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Cells that contain melanosomes are called melanocytes, and also the retinal pigment epithelium cells, whereas cells that have merely engulfed the melanosomes are called melanophages.
Additional recommended knowledge
Melanosomes are bound by a lipid 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
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 center of the cell and increasing the cell's effectiveness in absorbing light.
This happens slowly in dermal melanocytes in response to ultraviolet light, as well as production of new melanosomes and increased 'donation' of melanosomes to adjacent keratinocytes, the normal skin surface cells.
These changes, together, 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 signaling.
The beautiful and rapid colour changes seen in many cephalopods (octopuses and squid) are based, however, on a different system, the chromatophore organ.
Melanosomes are believed to template melanin polymerization by way of amyloidogenesis of the protein pMel, which is present in abundant quantities in melanosomes.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Melanosome". A list of authors is available in Wikipedia.|