Chromatophore
Chromatophores are cells that produce color, of which many types are pigment-containing cells, or groups of cells, found in a wide range of animals including amphibians, fish, reptiles, crustaceans and cephalopods. Mammals and birds, in contrast, have a class of cells called melanocytes for coloration.
Chromatophores are largely responsible for generating skin and
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 or metachrosis.[1] Cephalopods, such as the octopus, have complex chromatophore organs controlled by muscles to achieve this, whereas vertebrates such as chameleons generate a similar effect by cell signalling. Such signals can be hormones or neurotransmitters and may be initiated by changes in mood, temperature, stress or visible changes in the local environment.[citation needed] Chromatophores are studied by scientists to understand human disease and as a tool in drug discovery.
Human discovery
The octopus ... seeks its prey by so changing its colour as to render it like the colour of the stones adjacent to it; it does so also when alarmed.
Giosuè Sangiovanni was the first to describe invertebrate pigment-bearing cells as cromoforo in an Italian science journal in 1819.[3]
Charles Darwin described the colour-changing abilities of the cuttlefish in The Voyage of the Beagle (1860):[4]
These animals also escape detection by a very extraordinary, chameleon-like power of changing their colour. They appear to vary their tints according to the nature of the ground over which they pass: when in deep water, their general shade was brownish purple, but when placed on the land, or in shallow water, this dark tint changed into one of a yellowish green. The colour, examined more carefully, was a French grey, with numerous minute spots of bright yellow: the former of these varied in intensity; the latter entirely disappeared and appeared again by turns. These changes were effected in such a manner that clouds, varying in tint between a hyacinth red and a chestnut-brown, were continually passing over the body. Any part, being subjected to a slight shock of galvanism, became almost black: a similar effect, but in a less degree, was produced by scratching the skin with a needle. These clouds, or blushes as they may be called, are said to be produced by the alternate expansion and contraction of minute vesicles containing variously coloured fluids.
Classification of chromatophore
The term chromatophore was adopted (following Sangiovanni's chromoforo) 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 chrōma (χρῶμα) meaning "colour," and phoros (φόρος) meaning "bearing". In contrast, the word chromatocyte (kytos (κύτος) meaning "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 was only in the 1960s that chromatophores were well enough understood to enable them to be classified based on their appearance. This classification system persists to this day, even though the biochemistry of the pigments may be more useful to a scientific understanding of how the cells function.[5]
Colour-producing molecules fall into two distinct classes:
Whereas all chromatophores contain pigments or reflecting structures (except when there has been a
Xanthophores and erythrophores
Chromatophores that contain large amounts of
Most chromatophores can generate pteridines from guanosine triphosphate, but xanthophores appear to have supplemental biochemical pathways enabling them to accumulate yellow pigment. In contrast, carotenoids are metabolised 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 frogs' diet meant that the red/orange carotenoid colour 'filter' was not present in their erythrophores. This made the frogs appear blue instead of green.[8]
Iridophores and leucophores
Iridophores, sometimes also called guanophores, are chromatophores that reflect light using plates of crystalline chemochromes made from guanine.[9] When illuminated they generate iridescent colours because of the constructive interference of light. Fish iridophores are typically stacked guanine plates separated by layers of cytoplasm to form microscopic, one-dimensional, Bragg mirrors. Both the orientation and the optical thickness of the chemochrome determines the nature of the colour observed.[10] By using biochromes as coloured filters, iridophores create an optical effect known as Tyndall or Rayleigh scattering, producing bright-blue or -green colours.[11]
A related type of chromatophore, the leucophore, is found in some fish, in particular in the
Melanophores
Melanophores contain
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 that of
Cyanophores
Nearly all the vibrant blues in animals and plants are created by
Pigment translocation
Many species are able to translocate the pigment inside their chromatophores, resulting in an apparent change in body colour. This process, known as
Both types of melanophore are important in physiological colour change. Flat dermal melanophores often overlay other chromatophores, so when the pigment is dispersed throughout the cell the skin appears dark. When the pigment is aggregated toward the centre of the cell, the pigments in other chromatophores are exposed to light and the skin takes on their hue. Likewise, 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 chromatophores 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.[18][19]
The control and mechanics of rapid pigment translocation has been well studied in a number of different species, in particular amphibians and
Numerous melanocortin, MCH and melatonin receptors have been identified in fish
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
Development
During vertebrate
When and how
Practical applications
Chromatophores are sometimes used in applied research. For example, zebrafish larvae are used to study how chromatophores organise and communicate to accurately generate the regular horizontal striped pattern as seen in adult fish.
Chromatophores are also used as a
Cephalopod chromatophores
Octopuses and most cuttlefish[43] can operate chromatophores in complex, undulating chromatic displays, resulting in a variety of rapidly changing colour schemata. The nerves that operate the chromatophores are thought to be positioned in the brain in a pattern isomorphic to that of the chromatophores they each control. This means the pattern of colour change functionally matches the pattern of neuronal activation. This may explain why, as the neurons are activated in iterative signal cascade, one may observe waves of colour changing.[44] Like chameleons, cephalopods use physiological colour change for social interaction. They are also among the most skilled at camouflage, having the ability to match both the colour distribution and the texture of their local environment with remarkable accuracy.
See also
Notes
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- ^ Aristotle. Historia Animalium. IX, 622a: 2-10. About 400 BC. Cited in Luciana Borrelli, Francesca Gherardi, Graziano Fiorito. A catalogue of body patterning in Cephalopoda. Firenze University Press, 2006. Abstract Archived 2018-02-06 at the Wayback Machine Google books
- ^ Sangiovanni, G (1819). "Descrizione di un particolare sistema di organi cromoforo espansivo-dermoideo e dei fenomeni che esso produce, scoperto nei molluschi cefaloso". G. Enciclopedico Napoli. 9: 1–13.
- ^ Darwin, Charles (1860). "Chapter 1. Habits of a Sea-slug and Cuttle-fish". Journal Of Researches Into The Natural History And Geology Of The Countries Visited During The Voyage Round The World Of H.M.S. 'Beagle' Under The Command Of Captain Fitz Roy, R.N. John Murray, London. p. 7.
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- ^ Meyer-Rochow, VB (2001). "Fish chromatophores as sensors of environmental stimuli". In Kapoor BG & Hara TJ (ed.). Sensory Biology of Jawed Fishes. Science Publishers Enfield (NH), USA. pp. 317–334.
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- ^ Hansford, Dave (August 6, 2008). "Cuttlefish Change Color, Shape-Shift to Elude Predators". National Geographic News. Wellington, New Zealand. Archived from the original on August 10, 2008.
[...] cuttlefish have instead relied on invisibility, a talent that may have applications for human technology. Norman said the military has shown interest in cuttlefish camouflage with a view to one day incorporating similar mechanisms in soldiers' uniforms.
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