In medicine, melanins are the pigments produced naturally in the body that give the skin, hair and eyes their color. Melanins are produced in cells called melanocytes and released into the surrounding cells. In pigmented people, the pigment takes on the role of a UV filter.
What are melanins?
Melanins are reddish, black and brown pigments. In humans, they color the eyes, skin and hair. Melanins are also found in animals. In animals, they determine the color of fur and feathers. In squid, they also form the pigment of ink. To form melanins, an enzymatic oxidation takes place. The starting material of this reaction is the so-called tyrosine. In vertebrates, the biosynthesis for the production of melanins takes place in the basal cell layer of the epidermis and in the retina of the eyes. The producing cells are also called melanocytes and transport the pigments via their dendrites to the surrounding keratinocytes. In humans, melanin exists in two different variants. While pheomelanin is brownish red in color, eumelanin is brownish black in color. Variants of a different color are also called allomelanins and are found in bacteria, fungi and plants.
Anatomy and structure
In human skin and hair, melanins are present as hybrids of eumelanins and phaeomelanins. The proportions of the two subgroups, along with other factors, determine human skin type. For example, people with very red hair, fair skin and freckles have a particularly high content of phaeomelanins. In contrast, eumelanins predominate in people with black hair and dark skin. Eumelanin is formed by the oxidation of the amino acid tyrosine. These melanins are therefore derivatives of the same synthetic pathway that the dopamine precursor L-dopa passes through. Phaeomelanin, on the other hand, contains sulfur. Allomelanins as found in plants and microorganisms are derived from hydroxylbenzenes. In most cases, melanins are protein-bound or at least linked to lipids.
Function and roles
According to contemporary medicine, melanins serve primarily to protect against UV radiation. This assumption is preceded by the observation that dark-skinned people are significantly less likely to develop malignant melanomas and thus black skin cancer. In addition to the stimulating hormone, sun exposure also stimulates the production of melanin in melanocytes. Melanin has been shown to play the role of a UV filter. Radiant energy is transformed into mere heat in an internal conversion. The electronically excited state of molecules is consequently transformed into vibrational states during internal conversion. Around 99 percent of the radiation energy can be rendered harmless in this way. The lifetime of the molecule in the excited state is shortened and free radicals cannot form in this way. Since redheads with pale skin have a disproportionately higher risk of skin cancer than pigmented people, the sun protection provided by their melanin type is probably less effective. Pigment production occurs in the rough endoplasmic reticulum of melanocytes. In the Golgi apparatus of melanocytes, the amino acid tyrosinase is stored and sequestered into vesicles. Tyrosine migrates into these vesicles and a maturation process begins. With the help of a protein, tyrosinase becomes DOPA and finally melanin. A mature melanosome migrates into the dendrites of the melanocytes and is released into the five to eight surrounding cells. This process is activated by UV radiation or the hormone MSH.
Diseases
Hyperpigmentation is overpigmentation of the skin. In this disease, excessive pigment is deposited in the epidermis. Either only parts of the skin are affected or the entire body. The deposited pigments can be either the body’s own pigments or exogenous pigments. Exogenous deposits occur, for example, with carbon deposits from tattoos. A special form of hyperpigmentation is present in the postinflammatory form. In this case, melanocytes are not activated by sunlight, but by enzymes in the context of local inflammation. The opposite of hyperpigmentation is called hypopigmentation. The formation of the pigment melanin is disturbed in the context of many hypopigmentations. In albinism, for example, disorders occur in the biosynthesis of melanin. An intermediate product of melanin biosynthesis loses its functionality and the conversion to melanin is disturbed.Congenital hyperpigmentation, on the other hand, is known as a birthmark. Moles can be either clearly demarcated or irregular in appearance. Clearly demarcated moles are usually not at risk for degeneration. Diffuse moles or those with an extremely dark color, on the other hand, are prone to degeneration. Over time, they can develop into melanomas, i.e. black skin cancer. Black skin cancer is a malignant tumor of melanocytes that spreads metastases through the lymphatic and blood systems. In over half of all cases, melanoma arises from an irregular nevus cell nevus. Malignant melanomas do not only occur on the skin. Such formations can also develop in the mucous membranes of the internal organs or on the conjunctiva. However, these mucosal melanomas are much rarer than melanomas of the skin. Light-skinned people have a disproportionately higher risk of developing melanoma of the skin because of the lack of UV filters. In contrast, pigmented people often develop mucosal melanomas because their mucous membranes and conjunctiva are not pigmented and therefore lack UV protection.
