Melanin granule

K. Akeo, S. Amaki, T. Suzuki, and T. Hiramitsu, Melanin granules prevent the cytotoxic effects of L-DOPA on retinal pigment epithelial cells in vitro by regulation of NO and superoxide radicals. Pigm. Cell. Res. 13, 80-88 (2000). 

Colchicine inhibits the release of histamine-containing granules from mast cells, the secretion of insulin from P-cells of pancreatic islets, and the movement of melanin granules in melanophores. Although it is questionable whether these effects occur at clinically achieved concentrations of colchicine, all these processes may involve the translocation of granules by the microtubular system. 

In beagle dogs, intravenously injected radium-226 was deposited in the melanin granules of pigmented cells and rodlike organelles of the tapetum in the eye (a structure that humans do not have). Retention in the eye varied inversely with dose. At doses from 0.062 to 1.1 pCi/kg (2.3 to 41 kBq/kg), loss of pigment at the higher doses and melanosis and intraocular melanoma formation at the lower doses were observed (Taylor et al. 1972). 

Melanin compounds may appear brown, black, or red. The type of melanin determines hair color, and the density of melanin granules determines the shade. Dark shades of dyed hair contain higher concentrations of dyes. Most hair colors are combinations of organic compounds chosen to produce particular shades. Resorcinol produces a yellow color aminohydroxy-toluene produces a redder hair, and nitrophenylenediamine dye results in very red hair. Graded dyes favored by men often contain lead acetate. The lead ions penetrate into hair and form lead sulfide (PbS), a dark-colored compound. 

Melanin granules are secreted by melanocytes in the hair papilla and distributed to keratin in the hair cortex and inner layers of the hair sheath during normal development. Melanogenesis is subject to hormonal control and has been the focus of intensive genetic studies. Two main forms of melanin exist in human skin—eumelanin and phaeomelanin, both of which are derived from tyrosine through the action of tyrosinase (a cupro-enzyme) and possibly other key enzymes (with nickel, chromium, iron, and manganese as cofactors). Tyrosine is converted to dihydroxyphenylalanine and, via a series of intermediate steps, to indole-5,6-quinone, which polymerizes to eumelanin. Phaeomelanins are produced by a similar mechanism but with the incorporation of sulfur (as cysteine) by a nonenzymatic step in the oxidation process. 

Barnicot, N. A. and Birbeck, M. S. C., The electron microscopy of human melanocytes and melanin granules, in The Biology of Hair Growth, Montagna, W. and Ellis, R. A., Eds., Academic Press, New York, 1958, 239. 

The waste products are partially deposited on the Bruch s membrane (Young, 1987) in the form of drusen. The accumulation of lipofuscin in RPE cells appears detrimental to its function (Flood et al., 1984) and causes photoreceptor death (Dorey et ak, 1989). With age, the number of RPE cells decreases in the central retina, and they become pleomorphic (Dorey et ak, 1989). Other changes are also frequent. They include atrophy, depigmentation, hyperplasia, hypertrophy, and cell migration. The melanin concentration in the RPE cells decreases with age, especially in Caucasians, but also in blacks (Feeney-Burns et al., 1984). The melanin granules are slowly (over decades) digested by lysosomes (Bums and Feeney-Burns 1980). 

Subsequent exposure to aqueous alkaline solution or to peroxide solutions leads to rapid dissolution of the affected areas even during short-term exposure to peroxide solutions (bleaching solutions) see Figure 4-10 and 4-11. Longer exposure of these ultraviolet radiated fibers with oxidizing solutions leads to dissolution/elimination of scale differentiation and dissolution of the melanin granules, see Figure 4-12. 

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