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.
There are five basic types of melanin: eumelanin, pheomelanin, neuromelanin, allomelanin and pyomelanin. 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. Allomelanin and pyomelanin are two types of nitrogen-free melanin.
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. 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.
In humans, melanin is the primary determinant of skin color. 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 .
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.
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.
Eumelanin polymers have long been thought to comprise numerous cross-linked 5,6-dihydroxyindole (DHI) and 5,6-dihydroxyindole-2-carboxylic acid (DHICA) polymers.
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.
Pheomelanins (or phaeomelanins) impart a range of yellowish to reddish colors. 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, instead of DHI and DHICA, when the amino acid L-cysteine is present.
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.Therefore, it may play crucial roles in apoptosis and the related Parkinson’s disease.
Numerous steps are involved in the biosynthesis of melanin. The first step is catalysis of the chemical L-3,4-dihydroxyphenylalanine by tyrosinase. A lack of tyrosine can lead to albinism.
These melanosomes leave the melanocytes and move into other cells in the epidermis. Mostly brown or black in colour, melanin deposits determine the skin pigment which varies depending on the number and distribution of the melanosomes.
Melanin is found in several areas of the human body including:
- Skin where it provides skin color
- Pupils or irises of the eyes
- Stria vascularis of the inner ear
- Areas of the brain, the substantia nigra and locus coeruleus
- The medulla and zona reticularis of the adrenal gland
Types of melanin
Some of the different types of melanin include eumelanin, pheomelanin and neuromelanin. It is particularly abundant among black populations and provides black and brown pigment to the hair, skin and eyes.
When eumelanin is present only in small amounts, hair may be blonde.
This type of melanin is not as protective against UV- radiation induced cancer as eumelanin.
This review presents a general view of all types of melanin in all types of organisms. In that second phase, quinones derived from phenol oxidation play a crucial role. Concerning functions, all melanins show a common feature, a protective role. But they are not merely photoprotective pigments against UV sunlight.
In pathogenic microorganisms, melanization becomes a virulence factor since melanin protects microbial cells from defense mechanisms in the infected host. Then, their redox, metal chelating, or free radical scavenging properties are more important than light absorption capacity.
These pigments sometimes behave as a double-edged sword, and inhibition of melanogenesis is desirable in different cells. Melanin biochemistry is an active field of research from dermatological, biomedical, cosmetical, and microbiological points of view, as well as fruit technology.
Antecedents, Concept, Types, and Occurrence
Melanin is the generic name used to refer to perhaps the most ubiquitous, resistant, heterogeneous. And ancient pigments found in nature. Melanin appeared very early in most living kingdoms on the Earth. These recent findings will probably make melanin a new biomarker in life evolution.
The name “melanin” comes from the ancient Greek melanos, meaning “dark,” and, according to Borovansky , the term was probably first applied by the Swedish chemist Berzelius in 1840 to call a dark pigment extracted from eye membranes . However, first references of human skin pigmentation and somehow to the existence of melanin without using the current name are very old.
Pharaonic medicine in the Ebers Papyrus (1550 BC) described some diseases affecting skin color , and one of them was probably vitiligo, although that term appeared much later, derived from the Latin word “vitellus” meaning “veal” or pale pink skin . The first relatively detailed written description on skin pigmentation in mankind came from Herodotus in Greece, who described the darker skin of Persians, Ethiopians, and Indians in relation with Greeks.
According to the Bible, the varieties of human color could have descended from Noah’s three sons Shem, Japheth, and Ham (which means black).Aristotle’s explanation is somehow the first link between sunlight and tanning or stimulation of cutaneous melanogenesis.
In the modern era, there were a great number of anatomical descriptions by eminent scientists of those centuries, such as Malpighi in Naples and many others (see  for details). In those centuries, the most common idea in Europe. And the Arabic civilization about the origin of the dark pigment was that. The first chemical analyses by Berzelius and others established very significant differences between melanin and hemoglobin, making it unlikely or ruling out such relationship.
last 50 years
Structurally, melanins are a group of complex pigments with a structure relative diverse and undefined.
To better comprehension of the melanin concept and structure. Animal melanin, plant melanin, fungal melanin, bacterial melanin, and synthetic melanin. The phenolic units of the polymer and the properties of the oxidases needed for the polymer formation show significant differences.
During the study and extraction of different melanins from these sources, particular names have appeared. Mainly eumelanin, pheomelanin, neuromelanin, allomelanin, and pyomelanin. But there is not always a good correlation among the source and type of melanin.
Melanin is Made in Melanocytes
You’ve heard of cells before, right? They’re all over your body. Cells make up your skin, heart, stomach, everything.
Different types of cells do different things. Some make important products that keep your body running. For example, the cells in your stomach have to make the acids and enzymes that break down the food you eat.
Melanin is made in special cells called melanocytes. There are melanocytes in your skin and eyes, as well as in hair roots. These cells are the reason skin, eye and hair have color.
To start thinking about how melanin is made, let’s imagine each cell as a factory. There are lots of different steps to make one product at the end.
Making a car, for example, is a long process.
Same thing with making melanin. Except that instead of workers, a melanocyte has proteins. These proteins act like workers in an assembly line and carry out the steps for making pigment in your body. They start out with a chemical called tyrosine.
One set of proteins changes the tyrosine bit by bit. Another set links these modified tyrosines together to make a long chain. The end result is melanin.
But making melanin doesn’t just happen anywhere in the cell. The whole process happens in special compartments called melanosomes.
There are certain types of proteins that move components in or out of melanosomes. Kind of like delivery trucks that bring parts to factories.
Remember when we were talking about how different gene versions can make slightly different versions of the same protein? Well, it is the differences in the genes for melanosome proteins that determine eye color.
One wrong step in an assembly can shut down the whole car factory. The same is true for melanin production.
Blue Eyes Result from Differences in Melanosome Genes
One of the spots where melanin production can break down is in the delivery truck proteins. Remember, these are key proteins that move stuff in and out of melanosomes. OCA2 is a pretty famous example.
Sometimes blue eyes happen when there is a difference in the OCA2 gene so the deliveries don’t get made. This is like a delivery truck with a flat tire or an engine blow out.
TYR and TYRP1
OCA2 isn’t the only important gene. For example, TYR and TYRP1 are two of the assembly line worker proteins in melanin production.
Mutations in their genes reduce the amount of pigment made. This results in a lighter eye color. Depending on the mutation, this can mean blue eyes or even less pigment everywhere (albinism).
There are other genes that can cause blue eyes. But they all have something in common: they affect melanin, melanosomes, or melanocytes, the pigment-producing factories in your body.
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