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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 , fish, , and . and , in contrast, have a class of cells called for coloration.
Chromatophores are largely responsible for generating skin and eye color in animals and are generated in the neural crest during embryonic development. Mature chromatophores are grouped into subclasses based on their colour under white light: xanthophores (yellow), erythrophores (red), iridophores (reflective / iridescence), leucophores (white), melanophores (black/brown), and cyanophores (blue). While most chromatophores contain pigments that absorb specific wavelengths of light, the color of leucophores and iridophores is produced by their respective scattering and optical interference properties.
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. Cephalopods, such as the octopus, have complex chromatophore organs controlled by muscles to achieve this, whereas vertebrates such as generate a similar effect by cell signaling. Such signals can be or and may be initiated by changes in mood, temperature, stress or visible changes in the local environment. Chromatophores are studied by scientists to understand human disease and as a tool in drug discovery.
Giosuè Sangiovanni was the first to describe invertebrate pigment-bearing cells as cromoforo in an Italian science journal in 1819.
Charles Darwin described the colour-changing abilities of the cuttlefish in The Voyage of the Beagle (1860):
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.
Colour-producing molecules fall into two distinct classes: biochromes and structural colours or "schemochromes".Fox, DL. Animal Biochromes and Structural Colors: Physical, Chemical, Distributional & Physiological Features of Colored Bodies in the Animal World. University of California Press, Berkeley, 1976. The biochromes include true pigments, such as and . These pigments selectively absorb parts of the visible spectrum that makes up white light while permitting other to reach the eye of the observer. Structural colours are produced by various combinations of diffraction, reflection or scattering of light from structures with a scale around a quarter of the wavelength of light. Many such structures interfere with some wavelengths (colours) of light and transmit others, simply because of their scale, so they often produce iridescence by creating different colours when seen from different directions.
Whereas all chromatophores contain pigments or reflecting structures (except when there has been a mutation, as in albinism), not all pigment-containing cells are chromatophores. Haem, for example, is a biochrome responsible for the red appearance of blood. It is found primarily 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.
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 metabolism 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.Bagnara JT. Comparative Anatomy and Physiology of Pigment Cells in Nonmammalian Tissues. In: The Pigmentary System: Physiology and Pathophysiology, Oxford University Press, 1998.
A related type of chromatophore, the leucophore, is found in some fish, in particular in the tapetum lucidum. Like iridophores, they utilize crystalline (often guanine) to reflect light. Unlike iridophores, leucophores have more organized crystals that reduce diffraction. Given a source of white light, they produce a white shine. As with xanthophores and erythrophores, in fish the distinction between iridophores and leucophores is not always obvious, but, in general, iridophores are considered to generate iridescent or , whereas leucophores produce reflective white hues.
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 . In addition to eumelanin, melanocytes can generate a yellow/red pigment called phaeomelanin.
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.
The control and mechanics of rapid pigment translocation has been well studied in a number of different species, in particular amphibians and teleost fish. It has been demonstrated that the process can be under Hormone or neurotransmitter control or both and for many species of bony fishes it is known that chromatophores can respond directly to environmental stimuli like visible light, UV-radiation, temperature, pH, chemicals, etc. Neurochemicals that are known to translocate pigment include noradrenaline, through its adrenoceptor on the surface on melanophores. The primary hormones involved in regulating translocation appear to be the , 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 colour and hair colour in humans. It has been demonstrated that MC1R is required in zebrafish for dispersion of melanin. 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 such as protein kinase A to drive Moving proteins carrying pigment containing vesicles along both and .
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, SOX genes, and Microphthalmia are important in controlling chromatophore differentiation. If these proteins are defective, chromatophores may be regionally or entirely absent, resulting in a leucistic disorder.
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 be involved in processes such as appetite suppression and Sun tanning, making them attractive targets for pharmaceutical. 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 , 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, which could as in cuttlefish make objects nearly invisible.Lee I. Nanotubes for noisy signal processing PhD Thesis. 2005; University of Southern California.
Octopuses and most cuttlefish 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 Action potential. This may explain why, as the neurons are activated in iterative signal cascade, one may observe waves of colour changing. 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.
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 indole 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 (wine) red-colour pigment was identified in the melanophores of Phyllomedusinae. Some species of anole lizards, such as the Anolis grahami, use melanocytes in response to certain signals and hormonal changes, and is capable of becoming colors ranging from bright blue, brown, and black. This was subsequently identified as pterorhodin, a pteridine dimer that accumulates around eumelanin core, and it is also present in a variety of tree frog species from Australia and Papua New Guinea. 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.
Cyanophores
Nearly all the vibrant blues in animals and plants are created by structural coloration rather than by pigments. However, some types of Synchiropus splendidus do possess vesicles of a cyan biochrome of unknown chemical structure in cells named cyanophores. Although they appear unusual in their limited taxonomic range, there may be cyanophores (as well as further unusual chromatophore types) in other fish and amphibians. For example, brightly coloured chromatophores with undefined pigments are found in both poison dart frogs and , and atypical chromatophores, named erythro-iridophores have been described in Pseudochromis diadema.
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 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.
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 , 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.
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. Fish melanophores and iridophores have been found to contain the smooth muscle regulatory proteins calponin and caldesmon. 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.
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. This is seen as a useful Animal 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.
Cephalopod chromatophores
Coleoidea cephalopods (including octopuses, and cuttlefish) have complex multicellular organs that they use to change colour rapidly, producing a wide variety of bright colours and patterns. Each chromatophore unit is composed of a single chromatophore cell and numerous muscle, nerve, glial cell, 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 translucent, reflectivity, or opacity. This differs from the mechanism used in fish, amphibians, and reptiles in that the shape of the sacculus is changed, rather than translocating pigment vesicles within the cell. However, a similar effect is achieved. The energy cost of the complete activation of the chromatophore system is very high, being nearly as much as all the energy used by an octopus at rest.
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