   #copyright

Chromatophore

2007 Schools Wikipedia Selection. Related subjects: General Biology

   Zebrafish chromatophores mediate background adaptation on exposure to
   dark (top) and light environments (bottom).
   Enlarge
   Zebrafish chromatophores mediate background adaptation on exposure to
   dark (top) and light environments (bottom).

   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. Mature chromatophores are grouped into subclasses based on
   their colour (more properly " hue") under white light: xanthophores
   (yellow), erythrophores (red), iridophores ( reflective / iridescent),
   leucophores (white), melanophores (black/brown) and cyanophores (blue).
   The term can also refer to coloured, membrane associated vesicles found
   in some forms of photosynthetic bacteria.

   Some species can rapidly change colour through mechanisms that
   translocate pigment and reorient reflective plates within
   chromatophores. This process, often used as a type of camouflage, is
   called physiological colour change. Cephalopods such as octopus have
   complex chromatophore organs controlled by muscles to achieve this,
   while vertebrates such as chameleons generate a similar effect by cell
   signaling. Such signals can be hormones or neurotransmitters and may be
   initiated by changes in mood, temperature, stress or visible changes in
   local environment.

   Unlike cold-blooded animals, mammals and birds have only one class of
   chromatophore-like cell type: the melanocyte. The cold-blooded
   equivalent, melanophores, are studied by scientists to understand human
   disease and used as a tool in drug discovery.

Classification

   Invertebrate pigment-bearing cells were first described as chromoforo
   in an Italian science journal in 1819. The term chromatophore was
   adopted later as the name for pigment bearing cells derived from the
   neural crest of cold-blooded vertebrates and cephalopods. The word
   itself comes from the Greek words khrōma (χρωμα) meaning "colour," and
   phoros (φορος) meaning "bearing". In contrast, the word chromatocyte
   (cyte or κυτε being Greek for "cell") was adopted for the cells
   responsible for colour found in birds and mammals. Only one such cell
   type, the melanocyte, has been identified in these animals.

   It wasn't until the 1960s that the structure and colouration of
   chromatophores were understood well enough to allow the development of
   a system of sub-classification based on their appearance. This
   classification system persists to this day even though more recent
   studies have revealed that certain biochemical aspects of the pigments
   may be more useful to a scientific understanding of how the cells
   function.

   The colour-related biochemicals fall into distinct classes: biochromes
   and schemochromes. The biochromes include true pigments, such as
   carotenoids and pteridines. These pigments selectively absorb parts of
   the visible light spectrum that makes up white light while permitting
   other wavelengths to reach the eye of the observer. Schemochromes have
   a significant effect on the perceived colours of cells although they
   are not actually pigments themselves. Instead, the schemochromes,
   though colourless, produce iridescent colours, notably silver and gold,
   by diffusion, interference, and scattering of light.

   While all chromatophores contain pigments or reflecting structures
   (except when there has been a genetic mutation resulting in a disorder
   like albinism), not all pigment containing cells are chromatophores.
   Haem, for example, is a biochrome responsible for the red appearance of
   blood. It is primarily found in red blood cells (erythrocytes), which
   are generated in bone marrow throughout the life of an organism, rather
   than being formed during embryological development. Therefore
   erythrocytes are not classified as chromatophores.
   A veiled chameleon, Chamaeleo calyptratus. Structural green and blue
   colours are generated by overlaying chromatophore types to reflect
   filtered light.
   Enlarge
   A veiled chameleon, Chamaeleo calyptratus. Structural green and blue
   colours are generated by overlaying chromatophore types to reflect
   filtered light.

Xanthophores and erythrophores

   Chromatophores that contain large amounts of yellow pteridine pigments
   are named xanthophores and those with an excess of red/ orange
   carotenoids termed erythrophores. It was discovered that pteridine and
   carotenoid containing vesicles are sometimes found within the same
   cell, and that the overall colour depends on the ratio of red and
   yellow pigments. Therefore the distinction between these chromatophore
   types is essentially arbitrary. The capacity to generate pteridines
   from guanosine triphosphate is a feature common to most chromatophores,
   but xanthophores appear to have supplemental biochemical pathways that
   result in an excess accumulation of yellow pigment. In contrast,
   carotenoids are metabolised from the diet and transported to
   erythrophores. This was first demonstrated by rearing normally green
   frogs on a diet of carotene-restricted crickets. The absence of
   carotene in the frog's diet meant the red/orange carotenoid colour
   'filter' was not present in erythrophores. This resulted in the frog
   appearing blue in colour, instead of green.

Iridophores and leucophores

   Iridophores, sometimes also called guanophores, are pigment cells that
   reflect light using plates of crystalline schemochromes made from
   guanine. When illuminated they generate iridescent colours because of
   the diffraction of light within the stacked plates. Orientation of the
   schemochrome determines the nature of the colour observed. By using
   biochromes as coloured filters, iridophores create an optical effect
   known as Tyndall or Rayleigh scattering, producing bright blue or green
   colours. A related type of chromatophore, the leucophore, is found in
   some fish species. Like iridophores, they utilize crystalline purines
   to reflect light, providing the bright white colour seen in some fish.
   As with xanthophores and erythrophores, the distinction between
   iridophores and leucophores in fish is not always obvious, but
   generally iridophores are considered to generate iridescent or metallic
   colours while leucophores produce reflective white hues.

Melanophores

   Melanophores contain eumelanin, a type of melanin, that appears black
   or dark brown because of its light absorbing qualities. It is packaged
   in vesicles called melanosomes and distributed throughout the cell.
   Eumelanin is generated from tyrosine in a series of catalysed chemical
   reactions. It is a complex chemical containing units of dihydroxyindole
   and dihydroxyindole-2- carboxylic acid with some pyrrole rings. The key
   enzyme in melanin synthesis is tyrosinase. When this protein is
   defective, no melanin can be generated resulting in certain types of
   albinism. In some amphibian species there are other pigments packaged
   alongside eumelanin. For example, a novel deep red coloured pigment was
   identified in the melanophores of phyllomedusine frogs. This was
   subsequently identified as pterorhodin, a pteridine dimer that
   accumulates around eumelanin. While it is likely that other lesser
   studied species have complex melanophore pigments, it is nevertheless
   true that the majority of melanophores studied to date do contain
   eumelanin exclusively.

   Humans have only one class of pigment cell, the mammalian equivalent of
   melanophores, to generate skin, hair and eye colour. For this reason,
   and because the large number and contrasting colour of the cells
   usually make them very easy to visualise, melanophores are by far the
   most widely studied chromatophore. However, there are differences
   between the biology of melanophores and melanocytes. In addition to
   eumelanin, melanocytes can generate a yellow/red pigment called
   phaeomelanin.

Cyanophores

   In 1995 it was demonstrated that the vibrant blue colours in some types
   of mandarin fish are not generated by schemochromes. Instead, a cyan
   biochrome of unknown chemical nature is responsible. This pigment,
   found within vesicles in at least two species callionymid fish, is
   highly unusual in the animal kingdom, as all other blue colourings thus
   far investigated are schemochromatic. Therefore a novel chromatophore
   type, the cyanophore, was proposed. Although they appear unusual in
   their taxonomic restriction, there may be cyanophores (as well as
   further unusual chromatophore types) in other fish and amphibians. For
   example, bright coloured chromatophores with undefined pigments have
   been observed in both poison dart frogs and glass frogs.

Pigment translocation

   Many species have the ability to translocate the pigment inside
   chromatophores, resulting in an apparent change in colour. This
   process, known as physiological colour change, is most widely studied
   in melanophores, since melanin is the darkest and most visible pigment.
   In most species with a relatively thin dermis, the dermal melanophores
   tend to be flat and cover a large surface area. However, in animals
   with thick dermal layers, such as adult reptiles, dermal melanophores
   often form three-dimensional units with other chromatophores. These
   dermal chromatophore units (DCU) consist of an uppermost xanthophore or
   erythrophore layer, then an iridophore layer, and finally a basket-like
   melanophore layer with processes covering the iridophores.

   Both types of dermal melanophores are important in physiological colour
   change. Flat dermal melanophores will often overlay other
   chromatophores so when the pigment is dispersed throughout the cell the
   skin appears dark. When the pigment is aggregated towards the centre of
   the cell, the pigments in other chromatophores are exposed to light and
   the skin takes on their hue. Similarly, after melanin aggregation in
   DCUs, the skin appears green through xanthophore (yellow) filtering of
   scattered light from the iridophore layer. On the dispersion of
   melanin, the light is no longer scattered and the skin appears dark. As
   the other biochromatic chomatophores are also capable of pigment
   translocation, animals with multiple chromatophore types can generate a
   spectacular array of skin colours by making good use of the divisional
   effect.^,
   A single zebrafish melanophore imaged by time-lapse photography during
   pigment aggregation
   Enlarge
   A single zebrafish melanophore imaged by time-lapse photography during
   pigment aggregation

   The control and mechanics of rapid pigment translocation has been well
   studied in a number of different species, particularly amphibians and
   teleost fish. ^, It has been demonstrated that the process can be under
   hormonal, neuronal control or both. Neurochemicals that are known to
   translocate pigment include noradrenaline, through its receptor on the
   surface on melanophores. The primary hormones involved in regulating
   translocation appear to be the melanocortins, melatonin and melanin
   concentrating hormone (MCH), that are produced mainly in the pituitary,
   pineal gland and hypothalamus respectively. These hormones may also be
   generated in a paracrine fashion by cells in the skin. At the surface
   of the melanophore the hormones have been shown to activate specific
   G-protein coupled receptors that, in turn, transduce the signal into
   the cell. Melanocortins result in the dispersion of pigment, while
   melatonin and MCH results in aggregation.

   Numerous melanocortin, MCH and melatonin receptors have been identified
   in fish and frogs, including a homologue of MC1R, a melanocortin
   receptor known to regulate skin and hair colour in humans. Inside the
   cell, cyclic adenosine monophosphate (cAMP) has been shown to be an
   important second messenger of pigment translocation. Through a
   mechanism not yet fully understood, cAMP influences other proteins to
   drive molecular motors carrying pigment containing vesicles along both
   microtubules and microfilaments.^,^,

Background adaptation

   Most fish, reptiles and amphibians undergo a limited physiological
   colour change in response to a change in environment. This type of
   camouflage, known as background adaptation, most commonly appears as a
   slight darkening or lightening of skin tone to approximately mimic the
   hue of the immediate environment. It has been demonstrated that the
   background adaptation process is vision dependent (it appears the
   animal needs to be able to see the environment to adapt to it), and
   that melanin translocation in melanophores is the major factor in
   colour change. Some animals, such as chameleons and anoles, have a
   highly developed background adaptation response capable of generating a
   number of different colours very rapidly. They have adapted the
   capability to change colour in response to temperature, mood, stress
   levels and social cues, rather than to simply mimic their environment.
   Transverse section of a developing vertebrate trunk showing the
   dorsolateral (red) and ventromedial (blue) routes of chromatoblast
   migration.
   Enlarge
   Transverse section of a developing vertebrate trunk showing the
   dorsolateral (red) and ventromedial (blue) routes of chromatoblast
   migration.

Development

   During vertebrate embryonic development, chromatophores are one of a
   number of cell types generated in the neural crest, a paired strip of
   cells arising at the margins of the neural tube. These cells have the
   ability to migrate long distances, allowing chromatophores to populate
   many organs of the body, including the skin, eye, ear and brain.
   Leaving the neural crest in waves, chromatophores take either a
   dorsolateral route through the dermis, entering the ectoderm through
   small holes in the basal lamina, or a ventromedial route between the
   somites and the neural tube. The exception to this is the melanophores
   of the retinal pigmented epithelium of the eye. These are not derived
   from the neural crest, instead an outpouching of the neural tube
   generates the optic cup which, in turn, forms the retina.

   When and how multipotent chromatophore precursor cells (called
   chromatoblasts) develop into their daughter subtypes is an area of
   ongoing research. It is known in zebrafish embryos, for example, that
   by 3 days after fertilization each of the cell classes found in the
   adult fish — melanophores, xanthophores and iridophores — are already
   present. Studies using mutant fish have demonstrated that transcription
   factors such as kit, sox10 and mitf are important in controlling
   chromatophore differentiation. If these proteins are defective,
   chromatophores may be regionally or entirely absent, resulting in a
   leucistic disorder.

Practical applications

   In addition to basic research into better understanding of
   chromatophores themselves, the cells are used for applied research
   purposes. For example, zebrafish larvae are used to study how
   chromatophores organise and communicate to accurately generate the
   regular horizontal striped pattern in seen in adult fish. This is seen
   as a useful model system for understanding patterning in the
   evolutionary developmental biology field. Chromatophore biology has
   also been used to model human condition or disease, including melanoma
   and albinism. Recently the gene responsible for the
   melanophore-specific golden zebrafish strain, Slc24a5, was shown to
   have a human equivalent that strongly correlates with skin colour.

   Chromatophores are also used as a biomarker of blindness in
   cold-blooded species, as animals with certain visual defects fail to
   background adapt to light environments. Human homologues of receptors
   that mediate pigment translocation in melanophores are thought to
   involved in processes such as appetite suppression and tanning, making
   them attractive targets for drugs. Therefore pharmaceutical companies
   have developed a biological assay for rapidly identifying potential
   bioactive compounds using melanophores from the African clawed frog.
   Other scientists have developed techniques for using melanophores as
   biosensors, and for rapid disease detection (based on the discovery
   that pertussis toxin blocks pigment aggregation in fish melanophores).
   Potential military applications of chromatophore mediated colour
   changes have been proposed, mainly as a type of active camouflage.

Cephalopod chromatophores

   An infant cuttlefish, using background adaptation to mimic the local
   environment
   Enlarge
   An infant cuttlefish, using background adaptation to mimic the local
   environment

   Coleoid cephalopods have complex multicellular 'organs' which they use
   to change colour rapidly. This is most notable in brightly coloured
   squid, cuttlefish and octopuses. Each chromatophore unit is composed of
   a single chromatophore cell and numerous muscle, nerve, glial and
   sheath cells. Inside the chromatophore cell, pigment granules are
   enclosed in an elastic sac, called the cytoelastic sacculus. To change
   colour the animal distorts the sacculus form or size by muscular
   contraction, changing its translucency, reflectivity or opacity. This
   differs from the mechanism used in fish, amphibians and reptiles, in
   that the shape of the sacculus is being changed rather than a
   translocation of pigment vesicles within the cell. However a similar
   effect is achieved.

   Octopuses operate chromatophores in complex, wavelike chromatic
   displays, resulting in a variety of rapidly changing colour schemes.
   The nerves that operate the chromatophores are thought to be positioned
   in the brain, in a similar order to the chromatophores they each
   control. This means the pattern of colour change matches the pattern of
   neuronal activation. This may explain why, as the neurons are activated
   one after another, the colour change occurs in waves. Like chameleons,
   cephalopods use physiological colour change for social interaction.
   They are also among the most skilled at background adaptation, having
   the ability to match both the colour and the texture of their local
   environment with remarkable accuracy.

Bacteria

   Chromatophores are also found in membranes of phototrophic bacteria.
   Used primarly for photosynthesis, they contain bacteriochlorophyll
   pigments and carotenoids. In purple bacteria, such as Rhodospirillum
   rubrum the light-harvesting proteins are intrinsic to the chromatophore
   membranes. However, in green sulphur bacteria they are arranged in
   specialised antenna complexes called chlorosomes.
   Retrieved from " http://en.wikipedia.org/wiki/Chromatophore"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
   Documentation License. See also our Disclaimer.
