Melanin

Melanin
One possible structure of Eumelanin
Material type	Heterogeneous biopolymer

Melanin (/ˈmɛlənɪn/ ^ⓘ; from Ancient Greek μέλας (mélas) 'black, dark') is a family of biomolecules organized as oligomers or polymers, which among other functions provide the pigments of many organisms.^[1] Melanin pigments are produced in a specialized group of cells known as melanocytes.

There are five basic types of melanin: eumelanin, pheomelanin, neuromelanin, allomelanin and pyomelanin.^[2] Melanin is produced through a multistage chemical process known as melanogenesis, where the oxidation of the amino acid tyrosine is followed by polymerization. Pheomelanin is a cysteinated form containing polybenzothiazine portions that are largely responsible for the red or yellow tint given to some skin or hair colors. Neuromelanin is found in the brain. Research has been undertaken to investigate its efficacy in treating neurodegenerative disorders such as Parkinson's.^[3] Allomelanin and pyomelanin are two types of nitrogen-free melanin.

The phenotypic color variation observed in the epidermis and hair of mammals is primarily determined by the levels of eumelanin and pheomelanin in the examined tissue. In an average human individual, eumelanin is more abundant in tissues requiring photoprotection, such as the epidermis and the retinal pigment epithelium.^[4] In healthy subjects, epidermal melanin is correlated with UV exposure, while retinal melanin has been found to correlate with age, with levels diminishing 2.5-fold between the first and ninth decades of life,^[5] which has been attributed to oxidative degradation mediated by reactive oxygen species generated via lipofuscin-dependent pathways.^[6] In the absence of albinism or hyperpigmentation, the human epidermis contains approximately 74% eumelanin and 26% pheomelanin, largely irrespective of skin tone, with eumelanin content ranging between 71.8–78.9%, and pheomelanin varying between 21.1–28.2%.^[7] Total melanin content in the epidermis ranges from around 0 μg/mg in albino epidermal tissue^[8] to >10 μg/mg in darker tissue.^[9]

In the human skin, melanogenesis is initiated by exposure to UV radiation, causing the skin to darken. Eumelanin is an effective absorbent of light; the pigment is able to dissipate over 99.9% of absorbed UV radiation.^[10] Because of this property, eumelanin is thought to protect skin cells from UVA and UVB radiation damage, reducing the risk of folate depletion and dermal degradation. 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.^[11]