Everything about Melanosome totally explained
» 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.
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's 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.
Melanosomes found in certain fish species contain pigments that control the color of the fish's scales. Molecular motors, when signaled, will either carry melanosomes containing pigments out to the periphery of the cell, or concentrate them at the center. The motors responsible for concentrating the melansomes at the center are dyneins, which move the melanosomes along microtubule tracts towards the minus end (for example the center of the cell). The motors responsible for dispersing the melanosomes to the periphery are kinesins, which are plus end directed motors. Since the plus end of microtubules are oriented towards the periphery, kinesins will carry melanosomes to the periphery. Dispersion of melanosomes to the periphery causes the cell to appear darker. Concentration of melanosomes towards the center will cause the cell to appear lighter color. This is how a protective system works for the fish on a molecular level.
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.
Further Information
Get more info on 'Melanosome'.
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