more melanin had a heightened ability to trap heat but an increased predation rate due to a weaker and less effective aposematic signal.^[29]

Plants

Chemical structure of indole-5,6-quinone

Melanin produced by plants are sometimes referred to as 'catechol melanins' as they can yield catechol on alkali fusion. It is commonly seen in the enzymatic browning of fruits such as bananas. Chestnut shell melanin can be used as an antioxidant and coloring agent.^[30] Biosynthesis involves the oxidation of indole-5,6-quinone by the tyrosinase type polyphenol oxidase from tyrosine and catecholamines leading to the formation of catechol melanin. Despite this many plants contain compounds which inhibit the production of melanins.^[31]

Biosynthetic pathways

The first step of the biosynthetic pathway for both eumelanins and pheomelanins is catalysed by tyrosinase.^[32]

Tyrosine → DOPA → dopaquinone

Dopaquinone can combine with cysteine by two pathways to benzothiazines and pheomelanins

Dopaquinone + cysteine → 5-S-cysteinyldopa → benzothiazine intermediate → pheomelanin

Dopaquinone + cysteine → 2-S-cysteinyldopa → benzothiazine intermediate → pheomelanin

Also, dopaquinone can be converted to leucodopachrome and follow two more pathways to the eumelanins

Dopaquinone → leucodopachrome → dopachrome → 5,6-dihydroxyindole-2-carboxylic acid → quinone → eumelanin

Dopaquinone → leucodopachrome → dopachrome → 5,6-dihydroxyindole → quinone → eumelanin

Detailed metabolic pathways can be found in the KEGG database (see External links).

Microscopic appearance

Melanin is brown, non-refractile, and finely granular with individual granules having a diameter of less than 800 nanometers. This differentiates melanin from common blood breakdown pigments, which are larger, chunky, and refractile, and range in color from green to yellow or red-brown. In heavily pigmented lesions, dense aggregates of melanin can obscure histologic detail. A dilute solution of potassium permanganate is an effective melanin bleach.^[33]

Genetic disorders and disease states

There are approximately nine types of oculocutaneous albinism, which is mostly an autosomal recessive disorder. Certain ethnicities have higher incidences of different forms. For example, the most common type, called oculocutaneous albinism type 2 (OCA2), is especially frequent among people of black African descent. It is an autosomal recessive disorder characterized by a congenital reduction or absence of melanin pigment in the skin, hair, and eyes. The estimated frequency of OCA2 among African-Americans is 1 in 10,000, which contrasts with a frequency of 1 in 36,000 in white Americans.^[34] In some African nations, the frequency of the disorder is even higher, ranging from 1 in 2,000 to 1 in 5,000.^[35] Another form of Albinism, the "yellow oculocutaneous albinism", appears to be more prevalent among the Amish, who are of primarily Swiss and German ancestry. People with this IB variant of the disorder commonly have white hair and skin at birth, but rapidly develop normal skin pigmentation in infancy.^[35]

Ocular albinism affects not only eye pigmentation but visual acuity, as well. People with albinism typically test poorly, within the 20/60 to 20/400 range. In addition, two forms of albinism, with approximately 1 in 2700 most prevalent among people of Puerto Rican origin, are associated with mortality beyond melanoma-related deaths.

The connection between albinism and deafness is well known, though poorly understood. In his 1859 treatise On the Origin of Species, Charles Darwin observed that "cats which are entirely white and have blue eyes are generally deaf".^[36] In humans, hypopigmentation and deafness occur together in the rare Waardenburg's syndrome, predominantly observed among the Hopi in North America.^[37] The incidence of albinism in Hopi Indians has been estimated as approximately 1 in 200 individuals. Similar patterns of albinism and deafness have been found in other mammals, including dogs and rodents. However, a lack of melanin per se does not appear to be directly responsible for deafness associated with hypopigmentation, as most individuals lacking the enzymes required to synthesize melanin have normal auditory function.^[38] Instead the absence of melanocytes in the stria vascularis of the inner ear results in cochlear impairment,^[39] though why this is, is not fully understood.

In Parkinson's disease, a disorder that affects neuromotor functioning, there is decreased neuromelanin in the substantia nigra and locus coeruleus as consequence of specific dropping out of dopaminergic and noradrenergic pigmented neurons. This results in diminished dopamine and norepinephrine synthesis. While no correlation between race and the level of neuromelanin in the substantia nigra has been reported, the significantly lower incidence of Parkinson's in blacks than in whites has "prompt[ed] some to suggest that cutaneous melanin might somehow serve to protect the neuromelanin in substantia nigra from external toxins."^[40]

In addition to melanin deficiency, the molecular weight of the melanin polymer may be decreased by various factors such as oxidative stress, exposure to light, perturbation in its association with melanosomal matrix proteins, changes in pH, or in local concentrations of metal ions. A decreased molecular weight or a decrease in the degree of polymerization of ocular melanin has been proposed to turn the normally anti-oxidant polymer into a pro-oxidant. In its pro-oxidant state, melanin has been suggested to be involved in the causation and progression of macular degeneration and melanoma.^[41] Rasagiline, an important monotherapy drug in Parkinson's disease, has melanin binding properties, and melanoma tumor reducing properties.^[42]

Higher eumelanin levels also can be a disadvantage, however, beyond a higher disposition toward vitamin D deficiency. Dark skin is a complicating factor in the laser removal of port-wine stains. Effective in treating white skin, in general, lasers are less successful in removing port-wine stains in people of Asian or African descent. Higher concentrations of melanin in darker-skinned individuals simply diffuse and absorb the laser radiation, inhibiting light absorption by the targeted tissue. In similar manner, melanin can complicate laser treatment of other dermatological conditions in people with darker skin.

Freckles and moles are formed where there is a localized concentration of melanin in the skin. They are highly associated with pale skin.

Nicotine has an affinity for melanin-containing tissues because of its precursor function in melanin synthesis or its irreversible binding of melanin. This has been suggested to underlie the increased nicotine dependence and lower smoking cessation rates in darker pigmented individuals.^[43]

Human adaptation

Physiology

Melanocytes insert granules of melanin into specialized cellular vesicles called melanosomes. These are then transferred into the keratinocyte cells of the human epidermis. The melanosomes in each recipient cell accumulate atop the cell nucleus, where they protect the nuclear DNA from mutations caused by the ionizing radiation of the sun's ultraviolet rays. In general, people whose ancestors lived for long periods in the regions of the globe near the equator have larger quantities of eumelanin in their skins. This makes their skins brown or black and protects them against high levels of exposure to the sun, which more frequently result in melanomas in lighter-skinned people.^[44]

Not all the effects of pigmentation are advantageous. Pigmentation increases the heat load in hot climates, and dark-skinned people absorb 30% more heat from sunlight than do very light-skinned people, although this factor may be offset by more profuse sweating. In cold climates dark skin entails more heat loss by radiation. Pigmentation also hinders synthesis of vitamin D, so that in areas of poor nutrition darker skinned children are more liable to rickets than lighter skinned children. Since pigmentation appears to be not entirely advantageous to life in the tropics, other hypotheses about its biological significance have been advanced, for example a secondary phenomenon induced by adaptation to parasites and tropical diseases.^[45]

Evolutionary origins

Early humans evolved to have dark skin color around 1.2 million years ago, as an adaptation to a loss of body hair that increased the effects of UV radiation. Before the development of hairlessness, early humans had reasonably light skin underneath their fur, similar to that found in other primates.^[46] The most recent scientific evidence indicates that anatomically modern humans evolved in Africa between 200,000 and 100,000 years,^[47] and then populated the rest of the world through one migration between 80,000 and 50,000 years ago, in some areas interbreeding with certain archaic human species (Neanderthals, Denisovans, and possibly others).^[48] It seems likely that the first modern humans had relatively large numbers of eumelanin-producing melanocytes, producing darker skin similar to the indigenous people of Africa today. As some of these original people migrated and settled in areas of Asia and Europe, the selective pressure for eumelanin production decreased in climates where radiation from the sun was less intense. This eventually produced the current range of human skin color. Of the two common gene variants known to be associated with pale human skin, Mc1r does not appear to have undergone positive selection,^[49] while SLC24A5 has undergone positive selection.^[50]

Effects

As with peoples having migrated northward, those with light skin migrating toward the equator acclimatize to the much stronger solar radiation. Nature selects for less melanin when ultraviolet radiation is weak. Most people's skin darkens when exposed to UV light, giving them more protection when it is needed. This is the physiological purpose of sun tanning. Dark-skinned people, who produce more skin-protecting eumelanin, have a greater protection against sunburn and the development of melanoma, a potentially deadly form of 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.^[51] Some Northwestern Europeans have substantially lost the ability to tan as a result of relaxed natural selection. Their skin burns and peels rather than tans. This is due to the fact that they produce a defective form of a skin protein Mc1r (melanocortin-1 receptor) which is necessary for the production of melanin. They are at a distinct disadvantage in tropical and subtropical environments. Not only do they suffer the discomfort of readily burning, but they are at a much higher risk for skin cancer; the same is true of albinos.^[52]

Melanin in the eyes, in the iris and choroid, helps protect them from ultraviolet and high-frequency visible light; people with gray, blue, and green eyes are more at risk of sun-related eye problems. Further, the ocular lens yellows with age, providing added protection. However, the lens also becomes more rigid with age, losing most of its accommodation — the ability to change shape to focus from far to near — a detriment due probably to protein crosslinking caused by UV exposure.

Recent research suggests that melanin may serve a protective role other than photoprotection.^[53] Melanin is able to effectively chelate metal ions through its carboxylate and phenolic hydroxyl groups, in many cases much more efficiently than the powerful chelating ligand ethylenediaminetetraacetate (EDTA). Thus, it may serve to sequester potentially toxic metal ions, protecting the rest of the cell. This hypothesis is supported by the fact that the loss of neuromelanin observed in Parkinson's disease is accompanied by an increase in iron levels in the brain.

Physical properties and technological applications

Evidence exists in support of a highly cross-linked heteropolymer bound covalently to matrix scaffolding melanoproteins.^[54] It has been proposed that the ability of melanin to act as an antioxidant is directly proportional to its degree of polymerization or molecular weight.^[55] Suboptimal conditions for the effective polymerization of melanin monomers may lead to formation of lower-molecular-weight, pro-oxidant melanin that has been implicated in the causation and progression of macular degeneration and melanoma.^[56] Signaling pathways that upregulate melanization in the retinal pigment epithelium (RPE) also may be implicated in the downregulation of rod outer segment phagocytosis by the RPE. This phenomenon has been attributed in part to foveal sparing in macular degeneration.^[57]

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^ Wade, Nicholas (2003-08-19). "Why Humans and Their Fur Parted Ways". The New York Times. ISSN 0362-4331. Retrieved 2019-08-24.
^ Tishkoff SA, Reed FA, Friedlaender FR, et al. (May 2009). "The genetic structure and history of Africans and African Americans". Science. 324 (5930): 1035–44. Bibcode:2009Sci...324.1035T. doi:10.1126/science.1172257. PMC 2947357. PMID 19407144.
^ "A Single Migration From Africa Populated the World, Studies Find". New York Times. 2016-09-22.
^ Harding RM; Healy E; Ray AJ; et al. (April 2000). "Evidence for variable selective pressures at MC1R". American Journal of Human Genetics. 66 (4): 1351–61. doi:10.1086/302863. PMC 1288200. PMID 10733465. {{cite journal}}: Unknown parameter |name-list-format= ignored (|name-list-style= suggested) (help)
^ Lamason RL, Mohideen MA, Mest JR, et al. (December 2005). "SLC24A5, a putative cation exchanger, affects pigmentation in zebrafish and humans". Science. 310 (5755): 1782–6. Bibcode:2005Sci...310.1782L. doi:10.1126/science.1116238. PMID 16357253. S2CID 2245002.
^ Jablonski, Nina G.; Chaplin, George (2010-05-11). "Human skin pigmentation as an adaptation to UV radiation". Proceedings of the National Academy of Sciences. 107 (Supplement 2): 8962–8968. Bibcode:2010PNAS..107.8962J. doi:10.1073/pnas.0914628107. PMC 3024016. PMID 20445093.
^ https://www2.palomar.edu/anthro/adapt/adapt_4.htm
^ Liu Y, Hong L, Kempf VR, Wakamatsu K, Ito S, Simon JD (June 2004). "Ion-exchange and adsorption of Fe(III) by Sepia melanin". Pigment Cell Research. 17 (3): 262–9. doi:10.1111/j.1600-0749.2004.00140.x. PMID 15140071.
^ Donatien PD, Orlow SJ (August 1995). "Interaction of melanosomal proteins with melanin". European Journal of Biochemistry. 232 (1): 159–64. doi:10.1111/j.1432-1033.1995.tb20794.x. PMID 7556145.
^ Sarangarajan R, Apte SP (2005). "Melanin aggregation and polymerization: possible implications in age-related macular degeneration". Ophthalmic Research. 37 (3): 136–41. doi:10.1159/000085533. PMID 15867475. S2CID 27499198.
^ Meyskens FL, Farmer PJ, Anton-Culver H (April 2004). "Etiologic pathogenesis of melanoma: a unifying hypothesis for the missing attributable risk" (PDF). Clinical Cancer Research. 10 (8): 2581–3. doi:10.1158/1078-0432.ccr-03-0638. PMID 15102657. S2CID 26079375.
^ Sarangarajan R, Apte SP (2005). "Melanization and phagocytosis: implications for age related macular degeneration". Molecular Vision. 11: 482–90. PMID 16030499.

External links

"Absorption spectrum of melanin". Department of Computer Science and Technology.
"Tyrosine metabolism - Reference pathway". Kyoto Encyclopedia of Genes and Genomes.
"Melanogenesis - Reference pathway". Kyoto Encyclopedia of Genes and Genomes.

[1] Haining, Robert L.; Achat-Mendes, Cindy (March 2017). "Neuromelanin, one of the most overlooked molecules in modern medicine, is not a spectator". Neural Regeneration Research. 12 (3): 372–375. doi:10.4103/1673-5374.202928. PMC 5399705. PMID 28469642.{{cite journal}}: CS1 maint: unflagged free DOI (link)

[Meredith-2] Meredith P, Riesz J (2004). "Radiative relaxation quantum yields for synthetic eumelanin". Photochemistry and Photobiology. 79 (2): 211–6. arXiv:cond-mat/0312277. doi:10.1111/j.1751-1097.2004.tb00012.x. PMID 15068035.

[Electrodial_Crocodile_Style-3] Brenner M, Hearing VJ (2008). "The protective role of melanin against UV damage in human skin". Photochemistry and Photobiology. 84 (3): 539–49. doi:10.1111/j.1751-1097.2007.00226.x. PMC 2671032. PMID 18435612.

[4] Solano, F. (2014). "Melanins: Skin Pigments and Much More—Types, Structural Models, Biological Functions, and Formation Routes". New Journal of Science. 2014: 1–28. doi:10.1155/2014/498276.

[5] Cichorek, Mirosława; Wachulska, Małgorzata; Stasiewicz, Aneta; Tymińska, Agata (February 20, 2013). "Skin melanocytes: biology and development". Advances in Dermatology and Allergology. 30 (1): 30–41. doi:10.5114/pdia.2013.33376. PMC 3834696. PMID 24278043.

[6] "oculocutaneous albinism". Genetics Home Reference. Retrieved 2017-09-25.

[7] Meredith, Paul; Sarna, Tadeusz (2006-12-01). "The physical and chemical properties of eumelanin". Pigment Cell Research. 19 (6): 572–594. doi:10.1111/j.1600-0749.2006.00345.x. PMID 17083485.

[8] Ito, S.; Wakamatsu, K. (December 2011). "Diversity of human hair pigmentation as studied by chemical analysis of eumelanin and pheomelanin". Journal of the European Academy of Dermatology and Venereology: JEADV. 25 (12): 1369–1380. doi:10.1111/j.1468-3083.2011.04278.x. ISSN 1468-3083. PMID 22077870.

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[10] "pheomelanin". MetaCyc Metabolic Pathway Database. 2010. ^{[full citation needed]}

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[47] Tishkoff SA, Reed FA, Friedlaender FR, et al. (May 2009). "The genetic structure and history of Africans and African Americans". Science. 324 (5930): 1035–44. Bibcode:2009Sci...324.1035T. doi:10.1126/science.1172257. PMC 2947357. PMID 19407144.

[48] "A Single Migration From Africa Populated the World, Studies Find". New York Times. 2016-09-22.

[49] Harding RM; Healy E; Ray AJ; et al. (April 2000). "Evidence for variable selective pressures at MC1R". American Journal of Human Genetics. 66 (4): 1351–61. doi:10.1086/302863. PMC 1288200. PMID 10733465. {{cite journal}}: Unknown parameter |name-list-format= ignored (|name-list-style= suggested) (help)

[50] Lamason RL, Mohideen MA, Mest JR, et al. (December 2005). "SLC24A5, a putative cation exchanger, affects pigmentation in zebrafish and humans". Science. 310 (5755): 1782–6. Bibcode:2005Sci...310.1782L. doi:10.1126/science.1116238. PMID 16357253. S2CID 2245002.

[51] Jablonski, Nina G.; Chaplin, George (2010-05-11). "Human skin pigmentation as an adaptation to UV radiation". Proceedings of the National Academy of Sciences. 107 (Supplement 2): 8962–8968. Bibcode:2010PNAS..107.8962J. doi:10.1073/pnas.0914628107. PMC 3024016. PMID 20445093.

[52] ttps://www2.palomar.edu/anthro/adapt/adapt_4.htm

[53] Liu Y, Hong L, Kempf VR, Wakamatsu K, Ito S, Simon JD (June 2004). "Ion-exchange and adsorption of Fe(III) by Sepia melanin". Pigment Cell Research. 17 (3): 262–9. doi:10.1111/j.1600-0749.2004.00140.x. PMID 15140071.

[54] Donatien PD, Orlow SJ (August 1995). "Interaction of melanosomal proteins with melanin". European Journal of Biochemistry. 232 (1): 159–64. doi:10.1111/j.1432-1033.1995.tb20794.x. PMID 7556145.

[55] Sarangarajan R, Apte SP (2005). "Melanin aggregation and polymerization: possible implications in age-related macular degeneration". Ophthalmic Research. 37 (3): 136–41. doi:10.1159/000085533. PMID 15867475. S2CID 27499198.

[56] Meyskens FL, Farmer PJ, Anton-Culver H (April 2004). "Etiologic pathogenesis of melanoma: a unifying hypothesis for the missing attributable risk" (PDF). Clinical Cancer Research. 10 (8): 2581–3. doi:10.1158/1078-0432.ccr-03-0638. PMID 15102657. S2CID 26079375.

[57] Sarangarajan R, Apte SP (2005). "Melanization and phagocytosis: implications for age related macular degeneration". Molecular Vision. 11: 482–90. PMID 16030499.

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@@ Line 29: / Line 29: @@
 '''Melanin''' ({{IPAc-en|audio=melanin-pronunciation.ogg|ˈ|m|ɛ|l|ə|n|ɪ|n}}; from {{lang-el|μέλας}} ''melas'', "black, dark") is a broad term for a group of natural [[pigment]]s found in most [[organism]]s. Melanin is produced through a multistage chemical process known as [[melanogenesis]], where the [[oxidation]] of the [[amino acid]] [[tyrosine]] is followed by [[polymerization]]. The melanin pigments are produced in a specialized group of cells known as [[melanocytes]].
-There are three basic types of melanin: [[eumelanin]], [[pheomelanin]], and [[neuromelanin]]. The most common type is eumelanin, of which there are two types— brown eumelanin and black eumelanin. Pheomelanin is a [[cysteine]]-derivative that contains poly[[benzothiazine]] portions that are largely responsible for the color of [[red hair]], among other pigmentation. Neuromelanin is found in the [[brain]]. Research has been undertaken to investigate its efficacy in treating neurodegenerative disorders such as [[Parkinson's]].<ref>{{Cite journal |last=Haining|first=Robert L. |last2=Achat-Mendes|first2=Cindy |date=March 2017|title=Neuromelanin, one of the most overlooked molecules in modern medicine, is not a spectator|journal=Neural Regeneration Research |volume=12|issue=3|pages=372–375|doi=10.4103/1673-5374.202928 |pmc=5399705|pmid=28469642}}</ref>
+There are three basic types of melanin: [[eumelanin]], [[pheomelanin]], and [[neuromelanin]]. The most common type is eumelanin, of which there are two types— brown eumelanin and black eumelanin. Pheomelanin is a [[cysteine]]-derivative that contains poly[[benzothiazine]] portions that are largely responsible for the color of [[red hair]], among other pigmentation. Neuromelanin is found in the [[brain]]. Research has been undertaken to investigate its efficacy in treating neurodegenerative disorders such as [[Parkinson's]].<ref>{{Cite journal |last1=Haining|first1=Robert L. |last2=Achat-Mendes|first2=Cindy |date=March 2017|title=Neuromelanin, one of the most overlooked molecules in modern medicine, is not a spectator|journal=Neural Regeneration Research |volume=12|issue=3|pages=372–375|doi=10.4103/1673-5374.202928 |pmc=5399705|pmid=28469642}}</ref>
 In the human skin, melanogenesis is initiated by exposure to [[UV radiation]], causing the skin to darken. Melanin is an effective absorbent of light; the pigment is able to dissipate over 99.9% of absorbed UV radiation.<ref name=Meredith>{{cite journal |vauthors=Meredith P, Riesz J |title=Radiative relaxation quantum yields for synthetic eumelanin |journal=Photochemistry and Photobiology |volume=79 |issue=2 |pages=211–6 |year=2004  |pmid=15068035 |doi=10.1111/j.1751-1097.2004.tb00012.x |arxiv=cond-mat/0312277 }}</ref> Because of this property, melanin is thought to protect skin cells from [[indirect DNA damage|UVB radiation damage]], reducing the risk of folate depletion and dermal degradation, and it is considered that exposure to UV radiation is associated with increased risk of [[malignant melanoma]], a cancer of melanocytes (melanin cells). Studies have shown a lower incidence for skin cancer in individuals with more concentrated melanin, i.e. darker [[skin tone]]. However, the relationship between skin pigmentation and [[photoprotection]] is still uncertain.<ref name="Electrodial Crocodile Style">{{cite journal |vauthors=Brenner M, Hearing VJ |title=The protective role of melanin against UV damage in human skin |journal=Photochemistry and Photobiology |volume=84 |issue=3 |pages=539–49 |year=2008 |pmid=18435612 |pmc=2671032 |doi=10.1111/j.1751-1097.2007.00226.x}}</ref>
@@ Line 38: / Line 38: @@
 In humans, melanin is the primary determinant of [[human skin color|skin color]]. It is also found in [[hair]], the pigmented tissue underlying the [[iris (anatomy)|iris]] of the eye, and the [[stria vascularis of cochlear duct|stria vascularis]] of the [[inner ear]]. In the brain, tissues with melanin include the [[adrenal medulla|medulla]] and pigment-bearing neurons within areas of the [[brainstem]], such as the [[locus coeruleus]] . It also occurs in the [[zona reticularis]] of the [[adrenal gland]].<ref>{{Cite journal|last=Solano|first=F.|date=2014|title=Melanins: Skin Pigments and Much More—Types, Structural Models, Biological Functions, and Formation Routes|journal= New Journal of Science|volume=2014|pages=1–28|doi=10.1155/2014/498276|doi-access=free}}</ref>
-The melanin in the skin is produced by [[melanocyte]]s, which are found in the [[Stratum germinativum|basal layer]] of the [[epidermis (skin)|epidermis]]. Although, in general, human beings possess a similar concentration of melanocytes in their skin, the melanocytes in some individuals and ethnic groups produce variable amounts of melanin. Some humans have very little or no melanin synthesis in their bodies, a condition known as [[albinism]].<ref>{{Cite journal|last=Cichorek|first=Mirosława|last2=Wachulska|first2=Małgorzata|last3=Stasiewicz|first3=Aneta|last4=Tymińska|first4=Agata|date=February 20, 2013|title=Skin melanocytes: biology and development|journal=Advances in Dermatology and Allergology |volume=30|issue=1|pages=30–41|doi=10.5114/pdia.2013.33376|pmc=3834696|pmid=24278043}}</ref>
+The melanin in the skin is produced by [[melanocyte]]s, which are found in the [[Stratum germinativum|basal layer]] of the [[epidermis (skin)|epidermis]]. Although, in general, human beings possess a similar concentration of melanocytes in their skin, the melanocytes in some individuals and ethnic groups produce variable amounts of melanin. Some humans have very little or no melanin synthesis in their bodies, a condition known as [[albinism]].<ref>{{Cite journal|last1=Cichorek|first1=Mirosława|last2=Wachulska|first2=Małgorzata|last3=Stasiewicz|first3=Aneta|last4=Tymińska|first4=Agata|date=February 20, 2013|title=Skin melanocytes: biology and development|journal=Advances in Dermatology and Allergology |volume=30|issue=1|pages=30–41|doi=10.5114/pdia.2013.33376|pmc=3834696|pmid=24278043}}</ref>
 Because melanin is an aggregate of smaller component molecules, there are many different types of melanin with different 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.<ref>{{Cite web|url=https://ghr.nlm.nih.gov/condition/oculocutaneous-albinism|title=oculocutaneous albinism|website=Genetics Home Reference|access-date=2017-09-25}}</ref>
@@ Line 44: / Line 44: @@
 === Eumelanin ===
 [[File:Eumelanine.svg|thumb|Part of the structural formula of eumelanin. "(COOH)" can be COOH or H, or (more rarely) other [[substituent]]s. The arrow denotes where the polymer continues.]]
-Eumelanin polymers have long been thought to comprise numerous cross-linked 5,6-dihydroxy[[indole]] ([[5,6-dihydroxyindole|DHI]]) and 5,6-dihydroxyindole-2-[[carboxylic acid]] ([[DHICA]]) polymers.<ref>{{Cite journal|last=Meredith|first=Paul|last2=Sarna|first2=Tadeusz|date=2006-12-01|title=The physical and chemical properties of eumelanin|journal=Pigment Cell Research|volume=19|issue=6|pages=572–594|doi=10.1111/j.1600-0749.2006.00345.x|pmid=17083485}}</ref>
+Eumelanin polymers have long been thought to comprise numerous cross-linked 5,6-dihydroxy[[indole]] ([[5,6-dihydroxyindole|DHI]]) and 5,6-dihydroxyindole-2-[[carboxylic acid]] ([[DHICA]]) polymers.<ref>{{Cite journal|last1=Meredith|first1=Paul|last2=Sarna|first2=Tadeusz|date=2006-12-01|title=The physical and chemical properties of eumelanin|journal=Pigment Cell Research|volume=19|issue=6|pages=572–594|doi=10.1111/j.1600-0749.2006.00345.x|pmid=17083485}}</ref>
-There are two types of eumelanin, which are brown eumelanin and black eumelanin. Those two types of eumelanin chemically differ from each other in their pattern of polymeric bonds. A small amount of black eumelanin in the absence of other pigments causes grey hair. A small amount of brown eumelanin in the absence of other pigments causes yellow (blond) hair. As the body ages, it continues to produce black eumelanin but stops producing brown eumelanin, resulting in the grey hair that is common in elderly people.<ref>{{Cite journal|last=Ito|first=S.|last2=Wakamatsu|first2=K.|date=December 2011|title=Diversity of human hair pigmentation as studied by chemical analysis of eumelanin and pheomelanin|journal=Journal of the European Academy of Dermatology and Venereology: JEADV|volume=25|issue=12|pages=1369–1380|doi=10.1111/j.1468-3083.2011.04278.x|issn=1468-3083|pmid=22077870}}</ref>
+There are two types of eumelanin, which are brown eumelanin and black eumelanin. Those two types of eumelanin chemically differ from each other in their pattern of polymeric bonds. A small amount of black eumelanin in the absence of other pigments causes grey hair. A small amount of brown eumelanin in the absence of other pigments causes yellow (blond) hair. As the body ages, it continues to produce black eumelanin but stops producing brown eumelanin, resulting in the grey hair that is common in elderly people.<ref>{{Cite journal|last1=Ito|first1=S.|last2=Wakamatsu|first2=K.|date=December 2011|title=Diversity of human hair pigmentation as studied by chemical analysis of eumelanin and pheomelanin|journal=Journal of the European Academy of Dermatology and Venereology: JEADV|volume=25|issue=12|pages=1369–1380|doi=10.1111/j.1468-3083.2011.04278.x|issn=1468-3083|pmid=22077870}}</ref>
 === Pheomelanin ===
 [[File:Pheomelanine.svg|thumb|Part of the structural formula of pheomelanin. "(COOH)" can be COOH or H, or (more rarely) other [[substituent]]s. The arrows denote where the polymer continues.]]
-Pheomelanins (or phaeomelanins) impart a range of yellowish to reddish colors.<ref>{{cite journal | vauthors = Slominski A, Tobin DJ, Shibahara S, Wortsman J | title = Melanin pigmentation in mammalian skin and its hormonal regulation | journal = Physiological Reviews | volume = 84 | issue = 4 | pages = 1155–228 | date = October 2004 | pmid = 15383650 | doi = 10.1152/physrev.00044.2003 | url = https://semanticscholar.org/paper/e5483673641f40a5a97f84fbf487d4ecd5a69933 }}</ref> Pheomelanins are particularly concentrated in the lips, nipples, glans of the penis, and vagina.<ref>{{Cite web|url=https://metacyc.org/META/NEW-IMAGE?type=COMPOUND&object=CPD-12380|title=pheomelanin|last=|first=|date=2010|website=MetaCyc Metabolic Pathway Database}} {{full citation needed|date=August 2014}}</ref> When a small amount of brown eumelanin in hair, which would otherwise cause blond hair, is mixed with red pheomelanin, the result is orange hair, which is typically called [[red hair|"red" or "ginger" hair]]. Pheomelanin is also present in the skin, and redheads consequently often have a more pinkish hue to their skin as well.
+Pheomelanins (or phaeomelanins) impart a range of yellowish to reddish colors.<ref>{{cite journal | vauthors = Slominski A, Tobin DJ, Shibahara S, Wortsman J | title = Melanin pigmentation in mammalian skin and its hormonal regulation | journal = Physiological Reviews | volume = 84 | issue = 4 | pages = 1155–228 | date = October 2004 | pmid = 15383650 | doi = 10.1152/physrev.00044.2003 | s2cid = 21168932 | url = https://semanticscholar.org/paper/e5483673641f40a5a97f84fbf487d4ecd5a69933 }}</ref> Pheomelanins are particularly concentrated in the lips, nipples, glans of the penis, and vagina.<ref>{{Cite web|url=https://metacyc.org/META/NEW-IMAGE?type=COMPOUND&object=CPD-12380|title=pheomelanin|last=|first=|date=2010|website=MetaCyc Metabolic Pathway Database}} {{full citation needed|date=August 2014}}</ref> When a small amount of brown eumelanin in hair, which would otherwise cause blond hair, is mixed with red pheomelanin, the result is orange hair, which is typically called [[red hair|"red" or "ginger" hair]]. Pheomelanin is also present in the skin, and redheads consequently often have a more pinkish hue to their skin as well.
 In chemical terms, pheomelanins differ from eumelanins in that the oligomer structure incorporates [[benzothiazine]] and [[benzothiazole]] units that are produced,<ref>{{cite journal |vauthors=Greco G, Panzella L, Verotta L, d'Ischia M, Napolitano A |title=Uncovering the structure of human red hair pheomelanin: benzothiazolylthiazinodihydroisoquinolines as key building blocks |journal=Journal of Natural Products |volume=74 |issue=4 |pages=675–82 |date=April 2011  |pmid=21341762 |doi=10.1021/np100740n}}</ref> instead of DHI and [[DHICA]], when the amino acid [[L-cysteine]] is present.
 ==== Trichochromes ====
-Trichochromes (formerly called trichosiderins) are pigments produced from the same [[metabolic pathway]] as the eumelanins and pheomelanins, but unlike those molecules they have low molecular weight. They occur in some red human hair.<ref>{{cite journal |title=Biochemical sites of gene action for melanogenesis in mammals |author1=Prota, G. |author2=Searle, A. G. |year=1978 |volume=10 |issue=1 |pages=1–8 |journal=Annales de Génétique et de Sélection Animale |doi=10.1186/1297-9686-10-1-1|url=http://www.gse-journal.org/10.1051/gse:19780101/pdf|doi-access=free }}</ref>
+Trichochromes (formerly called trichosiderins) are pigments produced from the same [[metabolic pathway]] as the eumelanins and pheomelanins, but unlike those molecules they have low molecular weight. They occur in some red human hair.<ref>{{cite journal |title=Biochemical sites of gene action for melanogenesis in mammals |author1=Prota, G. |author2=Searle, A. G. |year=1978 |volume=10 |issue=1 |pages=1–8 |journal=Annales de Génétique et de Sélection Animale |doi=10.1186/1297-9686-10-1-1|pmid=22896083 |url=http://www.gse-journal.org/10.1051/gse:19780101/pdf|doi-access=free }}</ref>
 === Neuromelanin ===
 {{main|Neuromelanin}}
-Neuromelanin (NM) is a dark insoluble polymer pigment produced in specific populations of [[Catecholaminergic|catecholaminergic neurons]] in the brain. Humans have the largest amount of NM, which is present in lesser amounts in other primates, and totally absent in many other species.<ref>{{cite journal|vauthors=Fedorow H, Tribl F, Halliday G, Gerlach M, Riederer P, Double KL |title= Neuromelanin in human dopamine neurons: comparison with peripheral melanins and relevance to Parkinson's disease |year = 2005| journal= Prog Neurobiol | volume= 75 | issue= 2 |pages = 109–124 |pmid=15784302 |pmc= |doi= 10.1016/j.pneurobio.2005.02.001}}</ref> The biological function remains unknown, although human NM has been shown to efficiently bind [[transition metals]] such as iron, as well as other potentially toxic molecules. Therefore, it may play crucial roles in [[apoptosis]] and the related [[Parkinson's disease]].<ref>{{cite journal| author= Double KL|title= Functional effects of neuromelanin and synthetic melanin in model systems |year =2006 | journal= J Neural Transm | volume= 113 | issue= 6 |pages = 751–756 |pmid= 16755379 |pmc= |doi= 10.1007/s00702-006-0450-5}}</ref>
+Neuromelanin (NM) is a dark insoluble polymer pigment produced in specific populations of [[Catecholaminergic|catecholaminergic neurons]] in the brain. Humans have the largest amount of NM, which is present in lesser amounts in other primates, and totally absent in many other species.<ref>{{cite journal|vauthors=Fedorow H, Tribl F, Halliday G, Gerlach M, Riederer P, Double KL |title= Neuromelanin in human dopamine neurons: comparison with peripheral melanins and relevance to Parkinson's disease |year = 2005| journal= Prog Neurobiol | volume= 75 | issue= 2 |pages = 109–124 |pmid=15784302 |pmc= |doi= 10.1016/j.pneurobio.2005.02.001|s2cid= 503902 }}</ref> The biological function remains unknown, although human NM has been shown to efficiently bind [[transition metals]] such as iron, as well as other potentially toxic molecules. Therefore, it may play crucial roles in [[apoptosis]] and the related [[Parkinson's disease]].<ref>{{cite journal| author= Double KL|title= Functional effects of neuromelanin and synthetic melanin in model systems |year =2006 | journal= J Neural Transm | volume= 113 | issue= 6 |pages = 751–756 |pmid= 16755379 |pmc= |doi= 10.1007/s00702-006-0450-5|s2cid= 23096297 }}</ref>
 == Other organisms ==
 Melanins have very diverse roles and functions in various organisms. A form of melanin makes up the ink used by many [[cephalopod]]s (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]]. Melanin also protects against damage from high temperatures, chemical stresses (such as [[heavy metals]] and [[oxidizing agents]]), and biochemical threats (such as host defenses against invading microbes).<ref name="Hamilton & Gomez">{{cite journal |vauthors=Hamilton AJ, Gomez BL |title=Melanins in fungal pathogens |journal=Journal of Medical Microbiology |volume=51 |issue=3 |pages=189–91 |date=March 2002  |pmid=11871612| doi = 10.1099/0022-1317-51-3-189|doi-access=free }}</ref> Therefore, in many pathogenic microbes (for example, in ''[[Cryptococcus neoformans]]'', a fungus) melanins appear to play important roles in [[virulence]] and [[pathogenicity]] by protecting the microbe against [[immune system|immune]] responses of its [[host (biology)|host]]. 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.<ref>{{cite journal |vauthors=Cerenius L, Söderhäll K |title=The prophenoloxidase-activating system in invertebrates |journal=Immunological Reviews |volume=198 |issue= |pages=116–26 |date=April 2004  |pmid=15199959 |doi=10.1111/j.0105-2896.2004.00116.x}}</ref> Some types of fungi, called [[radiotrophic fungus|radiotrophic fungi]], appear to be able to use melanin as a [[photosynthetic pigment]] that enables them to capture [[gamma rays]]<ref>{{cite journal |journal=Science News |title=Dark Power: Pigment seems to put radiation to good use |date=May 26, 2007 |volume=171 |issue=21 |pages=325 |first=Davide |last=Castelvecchi |doi=10.1002/scin.2007.5591712106}}</ref> and harness this energy for growth.<ref name="Dadachova et al.">{{cite journal  |vauthors=Dadachova E, Bryan RA, Huang X, etal |title=Ionizing radiation changes the electronic properties of melanin and enhances the growth of melanized fungi |journal=PLOS ONE |volume=2 |issue=5 |pages=e457 |year=2007 |pmid=17520016 |pmc=1866175 |doi=10.1371/journal.pone.0000457|bibcode=2007PLoSO...2..457D }}</ref>
-The darker [[feather]]s of [[bird]]s owe their color to melanin and are less readily degraded by bacteria than unpigmented ones or those containing [[carotenoid]] pigments.<ref>{{Cite journal|last=Gunderson|first=Alex R.|last2=Frame|first2=Alicia M.|last3=Swaddle|first3=John P.|last4=Forsyth|first4=Mark H.|date=2008-09-01|title=Resistance of melanized feathers to bacterial degradation: is it really so black and white?|journal=Journal of Avian Biology|volume=39|issue=5|pages=539–545|doi=10.1111/j.0908-8857.2008.04413.x}}</ref> Feathers that contain melanin are also 39% more resistant to abrasion than those that do not because melanin granules help fill the space between the [[keratin]] strands that form feathers.<ref>{{Cite journal|last=Bonser|first=Richard H. C.|date=1995|title=Melanin and the Abrasion Resistance of Feathers|url=https://sora.unm.edu/node/105022|journal=Condor|volume=97|issue=2|pages=590–591|doi=10.2307/1369048|jstor=1369048}}</ref><ref>{{Cite journal|last=Galván|first=Ismael|last2=Solano|first2=Francisco|date=2016-04-08|title=Bird Integumentary Melanins: Biosynthesis, Forms, Function and Evolution|journal=International Journal of Molecular Sciences|volume=17|issue=4|doi=10.3390/ijms17040520|pmc=4848976|pmid=27070583|page=520}}</ref> Pheomelanin synthesis in birds implies the consumption of cysteine, a semi‐essential amino acid that is necessary for the synthesis of the antioxidant glutathione (GSH) but that may be toxic if in excess in the diet. Indeed, carnivorous birds, which have a high protein content in their diet, exhibit pheomelanin‐based coloration.<ref>{{Cite journal|last=Rodríguez‐Martínez|first=Sol|last2=Galván|first2=Ismael|date=2020|title=Juvenile pheomelanin-based plumage coloration has evolved more frequently in carnivorous species|url=https://onlinelibrary.wiley.com/doi/abs/10.1111/ibi.12770|journal=Ibis|language=en|volume=162|issue=1|pages=238–244|doi=10.1111/ibi.12770|issn=1474-919X}}</ref>
+The darker [[feather]]s of [[bird]]s owe their color to melanin and are less readily degraded by bacteria than unpigmented ones or those containing [[carotenoid]] pigments.<ref>{{Cite journal|last1=Gunderson|first1=Alex R.|last2=Frame|first2=Alicia M.|last3=Swaddle|first3=John P.|last4=Forsyth|first4=Mark H.|date=2008-09-01|title=Resistance of melanized feathers to bacterial degradation: is it really so black and white?|journal=Journal of Avian Biology|volume=39|issue=5|pages=539–545|doi=10.1111/j.0908-8857.2008.04413.x}}</ref> Feathers that contain melanin are also 39% more resistant to abrasion than those that do not because melanin granules help fill the space between the [[keratin]] strands that form feathers.<ref>{{Cite journal|last=Bonser|first=Richard H. C.|date=1995|title=Melanin and the Abrasion Resistance of Feathers|url=https://sora.unm.edu/node/105022|journal=Condor|volume=97|issue=2|pages=590–591|doi=10.2307/1369048|jstor=1369048}}</ref><ref>{{Cite journal|last1=Galván|first1=Ismael|last2=Solano|first2=Francisco|date=2016-04-08|title=Bird Integumentary Melanins: Biosynthesis, Forms, Function and Evolution|journal=International Journal of Molecular Sciences|volume=17|issue=4|doi=10.3390/ijms17040520|pmc=4848976|pmid=27070583|page=520}}</ref> Pheomelanin synthesis in birds implies the consumption of cysteine, a semi‐essential amino acid that is necessary for the synthesis of the antioxidant glutathione (GSH) but that may be toxic if in excess in the diet. Indeed, carnivorous birds, which have a high protein content in their diet, exhibit pheomelanin‐based coloration.<ref>{{Cite journal|last1=Rodríguez‐Martínez|first1=Sol|last2=Galván|first2=Ismael|date=2020|title=Juvenile pheomelanin-based plumage coloration has evolved more frequently in carnivorous species|journal=Ibis|language=en|volume=162|issue=1|pages=238–244|doi=10.1111/ibi.12770|issn=1474-919X}}</ref>
-Melanin is also important in [[mammal]]ian pigmentation.<ref>{{cite journal|last1=Jimbow|first1=K|last2=Quevedo WC|first2=Jr|last3=Fitzpatrick|first3=TB|last4=Szabo|first4=G|title=Some aspects of melanin biology: 1950–1975|journal=The Journal of Investigative Dermatology|date=Jul 1976|volume=67|issue=1|pages=72–89|pmid=819593|doi=10.1111/1523-1747.ep12512500|doi-access=free}}</ref> The coat pattern of mammals is determined by the [[agouti gene]] which regulates the distribution of melanin.<ref>{{Cite book|url=https://books.google.com/?id=8DxdDgAAQBAJ&pg=PA377|title=Genetic Analysis: Genes, Genomes, and Networks in Eukaryotes|last=Meneely|first=Philip|date=2014|publisher=Oxford University Press|isbn=9780199681266}}</ref><ref>{{Cite journal|last=Griffiths|first=Anthony JF|last2=Miller|first2=Jeffrey H.|last3=Suzuki|first3=David T.|last4=Lewontin|first4=Richard C.|last5=Gelbart|first5=William M.|date=2000|title=Gene interaction in coat color of mammals|url=https://www.ncbi.nlm.nih.gov/books/NBK21804/}}</ref> The mechanisms of the gene have been extensively studied in mice to provide an insight into the diversity of mammalian coat patterns.<ref>{{Cite journal|last=Millar|first=S. E.|last2=Miller|first2=M. W.|last3=Stevens|first3=M. E.|last4=Barsh|first4=G. S.|date=October 1995|title=Expression and transgenic studies of the mouse agouti gene provide insight into the mechanisms by which mammalian coat color patterns are generated|journal=Development|volume=121|issue=10|pages=3223–3232|pmid=7588057}}</ref>
+Melanin is also important in [[mammal]]ian pigmentation.<ref>{{cite journal|last1=Jimbow|first1=K|last2=Quevedo WC|first2=Jr|last3=Fitzpatrick|first3=TB|last4=Szabo|first4=G|title=Some aspects of melanin biology: 1950–1975|journal=The Journal of Investigative Dermatology|date=Jul 1976|volume=67|issue=1|pages=72–89|pmid=819593|doi=10.1111/1523-1747.ep12512500|doi-access=free}}</ref> The coat pattern of mammals is determined by the [[agouti gene]] which regulates the distribution of melanin.<ref>{{Cite book|url=https://books.google.com/books?id=8DxdDgAAQBAJ&pg=PA377|title=Genetic Analysis: Genes, Genomes, and Networks in Eukaryotes|last=Meneely|first=Philip|date=2014|publisher=Oxford University Press|isbn=9780199681266}}</ref><ref>{{Cite journal|last1=Griffiths|first1=Anthony JF|last2=Miller|first2=Jeffrey H.|last3=Suzuki|first3=David T.|last4=Lewontin|first4=Richard C.|last5=Gelbart|first5=William M.|date=2000|title=Gene interaction in coat color of mammals|url=https://www.ncbi.nlm.nih.gov/books/NBK21804/}}</ref> The mechanisms of the gene have been extensively studied in mice to provide an insight into the diversity of mammalian coat patterns.<ref>{{Cite journal|last1=Millar|first1=S. E.|last2=Miller|first2=M. W.|last3=Stevens|first3=M. E.|last4=Barsh|first4=G. S.|date=October 1995|title=Expression and transgenic studies of the mouse agouti gene provide insight into the mechanisms by which mammalian coat color patterns are generated|journal=Development|volume=121|issue=10|pages=3223–3232|pmid=7588057}}</ref>
-Melanin in [[arthropods]] has been observed to be deposited in layers thus producing a [[Bragg reflector]] of alternating refractive index. When the scale of this pattern matches the wavelength of visible light, [[structural coloration]] arises: giving a number of species an [[iridescent]] color.<ref>{{Cite book|url=https://books.google.com/?id=VQHtCAAAQBAJ&pg=PA121|title=Biology of the Arthropod Cuticle|last=Neville|first=A. C.|date=2012|publisher=Springer Science & Business Media|isbn=9783642809101}}</ref>
+Melanin in [[arthropods]] has been observed to be deposited in layers thus producing a [[Bragg reflector]] of alternating refractive index. When the scale of this pattern matches the wavelength of visible light, [[structural coloration]] arises: giving a number of species an [[iridescent]] color.<ref>{{Cite book|url=https://books.google.com/books?id=VQHtCAAAQBAJ&pg=PA121|title=Biology of the Arthropod Cuticle|last=Neville|first=A. C.|date=2012|publisher=Springer Science & Business Media|isbn=9783642809101}}</ref>
 [[Arachnid]]s are one of the few groups in which melanin has not been easily detected, though researchers found data suggesting spiders do in fact produce melanin.<ref name="HsiungBlackledge2015">{{cite journal|last1=Hsiung|first1=B.-K.|last2=Blackledge|first2=T. A.|last3=Shawkey|first3=M. D.|title=Spiders do have melanin after all|journal=Journal of Experimental Biology|volume=218|issue=22|year=2015|pages=3632–3635|doi=10.1242/jeb.128801|pmid=26449977|doi-access=free}}</ref>
-Some moth species, including the [[Parasemia plantaginis|wood tiger moth]], convert resources to melanin in order to enhance their thermoregulation. As the wood tiger moth has populations over a large range of latitudes, it has been observed that more northern populations showed higher rates of melanization. In both yellow and white male phenotypes of the wood tiger moth, individuals with more melanin had a heightened ability to trap heat but an increased predation rate due to a weaker and less effective [[Aposematism|aposematic]] signal.<ref>{{Cite journal|last=Hegna|first=Robert H.|last2=Nokelainen|first2=Ossi|last3=Hegna|first3=Jonathan R.|last4=Mappes|first4=Johanna|date=2013|title=To quiver or to shiver: increased melanization benefits thermoregulation, but reduces warning signal efficacy in the wood tiger moth|journal=Proc. R. Soc. B|volume=280|issue=1755|pages=20122812|doi=10.1098/rspb.2012.2812|pmid=23363631|pmc=3574392}}</ref>
+Some moth species, including the [[Parasemia plantaginis|wood tiger moth]], convert resources to melanin in order to enhance their thermoregulation. As the wood tiger moth has populations over a large range of latitudes, it has been observed that more northern populations showed higher rates of melanization. In both yellow and white male phenotypes of the wood tiger moth, individuals with more melanin had a heightened ability to trap heat but an increased predation rate due to a weaker and less effective [[Aposematism|aposematic]] signal.<ref>{{Cite journal|last1=Hegna|first1=Robert H.|last2=Nokelainen|first2=Ossi|last3=Hegna|first3=Jonathan R.|last4=Mappes|first4=Johanna|date=2013|title=To quiver or to shiver: increased melanization benefits thermoregulation, but reduces warning signal efficacy in the wood tiger moth|journal=Proc. R. Soc. B|volume=280|issue=1755|pages=20122812|doi=10.1098/rspb.2012.2812|pmid=23363631|pmc=3574392}}</ref>
 === Plants ===
 [[File:Indole-5-6-quinone.png|thumb|Chemical structure of indole-5,6-quinone]]
-Melanin produced by plants are sometimes referred to as 'catechol melanins' as they can yield [[catechol]] on alkali fusion. It is commonly seen in the [[enzymatic browning]] of fruits such as bananas.  Chestnut shell melanin can be used as an antioxidant and coloring agent.<ref>{{cite journal|last1=Yao|first1=Zeng-Yu|last2=Qi|first2=Jian-Hua|title=Comparison of Antioxidant Activities of Melanin Fractions from Chestnut Shell|journal=Molecules|volume=21|issue=4|pages=487|date=April 22, 2016|accessdate=|doi=10.3390/molecules21040487|pmid=27110763|pmc=6273334}}</ref> Biosynthesis involves the oxidation of [[indole-5,6-quinone]] by the tyrosinase type [[polyphenol oxidase]] from [[tyrosine]] and [[catecholamines]] leading to the formation of catechol melanin. Despite this many plants contain compounds which inhibit the production of melanins.<ref>{{cite journal|last1=Kim|first1=Y.-J.|last2=Uyama|first2=H.|title=Tyrosinase inhibitors from natural and synthetic sources: structure, inhibition mechanism and perspective for the future|journal=Cellular and Molecular Life Sciences|date=15 May 2005|volume=62|issue=15|pages=1707–1723|doi=10.1007/s00018-005-5054-y|pmid=15968468}}</ref>
+Melanin produced by plants are sometimes referred to as 'catechol melanins' as they can yield [[catechol]] on alkali fusion. It is commonly seen in the [[enzymatic browning]] of fruits such as bananas.  Chestnut shell melanin can be used as an antioxidant and coloring agent.<ref>{{cite journal|last1=Yao|first1=Zeng-Yu|last2=Qi|first2=Jian-Hua|title=Comparison of Antioxidant Activities of Melanin Fractions from Chestnut Shell|journal=Molecules|volume=21|issue=4|pages=487|date=April 22, 2016|accessdate=|doi=10.3390/molecules21040487|pmid=27110763|pmc=6273334}}</ref> Biosynthesis involves the oxidation of [[indole-5,6-quinone]] by the tyrosinase type [[polyphenol oxidase]] from [[tyrosine]] and [[catecholamines]] leading to the formation of catechol melanin. Despite this many plants contain compounds which inhibit the production of melanins.<ref>{{cite journal|last1=Kim|first1=Y.-J.|last2=Uyama|first2=H.|title=Tyrosinase inhibitors from natural and synthetic sources: structure, inhibition mechanism and perspective for the future|journal=Cellular and Molecular Life Sciences|date=15 May 2005|volume=62|issue=15|pages=1707–1723|doi=10.1007/s00018-005-5054-y|pmid=15968468|s2cid=8280251}}</ref>
 == Biosynthetic pathways ==
@@ Line 111: / Line 111: @@
 In Parkinson's disease, a disorder that affects [[neuromotor]] functioning, there is decreased neuromelanin in the substantia nigra and locus coeruleus as consequence of specific dropping out of dopaminergic and noradrenergic pigmented neurons. This results in diminished [[dopamine]] and [[norepinephrine]] synthesis. While no correlation between race and the level of neuromelanin in the substantia nigra has been reported, the significantly lower incidence of Parkinson's in blacks than in whites has "prompt[ed] some to suggest that cutaneous melanin might somehow serve to protect the neuromelanin in substantia nigra from external toxins."<ref>{{Cite web|url=http://www.seniorpsychiatry.com/pages/articles/lewy.html|archiveurl=https://web.archive.org/web/20090721054743/http://www.seniorpsychiatry.com/pages/articles/lewy.html|url-status=dead|title=Lewy Body Disease|archivedate=July 21, 2009}}</ref>
-In addition to melanin deficiency, the molecular weight of the melanin polymer may be decreased by various factors such as oxidative stress, exposure to light, perturbation in its association with melanosomal [[matrix protein]]s, changes in [[pH]], or in local concentrations of metal ions. A decreased molecular weight or a decrease in the degree of polymerization of '''ocular melanin''' has been proposed to turn the normally anti-oxidant polymer into a [[pro-oxidant]]. In its pro-oxidant state, melanin has been suggested to be involved in the causation and progression of [[macular degeneration]] and [[melanoma]].<ref>{{cite journal |vauthors=Meyskens FL, Farmer P, Fruehauf JP |title=Redox regulation in human melanocytes and melanoma |journal=Pigment Cell Research |volume=14 |issue=3 |pages=148–54 |date=June 2001 |pmid=11434561 |doi=10.1034/j.1600-0749.2001.140303.x |url=https://escholarship.org/content/qt72b7988d/qt72b7988d.pdf?t=nhppep}}</ref> [[Rasagiline]], an important monotherapy drug in Parkinson's disease, has melanin binding properties, and melanoma tumor reducing properties.<ref>{{cite journal  |vauthors=Meier-Davis SR, Dines K, Arjmand FM, etal |title=Comparison of oral and transdermal administration of rasagiline mesylate on human melanoma tumor growth in vivo |journal=Cutaneous and Ocular Toxicology |volume=31 |issue=4 |pages=312–7 |date=December 2012  |pmid=22515841 |doi=10.3109/15569527.2012.676119}}</ref>
+In addition to melanin deficiency, the molecular weight of the melanin polymer may be decreased by various factors such as oxidative stress, exposure to light, perturbation in its association with melanosomal [[matrix protein]]s, changes in [[pH]], or in local concentrations of metal ions. A decreased molecular weight or a decrease in the degree of polymerization of '''ocular melanin''' has been proposed to turn the normally anti-oxidant polymer into a [[pro-oxidant]]. In its pro-oxidant state, melanin has been suggested to be involved in the causation and progression of [[macular degeneration]] and [[melanoma]].<ref>{{cite journal |vauthors=Meyskens FL, Farmer P, Fruehauf JP |title=Redox regulation in human melanocytes and melanoma |journal=Pigment Cell Research |volume=14 |issue=3 |pages=148–54 |date=June 2001 |pmid=11434561 |doi=10.1034/j.1600-0749.2001.140303.x |url=https://escholarship.org/content/qt72b7988d/qt72b7988d.pdf?t=nhppep}}</ref> [[Rasagiline]], an important monotherapy drug in Parkinson's disease, has melanin binding properties, and melanoma tumor reducing properties.<ref>{{cite journal  |vauthors=Meier-Davis SR, Dines K, Arjmand FM, etal |title=Comparison of oral and transdermal administration of rasagiline mesylate on human melanoma tumor growth in vivo |journal=Cutaneous and Ocular Toxicology |volume=31 |issue=4 |pages=312–7 |date=December 2012  |pmid=22515841 |doi=10.3109/15569527.2012.676119|s2cid=30344869 }}</ref>
 Higher eumelanin levels also can be a disadvantage, however, beyond a higher disposition toward vitamin D deficiency. Dark skin is a complicating factor in the laser removal of [[port-wine stain]]s. Effective in treating white skin, in general, lasers are less successful in removing port-wine stains in people of Asian or African descent. Higher concentrations of melanin in darker-skinned individuals simply diffuse and absorb the laser radiation, inhibiting light absorption by the targeted tissue. In similar manner, melanin can complicate laser treatment of other dermatological conditions in people with darker skin.
@@ Line 117: / Line 117: @@
 [[Freckles]] and [[mole (skin marking)|mole]]s are formed where there is a localized concentration of melanin in the skin. They are highly associated with pale skin.
-[[Nicotine]] has an affinity for melanin-containing tissues because of its precursor function in melanin synthesis or its irreversible binding of melanin. This has been suggested to underlie the increased [[Nicotine#Dependence|nicotine dependence]] and lower [[smoking cessation]] rates in darker pigmented individuals.<ref>{{cite journal |vauthors=King G, Yerger VB, Whembolua GL, Bendel RB, Kittles R, Moolchan ET |title=Link between facultative melanin and tobacco use among African Americans |journal=Pharmacology Biochemistry and Behavior |volume=92 |issue=4 |pages=589–96 |date=June 2009  |pmid=19268687 |doi=10.1016/j.pbb.2009.02.011}}</ref>
+[[Nicotine]] has an affinity for melanin-containing tissues because of its precursor function in melanin synthesis or its irreversible binding of melanin. This has been suggested to underlie the increased [[Nicotine#Dependence|nicotine dependence]] and lower [[smoking cessation]] rates in darker pigmented individuals.<ref>{{cite journal |vauthors=King G, Yerger VB, Whembolua GL, Bendel RB, Kittles R, Moolchan ET |title=Link between facultative melanin and tobacco use among African Americans |journal=Pharmacology Biochemistry and Behavior |volume=92 |issue=4 |pages=589–96 |date=June 2009  |pmid=19268687 |doi=10.1016/j.pbb.2009.02.011|s2cid=3070838 }}</ref>
 == Human adaptation ==
@@ Line 126: / Line 126: @@
 ===Evolutionary origins===
-[[Early humans]] evolved to have dark skin color around 1.2 million years ago, as an adaptation to a loss of body hair that increased the effects of UV radiation. Before the development of hairlessness, early humans had reasonably light skin underneath their fur, similar to that found in other [[primate]]s.<ref>{{Cite news|url=https://www.nytimes.com/2003/08/19/science/why-humans-and-their-fur-parted-ways.html|title=Why Humans and Their Fur Parted Ways|last=Wade|first=Nicholas|date=2003-08-19|work=The New York Times|access-date=2019-08-24|language=en-US|issn=0362-4331}}</ref> The most recent scientific evidence indicates that [[anatomically modern humans]] evolved in [[Africa]] between 200,000 and 100,000 years,<ref>{{cite journal  |vauthors=Tishkoff SA, Reed FA, Friedlaender FR, etal |title=The genetic structure and history of Africans and African Americans |journal=Science |volume=324 |issue=5930 |pages=1035–44 |date=May 2009  |pmid=19407144 |pmc=2947357 |doi=10.1126/science.1172257|bibcode=2009Sci...324.1035T }}</ref> and then populated the rest of the world through one migration between 80,000 and 50,000 years ago, in some areas [[Archaic human admixture with modern humans|interbreeding]] with certain [[archaic human]] species ([[Neanderthals]], [[Denisovans]], and possibly others).<ref>{{cite web|title=A Single Migration From Africa Populated the World, Studies Find|url=https://www.nytimes.com/2016/09/22/science/ancient-dna-human-history.html?_r=0|work=New York Times|date=2016-09-22}}</ref> It seems likely that the first modern humans had relatively large numbers of eumelanin-producing melanocytes, producing darker skin similar to the indigenous people of Africa today. As some of these original people migrated and settled in areas of [[Asia]] and [[Europe]], the selective pressure for eumelanin production decreased in climates where radiation from the sun was less intense. This eventually produced the current range of human skin color. Of the two common gene variants known to be associated with pale human skin, ''[[Mc1r]]'' does not appear to have undergone positive selection,<ref>{{cite journal |author=Harding RM |title=Evidence for variable selective pressures at MC1R |journal=American Journal of Human Genetics |volume=66 |issue=4 |pages=1351–61 |date=April 2000 |pmid=10733465 |pmc=1288200 |doi=10.1086/302863 |url= |name-list-format=vanc|author2=Healy E |author3=Ray AJ |display-authors=3 |last4=Ellis |first4=Nichola S. |last5=Flanagan |first5=Niamh |last6=Todd |first6=Carol |last7=Dixon |first7=Craig |last8=Sajantila |first8=Antti |last9=Jackson |first9=Ian J.}}</ref> while ''[[SLC24A5]]'' has undergone positive selection.<ref>{{cite journal  |vauthors=Lamason RL, Mohideen MA, Mest JR, etal |title=SLC24A5, a putative cation exchanger, affects pigmentation in zebrafish and humans |journal=Science |volume=310 |issue=5755 |pages=1782–6 |date=December 2005  |pmid=16357253 |doi=10.1126/science.1116238|bibcode=2005Sci...310.1782L }}</ref>
+[[Early humans]] evolved to have dark skin color around 1.2 million years ago, as an adaptation to a loss of body hair that increased the effects of UV radiation. Before the development of hairlessness, early humans had reasonably light skin underneath their fur, similar to that found in other [[primate]]s.<ref>{{Cite news|url=https://www.nytimes.com/2003/08/19/science/why-humans-and-their-fur-parted-ways.html|title=Why Humans and Their Fur Parted Ways|last=Wade|first=Nicholas|date=2003-08-19|work=The New York Times|access-date=2019-08-24|language=en-US|issn=0362-4331}}</ref> The most recent scientific evidence indicates that [[anatomically modern humans]] evolved in [[Africa]] between 200,000 and 100,000 years,<ref>{{cite journal  |vauthors=Tishkoff SA, Reed FA, Friedlaender FR, etal |title=The genetic structure and history of Africans and African Americans |journal=Science |volume=324 |issue=5930 |pages=1035–44 |date=May 2009  |pmid=19407144 |pmc=2947357 |doi=10.1126/science.1172257|bibcode=2009Sci...324.1035T }}</ref> and then populated the rest of the world through one migration between 80,000 and 50,000 years ago, in some areas [[Archaic human admixture with modern humans|interbreeding]] with certain [[archaic human]] species ([[Neanderthals]], [[Denisovans]], and possibly others).<ref>{{cite web|title=A Single Migration From Africa Populated the World, Studies Find|url=https://www.nytimes.com/2016/09/22/science/ancient-dna-human-history.html?_r=0|work=New York Times|date=2016-09-22}}</ref> It seems likely that the first modern humans had relatively large numbers of eumelanin-producing melanocytes, producing darker skin similar to the indigenous people of Africa today. As some of these original people migrated and settled in areas of [[Asia]] and [[Europe]], the selective pressure for eumelanin production decreased in climates where radiation from the sun was less intense. This eventually produced the current range of human skin color. Of the two common gene variants known to be associated with pale human skin, ''[[Mc1r]]'' does not appear to have undergone positive selection,<ref>{{cite journal |author=Harding RM |title=Evidence for variable selective pressures at MC1R |journal=American Journal of Human Genetics |volume=66 |issue=4 |pages=1351–61 |date=April 2000 |pmid=10733465 |pmc=1288200 |doi=10.1086/302863 |url= |name-list-format=vanc|author2=Healy E |author3=Ray AJ |display-authors=3 |last4=Ellis |first4=Nichola S. |last5=Flanagan |first5=Niamh |last6=Todd |first6=Carol |last7=Dixon |first7=Craig |last8=Sajantila |first8=Antti |last9=Jackson |first9=Ian J.}}</ref> while ''[[SLC24A5]]'' has undergone positive selection.<ref>{{cite journal  |vauthors=Lamason RL, Mohideen MA, Mest JR, etal |title=SLC24A5, a putative cation exchanger, affects pigmentation in zebrafish and humans |journal=Science |volume=310 |issue=5755 |pages=1782–6 |date=December 2005  |pmid=16357253 |doi=10.1126/science.1116238|bibcode=2005Sci...310.1782L |s2cid=2245002 }}</ref>
 ===Effects===
-As with peoples having migrated northward, those with light skin migrating toward the equator acclimatize to the much stronger solar radiation. Nature selects for less melanin when ultraviolet radiation is weak. Most people's skin darkens when exposed to UV light, giving them more protection when it is needed. This is the physiological purpose of [[sun tanning]]. Dark-skinned people, who produce more skin-protecting eumelanin, have a greater protection against [[sunburn]] and the development of melanoma, a potentially deadly form of skin cancer, as well as other health problems related to exposure to strong [[solar radiation]], including the [[photodegradation]] of certain [[vitamin]]s such as [[riboflavin]]s, [[carotenoid]]s, [[tocopherol]], and [[folate]].<ref>{{Cite journal|last=Jablonski|first=Nina G.|last2=Chaplin|first2=George|date=2010-05-11|title=Human skin pigmentation as an adaptation to UV radiation|journal=Proceedings of the National Academy of Sciences|volume=107|issue=Supplement 2|pages=8962–8968|doi=10.1073/pnas.0914628107|pmid=20445093|pmc=3024016|bibcode=2010PNAS..107.8962J}}</ref> Some [[Northwestern Europeans]] have substantially lost the ability to tan as a result of relaxed natural selection.  Their skin burns and peels rather than tans. This is due to the fact that they produce a defective form of a skin protein Mc1r (melanocortin-1 receptor) which is necessary for the production of melanin.  They are at a distinct disadvantage in tropical and subtropical environments.  Not only do they suffer the discomfort of readily burning, but they are at a much higher risk for skin cancer; the same is true of albinos.<ref>https://www2.palomar.edu/anthro/adapt/adapt_4.htm</ref>
+As with peoples having migrated northward, those with light skin migrating toward the equator acclimatize to the much stronger solar radiation. Nature selects for less melanin when ultraviolet radiation is weak. Most people's skin darkens when exposed to UV light, giving them more protection when it is needed. This is the physiological purpose of [[sun tanning]]. Dark-skinned people, who produce more skin-protecting eumelanin, have a greater protection against [[sunburn]] and the development of melanoma, a potentially deadly form of skin cancer, as well as other health problems related to exposure to strong [[solar radiation]], including the [[photodegradation]] of certain [[vitamin]]s such as [[riboflavin]]s, [[carotenoid]]s, [[tocopherol]], and [[folate]].<ref>{{Cite journal|last1=Jablonski|first1=Nina G.|last2=Chaplin|first2=George|date=2010-05-11|title=Human skin pigmentation as an adaptation to UV radiation|journal=Proceedings of the National Academy of Sciences|volume=107|issue=Supplement 2|pages=8962–8968|doi=10.1073/pnas.0914628107|pmid=20445093|pmc=3024016|bibcode=2010PNAS..107.8962J}}</ref> Some [[Northwestern Europeans]] have substantially lost the ability to tan as a result of relaxed natural selection.  Their skin burns and peels rather than tans. This is due to the fact that they produce a defective form of a skin protein Mc1r (melanocortin-1 receptor) which is necessary for the production of melanin.  They are at a distinct disadvantage in tropical and subtropical environments.  Not only do they suffer the discomfort of readily burning, but they are at a much higher risk for skin cancer; the same is true of albinos.<ref>https://www2.palomar.edu/anthro/adapt/adapt_4.htm</ref>
@@ Line 139: / Line 139: @@
 == Physical properties and technological applications ==
-Evidence exists in support of a highly cross-linked [[heteropolymer]] bound [[covalent bond|covalently]] to matrix scaffolding [[melanoprotein]]s.<ref>{{cite journal |vauthors=Donatien PD, Orlow SJ |title=Interaction of melanosomal proteins with melanin |journal=European Journal of Biochemistry |volume=232 |issue=1 |pages=159–64 |date=August 1995  |pmid=7556145 |doi=10.1111/j.1432-1033.1995.tb20794.x}}</ref> It has been proposed that the ability of melanin to act as an [[antioxidant]] is directly proportional to its degree of polymerization or [[molecular weight]].<ref>{{cite journal |vauthors=Sarangarajan R, Apte SP |title=Melanin aggregation and polymerization: possible implications in age-related macular degeneration |journal=Ophthalmic Research |volume=37 |issue=3 |pages=136–41 |year=2005 |pmid=15867475 |doi=10.1159/000085533}}</ref> Suboptimal conditions for the effective polymerization of melanin [[monomer]]s may lead to formation of lower-molecular-weight, pro-oxidant melanin that has been implicated in the causation and progression of [[macular degeneration]] and [[melanoma]].<ref>{{cite journal |vauthors=Meyskens FL, Farmer PJ, Anton-Culver H |title=Etiologic pathogenesis of melanoma: a unifying hypothesis for the missing attributable risk |journal=Clinical Cancer Research |volume=10 |issue=8 |pages=2581–3 |date=April 2004  |pmid=15102657 |doi=10.1158/1078-0432.ccr-03-0638|url=https://escholarship.org/content/qt6sz6501z/qt6sz6501z.pdf?t=nhskht}}</ref> [[Signal transduction|Signaling pathway]]s that [[upregulation|upregulate]] melanization in the [[retinal pigment epithelium]] (RPE) also may be implicated in the [[downregulation]] of [[rod cell|rod]] outer segment [[phagocytosis]] by the RPE. This phenomenon has been attributed in part to [[Fovea centralis|fovea]]l sparing in [[macular degeneration]].<ref>{{cite journal |vauthors=Sarangarajan R, Apte SP |title=Melanization and phagocytosis: implications for age related macular degeneration |journal=Molecular Vision |volume=11 |issue= |pages=482–90 |year=2005 |pmid=16030499 }}</ref>
+Evidence exists in support of a highly cross-linked [[heteropolymer]] bound [[covalent bond|covalently]] to matrix scaffolding [[melanoprotein]]s.<ref>{{cite journal |vauthors=Donatien PD, Orlow SJ |title=Interaction of melanosomal proteins with melanin |journal=European Journal of Biochemistry |volume=232 |issue=1 |pages=159–64 |date=August 1995  |pmid=7556145 |doi=10.1111/j.1432-1033.1995.tb20794.x}}</ref> It has been proposed that the ability of melanin to act as an [[antioxidant]] is directly proportional to its degree of polymerization or [[molecular weight]].<ref>{{cite journal |vauthors=Sarangarajan R, Apte SP |title=Melanin aggregation and polymerization: possible implications in age-related macular degeneration |journal=Ophthalmic Research |volume=37 |issue=3 |pages=136–41 |year=2005 |pmid=15867475 |doi=10.1159/000085533|s2cid=27499198 }}</ref> Suboptimal conditions for the effective polymerization of melanin [[monomer]]s may lead to formation of lower-molecular-weight, pro-oxidant melanin that has been implicated in the causation and progression of [[macular degeneration]] and [[melanoma]].<ref>{{cite journal |vauthors=Meyskens FL, Farmer PJ, Anton-Culver H |title=Etiologic pathogenesis of melanoma: a unifying hypothesis for the missing attributable risk |journal=Clinical Cancer Research |volume=10 |issue=8 |pages=2581–3 |date=April 2004  |pmid=15102657 |doi=10.1158/1078-0432.ccr-03-0638|s2cid=26079375 |url=https://escholarship.org/content/qt6sz6501z/qt6sz6501z.pdf?t=nhskht}}</ref> [[Signal transduction|Signaling pathway]]s that [[upregulation|upregulate]] melanization in the [[retinal pigment epithelium]] (RPE) also may be implicated in the [[downregulation]] of [[rod cell|rod]] outer segment [[phagocytosis]] by the RPE. This phenomenon has been attributed in part to [[Fovea centralis|fovea]]l sparing in [[macular degeneration]].<ref>{{cite journal |vauthors=Sarangarajan R, Apte SP |title=Melanization and phagocytosis: implications for age related macular degeneration |journal=Molecular Vision |volume=11 |issue= |pages=482–90 |year=2005 |pmid=16030499 }}</ref>
 == See also ==

v t e Pathology
Principles of pathology	Disease Infection Neoplasia Cause Pathogenesis Hemodynamics Ischemia Inflammation Cell damage Wound healing Cellular adaptation Atrophy Hypertrophy Hyperplasia Dysplasia Metaplasia Squamous Glandular Cell death Necrosis Coagulative necrosis Liquefactive necrosis Gangrenous necrosis Caseous necrosis Fat necrosis Fibrinoid necrosis Myocytolysis Programmed cell death Apoptosis Pyknosis Karyorrhexis Karyolysis Accumulations pigment Hemosiderin Lipochrome/Lipofuscin Melanin Steatosis
Anatomical pathology	Surgical pathology Cytopathology Autopsy Molecular pathology Forensic pathology Oral and maxillofacial pathology Gross processing Histopathology Immunohistochemistry Electron microscopy Immunofluorescence Fluorescence in situ hybridization
Clinical pathology	Clinical chemistry Hematopathology Transfusion medicine Medical microbiology Diagnostic immunology Immunopathology Enzyme assay Mass spectrometry Chromatography Flow cytometry Blood bank Microbiological culture Serology