Melanosome
Encyclopedia
In a biological cell
, a melanosome is an organelle
containing melanin
, the most common light-absorbing pigment found in the animal kingdom.
Cells that synthesize melanins are called melanocyte
s, and also the retinal pigment epithelium cells, whereas cells that have merely engulfed the melanosomes are called melanophages.
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.
) 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
.
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.
, 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 cephalopod
s (octopuses and squid
) are based, however, on a different system, the chromatophore
organ.
, have found fossil
ized feathers in rock formations dating from the Jurassic period (200 to 150 million years ago) to the late Tertiary period (65 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
.
ogenesis of the protein Pmel17
, which is present in abundant quantities in melanosomes.
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
, 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 bilayer....
containing melanin
Melanin
Melanin is a pigment that is ubiquitous in nature, being found in most organisms . In animals melanin pigments are derivatives of the amino acid tyrosine. The most common form of biological melanin is eumelanin, a brown-black polymer of dihydroxyindole carboxylic acids, and their reduced forms...
, the most common light-absorbing pigment found in the animal kingdom.
Cells that synthesize melanins are called melanocyte
Melanocyte
-External links: - "Eye: fovea, RPE" - "Integument: pigmented skin"...
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-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, 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 tyrosinaseTyrosinase
Tyrosinase also known as monophenol monooxygenase is an enzyme that catalyses the oxidation of phenols and is widespread in plants and animals...
) that synthesise the large polymers generically known as melanin
Melanin
Melanin is a pigment that is ubiquitous in nature, being found in most organisms . In animals melanin pigments are derivatives of the amino acid tyrosine. The most common form of biological melanin is eumelanin, a brown-black polymer of dihydroxyindole carboxylic acids, and their reduced forms...
.
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 congenital disorder characterized by the complete or partial absence of pigment in the skin, hair and eyes due to absence or defect of an enzyme involved in the production of melanin...
.
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
Dynein
Dynein is a motor protein in cells which converts the chemical energy contained in ATP into the mechanical energy of movement. Dynein transports various cellular cargo by "walking" along cytoskeletal microtubules towards the minus-end of the microtubule, which is usually oriented towards the cell...
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 fishFish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...
, 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
Scale (zoology)
In most biological nomenclature, a scale is a small rigid plate that grows out of an animal's skin to provide protection. In lepidopteran species, scales are plates on the surface of the insect wing, and provide coloration...
. Molecular motors
Molecular motors
Molecular motors are biological molecular machines that are the essential agents of movement in living organisms. Generally speaking, a motor may be defined as a device that consumes energy in one form and converts it into motion or mechanical work; for example, many protein-based 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
Dynein
Dynein is a motor protein in cells which converts the chemical energy contained in ATP into the mechanical energy of movement. Dynein transports various cellular cargo by "walking" along cytoskeletal microtubules towards the minus-end of the microtubule, which is usually oriented towards the cell...
, which move the melanosomes along microtubule
Microtubule
Microtubules are a component of the cytoskeleton. These rope-like polymers of tubulin can grow as long as 25 micrometers and are highly dynamic. The outer diameter of microtubule is about 25 nm. Microtubules are important for maintaining cell structure, providing platforms for intracellular...
tracts towards the minus end (i.e. the center of the cell). The motors responsible for dispersing the melanosomes to the periphery are kinesin
Kinesin
A kinesin is a protein belonging to a class of motor proteins found in eukaryotic cells. Kinesins move along microtubule filaments, and are powered by the hydrolysis of ATP . The active movement of kinesins supports several cellular functions including mitosis, meiosis and transport of cellular...
, which are plus end directed motors. Since the plus end of microtubules are oriented towards the periphery, kinesin
Kinesin
A kinesin is a protein belonging to a class of motor proteins found in eukaryotic cells. Kinesins move along microtubule filaments, and are powered by the hydrolysis of ATP . The active movement of kinesins supports several cellular functions including mitosis, meiosis and transport of cellular...
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
A cephalopod is any member of the molluscan class Cephalopoda . These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles modified from the primitive molluscan foot...
s (octopuses and squid
Squid
Squid are cephalopods of the order Teuthida, which comprises around 300 species. Like all other cephalopods, squid have a distinct head, bilateral symmetry, a mantle, and arms. Squid, like cuttlefish, have eight arms arranged in pairs and two, usually longer, tentacles...
) are based, however, on a different system, the chromatophore
Chromatophore
Chromatophores are pigment-containing and light-reflecting cells found in amphibians, fish, reptiles, crustaceans, and cephalopods. They are largely responsible for generating skin and eye colour in cold-blooded animals and are generated in the neural crest during embryonic development...
organ.
In fossils
Recent (2008) discoveries by Xu Xing, a Chinese paleontologistPaleontology
Paleontology "old, ancient", ὄν, ὀντ- "being, creature", and λόγος "speech, thought") is the study of prehistoric life. It includes the study of fossils to determine organisms' evolution and interactions with each other and their environments...
, have found fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
ized feathers in rock formations dating from the Jurassic period (200 to 150 million years ago) to the late Tertiary period (65 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
Iridescence
Iridescence is generally known as the property of certain surfaces which appear to change color as the angle of view or the angle of illumination changes...
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
Derek Briggs
Derek Ernest Gilmor Briggs is an Irish paleontologist and taphonomist based at Yale University. Briggs is one of three paleontologists who were key in the reinterpretation of the fossils of the Burgess Shale.-Professional achievements:...
of the Yale University
Yale University
Yale University is a private, Ivy League university located in New Haven, Connecticut, United States. Founded in 1701 in the Colony of Connecticut, the university is the third-oldest institution of higher education in the United States...
study team.
Melanosomes were used to discover the true colors of fossil Anchiornis huxleyi by the Beijing Museum of Natural History
Beijing Museum of Natural History
The Beijing Museum of Natural History is located at 126, Tian Qiao Nan Street, Chong Wen District, Beijing, 100050, and is the most popular natural history museum in China. It was originally founded in 1951 as the National Central Museum of Natural History, and its name changed to the Beijing...
.
Templating
Melanosomes are believed to template melanin polymerization by way of amyloidAmyloid
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 neurodegenerative diseases.-Definition:...
ogenesis of the protein Pmel17
SILV
Melanocyte protein Pmel 17 is a protein that in humans is encoded by the SILV 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.