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Melanosome

 

 
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Melanosome
 
 
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In a biological cell
Cell (biology)

The cell is the structural and functional unit of all known Life organisms. It is the smallest unit of an organism that is classified as living, and is often called the building bricks of life....
, a melanosome is an organelle
Organelle

In cell biology, an organelle is a specialized subunit within a cell that has a specific function, and is usually separately enclosed within its own lipid membrane....
 containing melanin
Melanin

Melanin is a class of compounds found in the plant, animal, and protista kingdom , where it serves predominantly as a pigment. The class of pigments are derivatives of the amino acid tyrosine....
, the most common light-absorbing pigment found in the animal kingdom.

Cells that contain melanosomes are called melanocyte
Melanocyte

Melanocytes are cell located in the bottom layer of the skin's epidermis , the middle layer of the eye , the inner ear, meninges, bones and heart....
s, and also the retinal pigment epithelium cells, whereas cells that have merely engulfed the melanosomes are called melanophages.

nosomes are bound by a lipid membrane and are, in general, rounded, sausage dog-like, or cigar-like in shape. The shape is constant for a given species and cell type.

They have a characteristic ultrastructure
Ultrastructure

Ultrastructure is the detailed structure of a biological specimen, such as a Cell , biological tissue, or Organ , that can be observed by electron microscopy....
 on electron microscopy, which varies according to the maturity of the melanosome, and, for research purposes, a numeric staging system is sometimes used.

are dependent for their pigment on a set of enzymes within the cell (especially tyrosinase
Tyrosinase

Tyrosinase is an enzyme that Catalysis the oxidation of phenols and is widespread in plants and animals. Tyrosinase is a copper-containing enzyme present in plant and animal tissues that catalyzes the production of melanin and other pigments from tyrosine by oxidation, as in the blackening of a peeled or sliced potato exposed to air....
) that synthesise the large polymers generically known as melanin
Melanin

Melanin is a class of compounds found in the plant, animal, and protista kingdom , where it serves predominantly as a pigment. The class of pigments are derivatives of the amino acid tyrosine....
.

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
Albinism

Albinism is a form of hypopigmentation congenital disorder, characterized by a partial or total lack of melanin Biological pigment in the eyes, skin and hair ....
.

ome melanocytes, the melanosomes remain static within the cell.






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Encyclopedia


In a biological cell
Cell (biology)

The cell is the structural and functional unit of all known Life organisms. It is the smallest unit of an organism that is classified as living, and is often called the building bricks of life....
, a melanosome is an organelle
Organelle

In cell biology, an organelle is a specialized subunit within a cell that has a specific function, and is usually separately enclosed within its own lipid membrane....
 containing melanin
Melanin

Melanin is a class of compounds found in the plant, animal, and protista kingdom , where it serves predominantly as a pigment. The class of pigments are derivatives of the amino acid tyrosine....
, the most common light-absorbing pigment found in the animal kingdom.

Cells that contain melanosomes are called melanocyte
Melanocyte

Melanocytes are cell located in the bottom layer of the skin's epidermis , the middle layer of the eye , the inner ear, meninges, bones and heart....
s, 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 dog-like, or cigar-like in shape. The shape is constant for a given species and cell type.

They have a characteristic ultrastructure
Ultrastructure

Ultrastructure is the detailed structure of a biological specimen, such as a Cell , biological tissue, or Organ , that can be observed by electron microscopy....
 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
Tyrosinase

Tyrosinase is an enzyme that Catalysis the oxidation of phenols and is widespread in plants and animals. Tyrosinase is a copper-containing enzyme present in plant and animal tissues that catalyzes the production of melanin and other pigments from tyrosine by oxidation, as in the blackening of a peeled or sliced potato exposed to air....
) that synthesise the large polymers generically known as melanin
Melanin

Melanin is a class of compounds found in the plant, animal, and protista kingdom , where it serves predominantly as a pigment. The class of pigments are derivatives of the amino acid tyrosine....
.

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
Albinism

Albinism is a form of hypopigmentation congenital disorder, characterized by a partial or total lack of melanin Biological pigment in the eyes, skin and hair ....
.

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
Fish

A fish is any marine biology vertebrate animal that is typically ectothermic , covered with scale , and equipped with two sets of paired fins and several unpaired fins....
, 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 (i.e. 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 cephalopod
Cephalopod

The cephalopods are the mollusc class Cephalopoda characterized by bilateral symmetry, a prominent head, and a modification of the mollusk foot, a muscular hydrostat, into the form of cephalopod arms or tentacles....
s (octopuses and squid) are based, however, on a different system, the chromatophore
Chromatophore

Chromatophores are Biological pigment-containing and light-reflecting cell found in amphibians, fish, reptiles, crustaceans, and cephalopods. They are largely responsible for generating skin and eye color in cold-blooded animals and are generated in the neural crest during embryonic development....
 organ.

Templating

Melanosomes are believed to template melanin polymerization by way of amyloidogenesis
Amyloid

Amyloids are insoluble fibrous protein aggregates sharing specific structural traits. Abnormal accumulation of amyloid in organs may lead to amyloidosis, and may play a role in various other neurodegenerative diseases....
 of the protein Pmel17
SILV

Silver homolog , also known as SILV, is a human gene. Its gene product may be referred to as silver, ME20, gp100 or Pmel17 and is a 100 kDa type I transmembrane protein....
, which is present in abundant quantities in melanosomes.

External links

- "Integument: pigmented skin"