Melanosome

Melanosome

A melanosome is an 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.[1][2]

Contents

  • Structure 1
  • Synthesis of melanin 2
  • Pseudopodia 3
  • In animals 4
  • In fossils 5
  • Templating 6
  • References 7
  • External links 8

Structure

Melanosomes are relatively large organelles, measuring up to 500 nm in diameter.[1] 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 Chédiak–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 Derek Briggs of the Yale University study team.[3][4]

Melanosomes were used to discover the true colors of fossil Anchiornis huxleyi by the Beijing Museum of Natural History.[5][6]

Templating

Melanosomes are believed to template melanin polymerization by way of amyloidogenesis of the protein Pmel17, which is present in abundant quantities in melanosomes.[7]

References

  1. ^ a b Wasmeier C, Hume AN, Bolasco G, Seabra MC (2008). "Melanosomes at a glance". J Cell Sci 121 (pt24): 3995–3999.  
  2. ^ Raposo G, Marks MS (2007). "Melanosomes--dark organelles enlighten endosomal membrane transport". Nat Rev Mol Cell Biol 8 (10): 786–797.  
  3. ^ Andrea Thompson (2008-07-08). "Feather Fossils Could Yield Dinosaur Colors".  
  4. ^ "Ancient Bird Feathers Had Iridescent Glow".  
  5. ^ Li, Q., Gao, K.-Q., Vinther, J., Shawkey, M.D., Clarke, J.A., D'Alba, L., Meng, Q., Briggs, D.E.G. and Prum, R.O. "Plumage color patterns of an extinct dinosaur." Science, 327(5971): 1369 - 1372. doi:10.1126/science.1186290 PMID 20133521
  6. ^ Jesus Diaz. "The Real Colors of a Dinosaur Revealed for the First Time". Gizmodo. Gawker Media. Retrieved 8 January 2015. .
  7. ^ Fowler DM, Koulov AV, Alory-Jost C, Marks MS, Balch WE, et al. 2005 Functional Amyloid Formation within Mammalian Tissue. PLoS Biol 4(1): e6. doi:10.1371/journal.pbio.0040006

External links

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