Melanosomes

Inhibition of the passage of melanosomes from melanocytes to keratinocytes... 

Rozanowska, M, Korytowski, W, Rozanowski, B, Skumatz, C, Boulton, ME, Burke, JM, and Sarna, T, 2002. Photoreactivity of aged human RPE melanosomes A comparison with lipofuscin. Invest Ophthalmol Vis Sci 43, 2088-2096. 

Rozanowski, B, Burke, JM, Boulton, ME, Sarna, T, and Rozanowska, M, 2008b. Human RPE melanosomes protect from photosensitized and iron-mediated oxidation but become pro-oxidant in the presence of iron upon photodegradation. Invest Ophthalmol Vis Sci 49, 2838-2847. 

Rozanowski, B, Cuenco, J, Davies, S, Shamsi, FA, Zadlo, A, Dayhaw-Barker, P, Rozanowska, M, Sarna, T, and Boulton, ME, 2008c. The phototoxicity of aged human retinal melanosomes. Photochem Photobiol 84, 650-657. 

A nonubiquitous organelle, the melanosome, is specialized for melanin synthesis. It is somewhat similar to the lysosome, and the resident proteins are derived from the endocytic pathway. A sorting signal, the NQPLLT, was found in the cytoplasmic protein of a human membrane protein (Vijayasaradhi et al., 1995). 

Vijayasaradhi, S., Xu, Y., Bouchard, B., and Houghton, A. (1995). Intracellular sorting and targeting of melanosomal membrane proteins identification of signals for sorting of the human brown locus protein, gp75. J. Cell Biol. 130, 807-820. 

Hydroquinone interferes with the production of the pigment melanin by epidermal melanocytes through at least two mechanisms it competitively inhibits tyrosinase, one of the principal enzymes responsible for converting tyrosine to melanin, and it selectively damages melanocytes and melanosomes (the organelles within which melanin is stored). 

Also present in the epidermis are embedded macrophage-like Langerhans cells as well as pigmented melanocytes, cells with highly branched dendrites, which lie just above the basal stem cell layer. Each melanocyte contains hundreds of pigmented organelles called melanosomes. They contain not only the black or reddish melanin pigments but also the enzymes needed to form them (Chapter 25).n o... 

The dendrites of a melanocyte contact about 36 keratinocytes and are able to transfer melanosomes to these adjacent cells. The numbers and sizes of the melanosomes as well as melanin structure determine differences in skin color.131 Similar cells in amphibians, the melanophores, also contain light receptors p Their melanosomes are not transferred to other cells but may be either clustered near the center of the cell or dispersed. The location can be changed quickly by transport of the melanosomes along a network of microtubules allowing the animals to change in response to changes in light color.0)... 

Diagram of a dendritic melanocyte surrounded by satellite keratinocytes. The Golgi area (G), where the melanosomes are synthesized, is shown around the nucleus. The other branched cell, higher in the epidermis, is a Langerhans cell with its tennis racquet-shaped granules. Courtesy of Dr. W. Quevedo, Jr. From Montagna et al.m... 

The principal pigment of human skin, hair, and eyes is melanin, which is synthesized in specialized cells, the melanocytes. They lie between the epidermis (outer layer) and the dermis (inner layer) as shown in Box 8-F. Melanocytes originate from embryonic nervous tissue and migrate into the skin by the third month of fetal life. They retain the highly branched morphology of neurons. Persons of different races all have the same numbers of melanocytes but the numbers and sizes of the pigmented melanosomes (Box 8-F) vary as does the content and chemical composition of the melanin.a d Melanosomes not only are found in the dendrites of the melanocytes but are transferred from them into adjacent epithelial cells.6 1... 

Leukoderma can occur, accompanied by normal melanocyte function but reduced phagocytic activity of the keratinocytes to eliminate the melanosomes (169). Depigmentation can occur at the site of injection of glucocorticoids. 

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