Melanosome

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 melanocytes), in the eye (in choroidal melanocytes and retinal pigment epithelial (RPE) cells), and in melanophores found in lower vertebrates.

Structure

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.

Synthesis of melanin

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.

Melanosome (geology)

In geology, a melanosome is a dark,mafic mineral band formed in migmatite which is melting into a eutaxitic texture ; often, this leads to the formation of granite. The melanosomes form bands with leucosomes, and in that context may be described as schlieren or migmatitic.

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