Skin pigment - Melanin pigment

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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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