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The name 'melanin' comes from the ancient Greek melanos, meaning 'black', and the term was probably first applied by the Swedish chemist Berzelius in 1840. Melanin has many biological functions, including colouring the skin and hairs or feathers, strengthening plant cell walls and insect cuticle, absorbing light (and so providing photoreceptor shieldlng, thermoregulation, photoprotectlon, storage), and it is an important free radical sink.
There are three types of melanin, the first two of which have been identified in fossils:
1. Eumelanin is the best-known form of melanin, providing primarily black colours. Eumelanin is found in the hair and skin of humans, and it colours the hair black, and brown.
Eumelanin exists in two forms, black eumelanin and brown eumelanin. Black melanin produces black colours when it is present in large quantities. Brown eumelanin may produce brown hair colours when it is present in abundance, but smaller amounts produce lighter brown. When there exists little quantities of brown melanin it will produce blond hair colours. The black variant of eumelanin is commonest in people of non-European descent, whereas ethnic Europeans more often have very little brown eumelanin variety, and predominately more Pheomelanin.
2. Phaeomelanin (= pheomelanin) produces reddish/Yellow colours. In humans, phaeomelanin is more abundant in the skin of women than men, and so their skin is slightly redder. This is not true for whites where Pheomelanin content in on average equal for white men as white women. It occurs also in hair, and provides the main colouring agent in ginger hair.
3. Neuromelanin is the dark pigment that produces a black colour in certain parts of the brain. We have now visual data confirming human neuromelanin is comprised of a combination of both Eumelanin and Pheomelanin.
The ink in the cuttlefish contains another type of melanin called sepiomelanin and the melanins in plants are called allomelanins. Some melanins in Fungi are the simplest of all, polyacetylene.
Chemistry of melanins
Molecular structure of phaeomelanin (A) and eumelanin (B). [Diagram from ]http://photoprotection.clinuvel.com/node/204.][/I]
Eumelanins are insoluble, nitrogenous pigments produced by the oxidative polymerization of 5,6-dihydroxyindoles derived enzymatically from tyrosine via DOPA.
Phaeomelanins are sulfur-containing, alkali-soluble, pigments produced by oxidative polymerization of cysteinyldopas via 1,4-benzothiazine intermediates.
Melanocytes and melanosomes
Dermal melanin is produced by melanocytes, specialized skin cells located in the basal layer of the epidermis (right). Melanocytes in the embryo derive from the neural crest, an active zone of development in the early animal embryo that provides key tissues in the head (bones, teeth, skin, nerves, sensory organs), the spinal cord, and key glands. Although almost everyone has the same amount of melanocytes, the amount and size of the melanosomes and melanin particles produced can differ immensely in humans, resulting in the different races of the world.
Melanocytes insert granules of melanin into specialized cellular vesicles, or organelles, called melanosomes. Melanocytes are branched structures, consisting of a central cell body and numerous branches, or dendrites. Melanosomes are produced inside the melanocytes, and they pass through several developmental stages, starting in the middle of the melanocyte cell, and migrating to the outer edge of the cell through the dendrites. Melanocytes may produce eumelanosomes or phaeomelanosomes at different times, switching from one to the other.
The melanosomes mature within the melanocytes, and pass to the outer tips of the dendrites, where they are transferred into the other skin cells of the epidermis.
Melanosome transfer is a cytophagic ("cell engulfing") process during which a portion of a melanocyte dendrite is pinched off by the epidermal cell so that melanosomes and melanocyte cytoplasm are incorporated into the keratinocyte. Keratinocytes are the cells that produce the protein keratin, the key structural component of hair and feathers. The melanosomes enter the developing hair at an early stage before it extends out through the skin.
Eumelanosomes are typically elongate (0.8-1 µm [800-1000 nm] long, and 200-400 nm wide) with rounded ends, whereas phaeomelanosomes are ovoid to sub-spherical and they vary more in size; most are between 500 and 700 nm long (occasionally up to 900 nm) and 300 and 600 nm wide. [Note, 1 nm, or nanometre, is one-millionth of a millimetre (one billionth of a metre); so 1 mm = 1,000,000 nm.]
Melanocytes are pigment-producing cells that originate from the dorsal portions of the closing neural tube in vertebrate embryos (Fig. 70-1).
They derive from pluripotent neural crest cells that differentiate into numerous cell lineages, including neurons, glia, smooth muscle, craniofacial bone, cartilage, and melanocytes (reviewed in refs. 2 and 3).
Progenitor melanoblasts migrate dorsolaterally between the mesodermal and ectodermal layers to reach their final destinations in the hair follicles and the skin as well as inner ear ochlea, choroids, ciliary body, and iris (reviewed in refs. 2 and 4).
Pigment producing cells can be found in fetal epidermis as early as the fiftieth day of gestation.
Mitf affects melanoblast differentiation by inducing the transcription of three enzymes that regulate melanin synthesis: tyrosinase, tyrosinase-related protein-1 (TRP-1), and 3,4 dihydroxyphenylalanine (DOPA)chrome tautomerase (TRP-2)(reviewed in ref. 12).
Melanocyte Stem Cells
Generally, stem cells are defined by their undifferentiated state and their capacity to develop into several differentiated cell types. They are quiescent, slow-cycling cells that frequently are found in niches where they are surrounded by differentiated cells that affect their behavior through the secretion of cytokines and growth factors (reviewed in refs. 29 and 30). Melanocyte stem cells reside in the hair follicle bulge (Fig. 70-3).
They express TRP-2 as well as the neural crest stem cell intermediate filament nestin in addition to other neural crest stem cell markers, including the transcription factors Sox10 and Pax5 that participate in the regulation of Mitf and TRP-2. Melanocyte stem cells can leave the bulge region and migrate/differentiate in the epidermis or the hair follicle.
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