Melanosome

For the geological term, see melanosome (geology)

In a biological cell, a melanosome is an organelle containing melanin, the most common light-absorbing pigment found in the animal kingdom.

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.

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Shape

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

They are dependent for their pigment on a set of enzymes within the cell (especially tyrosinase) that 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.

Pseudopodia

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 animals

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.

Templating

Melanosomes are believed to template melanin polymerization by way of amyloidogenesis of the protein pMel, which is present in abundant quantities in melanosomes.

References

• Fowler, et al. PLoS Biol. 2005 Nov 29;4(1)