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MelanosomeA melanosome is a cellular organelle containing melanin, the commonest light-absorbing pigment found in the animal kingdom. Cells which produce melanosomes are called melanocytes, whereas cells which have merely engulfed the melanosomes are called melanophages. Melanosomes are bound by a lipid membrane and are generally 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. They are dependent for their pigment on a set of enzymes within the cell (especially tyrosinase) which synthesise the large polymers generically known as melanin. Before it contains much pigment (sufficient to be seen on light microscopy), it is known as a pre-melanosome. Dysfunction or absence of the melanin-synthesising enzymes leads to various patterns of albinism. In some melanocytes, the melanosomes remain static within the cell. In other types of melanocyte, the cell can extend its surface as long pseudopodia, carrying melanosomes away from the centre of the cell and increasing the cell's effectiveness in absorbing light. For example, this happens slowly in dermal melanocytes in responsive to ultraviolet light, as well as production of new melanosomes and increased 'donation' of melanosomes to adjacent keratinocytes, the normal skin surface cells. Collectively these changes are responsible for 'tanning' after exposure to UV or sunlight. In many species of fish, amphibians, crustaceans and reptiles, melanosomes can be highly mobile within the cell in response to hormonal (or sometimes neural) control, and this leads to visible changes in colour that are used for behavioural signalling. The beautiful and rapid colour changes seen in many cephalopods (octopuses and squid) are based however on a different system, the chromatophore organ.
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