Chromatophore
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Chromatophore is the collective term for pigment containing and light reflecting cells found in amphibians, fish, reptiles, crustaceans and cephalopods.
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Classification
Chromforo was first used to describe invertebrate pigment cells in 1819 and the term chromatophore (Greek: khrōma = "color", phoros = "bearing") later adopted as a name for the pigment bearing cells of cold blooded vertebrates and cephalopods (in contrast to the chromato-cytes found in mammals and birds). By the 1960s, enough information about the structure and colour of chromatophores was available to sub-classify them according to appearance and, despite studies revealing the biochemical nature of the pigments within chromatophore types, this classification system persists. [1]
Xanthophores and erythrophores
Originally termed lipophores due to their fat-soluble content, chromatophores that contain large amounts of yellow pteridine pigments were renamed xanthophores and those with an excess of red/orange carotinoids termed erythrophores. [2] Soon after, it was discovered that pterinosome and carotinoid vesicles are found within the same cell, and that the manifest color depends on the ratio of red and yellow pigments. [3] Thus the distinction between these cell types is essentially arbitrary.
Iridophores and leucophores
Image:Chameleon - Tanzania - Usambara Mountains.jpg Biochromes, such as pteridines and carotinoids, selectively absorb a part of the visual spectrum that makes up white incident light, while they let the other wavelengths pass and reach the eye of the observer. Not all colours are generated in this manner, however. Some, most notably blues and greens, are generated by the scattering, interference and diffusion of light by crystalline structures called schemochromes.
Iridophores are lower vertebrate pigment cells that reflect light using plates of crystalline guanine schemochromes. [4] When illuminated they generate iridescent colors due to the diffraction of light within the stacked plates. Orientation of the schemochrome determines the nature of the structural color. [5] By using biochromes as filters, iridophores mediate an optical effect known as Tyndall or Rayleigh scattering, producing bright blue or green colors that are not modified by the angle of vision. [6] 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 color seen in some fish. As with xanthophores and erythrophores, the distinction between iridophores and leucophores in fish is not always obvious, but generally iridophores generate iridescent or metallic colours while leucophores produce structural white hues. [7]
Melanophores
The most widely studied chromatophore, due both to its extensive taxonomic distribution and apparent colour, is the melanophore. Eumelanin, the biochrome found in melanophores, is generated from tyrosine in a series of catalysed chemical reactions. [8] This type of melanin, when packaged in vesicles and distributed throughout the cell, appears black or dark brown, due to its light absorbing qualities. In some amphibian species, however, there are other pigments packaged alongside eumelanin. For example, a novel deep red colored pigment called was identified in the melanophores of phyllomedusine frogs. [9] This was subsequently identified as pterorhodin, a pterodine dimer that accumulates around eumelanin. While it is likely that other species have also evolved multiple melanophore pigments, it is nevertheless true that the majority of melanophores studied to date contain eumelanin exclusively. Image:Dendrobates pumilio.jpg
Cyanophores
In 1995 it was demonstrated that the spectacular blue colors of mandarin fish are not structural in nature. Instead, a cyan biochrome of unknown chemical nature is responsible. [10] This pigment, found within fibrous 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 cyanophores are unusual in their taxonomic restriction, there may be other unusual chromatophore types in lesser-studied fish and amphibians. Indeed, bright coloured chromatophores with undefined pigments have been observed in both poison dart frogs and glass frogs. [11]
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, as 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 [12].
Both types of dermal melanophores are extremely 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 thus the skin takes on their hue. Similarly, after melanin aggregation in DCUs, the skin appears green through xanthophore (yellow) filtering of scattered (blue) light from the iridphore layer. On the dispersion of melanin to the processes, the light is no longer scattered and the skin appears dark. As the other chomatophores are also capable of pigment translocation, by making good use of the divisional effect animals with multiple chromatophore types can generate a spectacular array of skin colours [13] [14] Image:Zfishchroma.jpg The control and mechanics of rapid pigment translocation has been well studied in a multitude of species, particularly amphibians [15] and teleost fish [16]. It is thought that both microtubules and microfilaments are involved in the rapid translocation of pigments across the chromatophore cytoskeleton [17] [18] [19]. It has also been demonstrated that the process can be under hormonal, neuronal control or both [20] [21].
Background Adaptation
Most fish, reptiles and amphibians animals undergo physiological colour change in response to a change in environment. Known as background adaptation, this most commonly manifests as a slight darkening or lightening of skin tone to approximately mimic the hue of the immediate environment, a type of camouflage. It has been demonstrated that the process can be under hormonal control, neuronal control or both [22] [23], that it is vision dependent (the animal needs to be able to see the environment to adapt to it) [24], and that melanin translocation in melanophores is the primary mechanism for colour change.
Some animals, such as chameleons and anoles, have a highly evolved background adaptation response capable of generating different colours very rapidly. They have adapted this capability to change colour in response to temperature, mood, stress levels and social cues, rather than to simply mimic their environment.
Cephalopod chromatophores
Most notable in squid, cuttlefish and octopuses, cephalopods have complex multicellular 'organs' which they use to change color rapidly. Each unit is composed of a single chromatophore cell and numerous muscle, nerve, glial and sheath cells. [25] To change color the animal distorts the chromatophore form or size by stretching or contraction, thus changing its translucency, reflectivity or opacity. Octopuses can operate each chromatophore individually resulting in a spectacular variety of color schemes. This differs from the mechanism used in fish, amphibians and reptiles, in that the shape of the cells is being changed rather than a translocation of pigment within the cell.