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Concept
Melanosome
Summary
A melanosome is an organelle found in animal cells and is the site for synthesis, storage and transport of melanin, the most common light-absorbing pigment found in the animal kingdom. Melanosomes are responsible for color and photoprotection in animal cells and tissues.
Melanosomes are synthesised in the skin in melanocyte cells, as well as the eye in choroidal melanocytes and retinal pigment epithelial (RPE) cells. In lower vertebrates, they are found in melanophores or chromatophores.
Melanosomes are relatively large organelles, measuring up to 500 nm in diameter. They are bound by a bilipid membrane and are, in general, rounded, sausage-like, or cigar-like in shape. The shape is constant for a given species and cell type. They have a characteristic ultrastructure on electron microscopy, which varies according to the maturity of the melanosome, and for research purposes a numeric staging system is sometimes used.
Melanosomes are dependent for their pigment on certain enzymes, especially tyrosinase, that synthesise the large polymers of melanin within the cell. Before it generates sufficient pigment to be seen on light microscopy it is known as a pre-melanosome.
Dysfunction or absence of the melanin-synthesising enzymes (in conditions such as Chédiak–Higashi syndrome) leads to various patterns of albinism.
In some melanocytes, the melanosomes remain static within the cell. In others the cell can extend its surface lengthwise as temporary projections known as pseudopodia, which carry melanosomes away from the center of the cell, thereby increasing the cell's effectiveness in absorbing light.
The pseudopodial process (aka the tanning process) happens slowly in dermal melanocytes in response to ultraviolet light and to production of new melanosomes and increased donation of melanosomes to adjacent keratinocytes, which are typical skin surface cells. Donation occurs when some keratinocytes engulf the end of the melanocyte pseudopodia, which contain many melanosomes.
Official source
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