Melanin in skin - Function of melanin

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The primary function of melanin is in skin pigmentation. Melanin has other important roles in the human body such as, sunlight absorption and sun screening, protection from ultraviolet radiation, charge-transfer redox activity, free radical scavenging, antimicrobial immune defense, immunomodulation and ion chelating.

Melanin in skin

Melanins are a broad spectrum of very common natural pigment compounds found in both the plant and animal kingdoms. It is a complex molecule derived from the amino acid tyrosine via the intermediate product L-DOPA, by the action of the tyrosinase enzymes. There are three basic types of melanin, namely, eumelanin, pheomelanin, and neuromelanin. Variation in human skin color is a result of density and relative proportions of contributing melanins.

Function of melanin

In plants and invertebrates, melanin is involved in immune defence. In certain pathogenic microbes, bacteria and fungi, these pigments function as defence mechanisms against the host's immune responses.

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Though many other functions are attributed to melanins in plant and animal kingdoms, herein we will consider their functions related to the human body.

Skin pigmentation

Melanin, whose primary function is in pigmentation, has many more important functions to perform in the human body.

Protection from UV radiation

Melanin functions as a sunscreen and as a broad spectrum sun radiation absorbent. The absorbed energy causes redox reactions, and both eumelanin and pheomelanin form free radical species. Eumelanin, being more photostable scavenges the originated oxidative species and neutralizes them. However, pheomelanin being more photolabile may not neutralize all the oxidative free radicals. Free radicals have the potential to cause DNA damage. Hence, fair individuals, having high amounts of pheomelanin, are more prone to skin cancers on sun exposure.

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Melanin in the eyes

Melanin in the iris, protects the retina from excessive light and photodamage. By absorbing the scattered light within the eyeball, it also helps in improving the visual acuity. Melanin in the retinal pigment epithelium (RPE) has antioxidant properties and neutralizes the photooxidative species. The antioxidative activity wanes with aging and one of causes of macular degeneration is oxidative stress.

Melanin in the ears

The melanin pigment is present in the inner ear with no known function. In Waardenburg syndrome, deafness occurs along with reduced inner ear pigmentation. In the inner ear, the loss of melanocytes in stria vascularis of cochlear duct is associated with loss of hearing.

Melanin in the brain

The substantia nigra pars compacta is a part of the midbrain, which appears darker than the surrounding areas due to the presence of neuromelanin in dopaminergic neurons. Degeneration of the pigmented neurons or depigmentation is found in persons affected with Parkinson's disease. It is hypothesized that neuromelanin may be neutralizing the oxidative stress and chelating the toxic metal ions in the brain. The decrease in neuromelanin, as observed in Parkinson's disease may be the cause of the associated degenerative symptoms.

Melanins also function as chelating agents. They can chelate metal ions, including toxic species, and protect the cells from damage. Melanins can also bind with a number of organic molecules and neutralize their damaging effects.

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Melanins behave as semiconductors. Thin melanin films having multifarious electrical and optical properties can be made by spray deposition. Having interesting biophysical properties, melanin opens up many new biotechnological applications.

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Reference: 1.Michaela Brenner, Vincent J. Hearing. The Protective Role of Melanin Against UV Damage in Human Skin. Photochem Photobiol. 2008; 84(3): 539–549. 2.F. Solano. Melanins: Skin Pigments and Much More—Types, Structural Models, Biological Functions, and Formation Routes. New Journal of Science, vol. 2014, Article ID 498276, 28 pages, 2014. doi:10.1155/2014/498276. 3.Burkhart, C. G. and Burkhart, C. N. The mole theory: primary function of melanocytes and melanin may be antimicrobial defense and immunomodulation (not solar protection). International Journal of Dermatology, 44: 340–342.

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Current topic: What is the function of melanin in skin.

Skin pigment - Melanin pigment

Home › Melanin pigment in skin

Melanin pigment

Melanin pigment gives color to skin, hair and eyes in humans.

The melanin skin pigment is derived from the amino acid tyrosine.
The melanin pigment is a polymer compound which is neither a protein nor an amino acid.
Apart from mammals many other organisms in the animal kingdom have been found to have this skin pigment.
Apart from its presence in the epidermis, iris and hair, melanin pigment is found in parts of the brain.

When the body is unable to produce the pigment due to certain physiological conditions, disorders like albinism, leucism and vitiligo occur. In certain diseases like tinea versicolor and in certain conditions like idiopathic guttate hypomelanosis, nevus depigmentosis and pityriasis alba there is partial loss of normal melanin pigment showing up as patches of white discoloration on skin.

Melanogenesis

Melanocytes are the pigment producing cells. They are present in the stratum basale, the innermost of the five layers of epidermis. The pigment granules are produced by melanocytes in melanosomes which are specialized membrane–bound cellular vesicles.

Human skin color variations

Melanin synthesis is carried with the help of enzymes present in the melanosomes. There are melanogenic regulators in melanocytes. This pigment is the end product of complex interaction between melanogenic regulators and factors derived from keratinocytes and fibroblasts.

Tyrosinase is an important enzyme in melanogenesis, transforming the tyrosine, the base amino acid. Tyrosinase is important for the synthesis of both eumelanin and pheomelanin.

When the melanosomes are full with the pigment granules, they are transferred to keratinocyte cells in the epidermal layer of skin. Melanosomes usually accumulate atop the nucleus of the keratinocyte cell, protecting it from the sun's radiation and genetic damage.

There is not much difference between the concentration of melanocytes between individuals of different ethnic groups. The variations in the human skin color is due to quantity, type, size and pattern of distribution of the pigment. The concentration and the type of pigment contributing to human complexion is determined by the genetic makeup of an individual.

The frequency of expression of the melanin-producing genes is controlled by the genotype of an individual. Melanogenesis is multidirectional in nature. Apart from the genetic makeup, the type and distribution of the pigment in the skin is modulated by environmental and endocrine factors.

Types of melanin pigments

Melanin is made up of smaller component molecules. Different types of melanin arise due to differing proportions and bonding patterns of these components. There are three types of pigments found in the human body. Eumelanin and pheomelanin are found in the epidermis and hair. Neuromelanin is present in the parts of the brain.

Eumelanin

Eumelanin, a brown-black polymer of dihydroxyindole carboxylic acids, is the most common skin pigment with black and brown subtypes imparting brown, dark brown and black color. Individuals with darker complexions produce more of the black eumelanin. Small amounts of brown eumelanin in hair with the lack of any other pigment produces blond hair. In grey hair small quantities of black eumelanin is present.

Pheomelanin

The color variations ranging from red, pink to yellow are imparted by the pheomelanin. Individuals with fair complexions produce more of the pheomelanin. Pheomelanin is concentrated in the lips, nipples and genital parts. Pheomelanin is present in large quantities in red hair.

Neuromelanin

Neuromelanin pigment is present in the parts of the brain like medulla, the locus coeruleus and the substantia nigra. Neuromelanin is found to effectively ligate metal ions through its carboxylate and phenolic hydroxyl groups and protect the neurons from potentially toxic metal ions. In Parkinson's disease and also in a variety of neurodegenerative diseases, there is loss of neuromelanin, followed by increase in the iron deposits in the brain.

Function of melanin

• Eumelanin gives photoprotection to the epidermis by absorbing/scattering UV radiation in the sunlight.
• It preserves folic acid from getting destroyed due to ultraviolet-A rays.
• The ratio of eumelanin to pheomelanin in the epidermis imparts color complexion.
• The area in the retina called fovea does not develop properly in the absence of the pigment.
• It is also important for the proper development of nerve connections between the retina and the brain.
• Tanning is a type of melanin production induced by DNA damage from ultraviolet (UV) radiation.
• Higher concentration of the pigment limits exposure to sunlight and interferes with vitamin D synthesis by the dermis especially in higher latitudes.
• Melanin in the iris protects the eyes from glare and radiation.

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Reference:

1.Slominski A, Tobin DJ, Shibahara S, Wortsman J. Melanin pigment in mammalian skin and its hormonal regulation. Physiol Rev. 2004 Oct;84(4):1155-228.

2.Sulaimon SS, Kitchell BE. The biology of melanocytes. Vet Dermatol. 2003 Apr;14(2):57-65.

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Current topic: Skin pigment melanin