Melanosome

A melanosome is an organelle found in animal cells, and is the cellular site of synthesis, storage and transport of melanin, the most common light-absorbing pigment found in the animal kingdom. These melanosomes are responsible for color and photoprotection in animal tissues.

Melanosomes are synthesised in the skin melanocytes, choroidal melanocytes and retinal pigment epithelial (RPE) cells in the eye, and in melanophores 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

They are dependent for their pigment on a set of enzymes within the cell (especially tyrosinase) that synthesise the large polymers 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 in conditions such as Chédiakâ€"Higashi syndrome 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. This donation comes about because some keratinocytes may engulf the end of the melanocyte pseudopodia, which contain many melanosomes. Cytoplasmic dynein will carry the vesicles containing the melanin to the center of the cell. This causes melanosomes to become sequestered around the keratinocyte's nucleus, providing optimal protection from UV rays. 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 melanosomes at the center are dynein, which move the melanosomes along microtubule tracts towards the minus end (i.e. the center of the cell). The motors responsible for dispersing the melanosomes to the periphery are kinesin, which are plus end directed motors. Since the plus end of microtubules are oriented towards the periphery, kinesin 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.

In fossils

Recent (2008) discoveries by Xu Xing, a Chinese paleontologist, include fossilized feathers in rock formations dating from the Jurassic period (200 to 150 million years ago) to the late Paleogene and Neogene periods (66 to 2 million years ago). The feathers contain preserved residues of carbon that were previously thought to be traces of bacteria that decomposed feather tissues but are in fact microscopic organic imprints of fossilized melanosomes. Some of these structures still maintain an iridescent color typical of feather and fur tissues. It is conjectured that these microscopic structures could be further studied to reveal the original colors and textures of softer tissues in fossils. "The discovery of ultra-structural detail in feather fossils opens up remarkable possibilities for the investigation of other features in soft-bodied fossils, like fur and even internal organs," said Derek Briggs of the Yale University study team.

Melanosomes were used to discover the true colors of fossil Anchiornis huxleyi by the Beijing Museum of Natural History.

Templating

Melanosomes are believed to template melanin polymerization by way of amyloidogenesis of the protein Pmel17, which is present in abundant quantities in melanosomes.

References

External links

• Histology image: 08103loa â€" Histology Learning System at Boston University - "Integument: pigmented skin"

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