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This article is about a type of cell or multicellular organ. Chromatophore_sentence_0

For other uses, see Chromatophore (disambiguation). Chromatophore_sentence_1

Chromatophores are pigment-containing, or groups of cells, found in a wide range of animals including amphibians, fish, reptiles, crustaceans and cephalopods. Chromatophore_sentence_2

Mammals and birds, in contrast, have a class of cells called melanocytes for coloration. Chromatophore_sentence_3

Chromatophores are largely responsible for generating skin and eye colour in ectothermic animals and are generated in the neural crest during embryonic development. Chromatophore_sentence_4

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). Chromatophore_sentence_5

Some species can rapidly change colour through mechanisms that translocate pigment and reorient reflective plates within chromatophores. Chromatophore_sentence_6

This process, often used as a type of camouflage, is called physiological colour change or metachrosis. Chromatophore_sentence_7

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. Chromatophore_sentence_8

Such signals can be hormones or neurotransmitters and may be initiated by changes in mood, temperature, stress or visible changes in the local environment. Chromatophore_sentence_9

Chromatophores are studied by scientists to understand human disease and as a tool in drug discovery. Chromatophore_sentence_10

Human discovery Chromatophore_section_0

Aristotle mentioned the ability of the octopus to change colour for both camouflage and signalling in his Historia animalium (ca 400 BC): Chromatophore_sentence_11

Giosuè Sangiovanni was the first to describe invertebrate pigment-bearing cells as cromoforo in an Italian science journal in 1819. Chromatophore_sentence_12

Charles Darwin described the colour-changing abilities of the cuttlefish in The Voyage of the Beagle (1860): Chromatophore_sentence_13

Classification Chromatophore_section_1

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. Chromatophore_sentence_14

The word itself comes from the Greek words chrōma (χρῶμα) meaning "colour," and phoros (φόρος) meaning "bearing". Chromatophore_sentence_15

In contrast, the word chromatocyte (kytos (κύτος) meaning "cell") was adopted for the cells responsible for colour found in birds and mammals. Chromatophore_sentence_16

Only one such cell type, the melanocyte, has been identified in these animals. Chromatophore_sentence_17

It was only in the 1960s that chromatophores were well enough understood to enable them to be classified based on their appearance. Chromatophore_sentence_18

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. Chromatophore_sentence_19

Colour-producing molecules fall into two distinct classes: biochromes and structural colours or "schemochromes". Chromatophore_sentence_20

The biochromes include true pigments, such as carotenoids and pteridines. Chromatophore_sentence_21

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. Chromatophore_sentence_22

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. Chromatophore_sentence_23

Many such structures interfere with some wavelengths (colours) of light and transmit others, simply because of their scale, so they often produce iridescence, creating different colours when seen from different directions. Chromatophore_sentence_24

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. Chromatophore_sentence_25

Haem, for example, is a biochrome responsible for the red appearance of blood. Chromatophore_sentence_26

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. Chromatophore_sentence_27

Therefore, erythrocytes are not classified as chromatophores. Chromatophore_sentence_28

Xanthophores and erythrophores Chromatophore_section_2

Chromatophores that contain large amounts of yellow pteridine pigments are named xanthophores; those with mainly red/orange carotenoids are termed erythrophores. Chromatophore_sentence_29

However, vesicles containing pteridine and carotenoids are sometimes found in the same cell, in which case the overall colour depends on the ratio of red and yellow pigments. Chromatophore_sentence_30

Therefore, the distinction between these chromatophore types is not always clear. Chromatophore_sentence_31

Most chromatophores can generate pteridines from guanosine triphosphate, but xanthophores appear to have supplemental biochemical pathways enabling them to accumulate yellow pigment. Chromatophore_sentence_32

In contrast, carotenoids are metabolised and transported to erythrophores. Chromatophore_sentence_33

This was first demonstrated by rearing normally green frogs on a diet of carotene-restricted crickets. Chromatophore_sentence_34

The absence of carotene in the frogs' diet meant that the red/orange carotenoid colour 'filter' was not present in their erythrophores. Chromatophore_sentence_35

This made the frogs appear blue instead of green. Chromatophore_sentence_36

Iridophores and leucophores Chromatophore_section_3

Iridophores, sometimes also called guanophores, are pigment cells that reflect light using plates of crystalline chemochromes made from guanine. Chromatophore_sentence_37

When illuminated they generate iridescent colours because of the diffraction of light within the stacked plates. Chromatophore_sentence_38

Orientation of the schemochrome determines the nature of the colour observed. Chromatophore_sentence_39

By using biochromes as coloured filters, iridophores create an optical effect known as Tyndall or Rayleigh scattering, producing bright-blue or -green colours. Chromatophore_sentence_40

A related type of chromatophore, the leucophore, is found in some fish, in particular in the tapetum lucidum. Chromatophore_sentence_41

Like iridophores, they utilize crystalline purines (often guanine) to reflect light. Chromatophore_sentence_42

Unlike iridophores, however, leucophores have more organized crystals that reduce diffraction. Chromatophore_sentence_43

Given a source of white light, they produce a white shine. Chromatophore_sentence_44

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 metallic colours, whereas leucophores produce reflective white hues. Chromatophore_sentence_45

Melanophores Chromatophore_section_4

See also: Melanocyte Chromatophore_sentence_46

Melanophores contain eumelanin, a type of melanin, that appears black or dark-brown because of its light absorbing qualities. Chromatophore_sentence_47

It is packaged in vesicles called melanosomes and distributed throughout the cell. Chromatophore_sentence_48

Eumelanin is generated from tyrosine in a series of catalysed chemical reactions. Chromatophore_sentence_49

It is a complex chemical containing units of dihydroxyindole and dihydroxyindole-2-carboxylic acid with some pyrrole rings. Chromatophore_sentence_50

The key enzyme in melanin synthesis is tyrosinase. Chromatophore_sentence_51

When this protein is defective, no melanin can be generated resulting in certain types of albinism. Chromatophore_sentence_52

In some amphibian species there are other pigments packaged alongside eumelanin. Chromatophore_sentence_53

For example, a novel deep (wine) red-colour pigment was identified in the melanophores of phyllomedusine frogs. Chromatophore_sentence_54

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. Chromatophore_sentence_55

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. Chromatophore_sentence_56

Humans have only one class of pigment cell, the mammalian equivalent of melanophores, to generate skin, hair, and eye colour. Chromatophore_sentence_57

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. Chromatophore_sentence_58

However, there are differences between the biology of melanophores and that of melanocytes. Chromatophore_sentence_59

In addition to eumelanin, melanocytes can generate a yellow/red pigment called phaeomelanin. Chromatophore_sentence_60

Cyanophores Chromatophore_section_5

Nearly all the vibrant blues in animals and plants are created by structural coloration rather than by pigments. Chromatophore_sentence_61

However, some types of Synchiropus splendidus do possess vesicles of a cyan biochrome of unknown chemical structure in cells named cyanophores. Chromatophore_sentence_62

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. Chromatophore_sentence_63

For example, brightly coloured chromatophores with undefined pigments are found in both poison dart frogs and glass frogs, and atypical chromatophores, named erythro-iridophores have been described in Pseudochromis diadema. Chromatophore_sentence_64

Pigment translocation Chromatophore_section_6

Many species are able to translocate the pigment inside their chromatophores, resulting in an apparent change in body colour. Chromatophore_sentence_65

This process, known as physiological colour change, is most widely studied in melanophores, since melanin is the darkest and most visible pigment. Chromatophore_sentence_66

In most species with a relatively thin dermis, the dermal melanophores tend to be flat and cover a large surface area. Chromatophore_sentence_67

However, in animals with thick dermal layers, such as adult reptiles, dermal melanophores often form three-dimensional units with other chromatophores. Chromatophore_sentence_68

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. Chromatophore_sentence_69

Both types of melanophore are important in physiological colour change. Chromatophore_sentence_70

Flat dermal melanophores often overlay other chromatophores, so when the pigment is dispersed throughout the cell the skin appears dark. Chromatophore_sentence_71

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. Chromatophore_sentence_72

Likewise, after melanin aggregation in DCUs, the skin appears green through xanthophore (yellow) filtering of scattered light from the iridophore layer. Chromatophore_sentence_73

On the dispersion of melanin, the light is no longer scattered and the skin appears dark. Chromatophore_sentence_74

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. Chromatophore_sentence_75

The control and mechanics of rapid pigment translocation has been well studied in a number of different species, in particular amphibians and teleost fish. Chromatophore_sentence_76

It has been demonstrated that the process can be under hormonal or neuronal 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 receptor on the surface on melanophores. Chromatophore_sentence_77

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. Chromatophore_sentence_78

These hormones may also be generated in a paracrine fashion by cells in the skin. Chromatophore_sentence_79

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. Chromatophore_sentence_80

Melanocortins result in the dispersion of pigment, while melatonin and MCH results in aggregation. Chromatophore_sentence_81

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. Chromatophore_sentence_82

It has been demonstrated that MC1R is required in zebrafish for dispersion of melanin. Chromatophore_sentence_83

Inside the cell, cyclic adenosine monophosphate (cAMP) has been shown to be an important second messenger of pigment translocation. Chromatophore_sentence_84

Through a mechanism not yet fully understood, cAMP influences other proteins such as protein kinase A to drive molecular motors carrying pigment containing vesicles along both microtubules and microfilaments. Chromatophore_sentence_85

Background adaptation Chromatophore_section_7

See also: Camouflage Chromatophore_sentence_86

Most fish, reptiles and amphibians undergo a limited physiological colour change in response to a change in environment. Chromatophore_sentence_87

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. Chromatophore_sentence_88

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. Chromatophore_sentence_89

Some animals, such as chameleons and anoles, have a highly developed background adaptation response capable of generating a number of different colours very rapidly. Chromatophore_sentence_90

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. Chromatophore_sentence_91

Development Chromatophore_section_8

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. Chromatophore_sentence_92

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. Chromatophore_sentence_93

Fish melanophores and iridophores have been found to contain the smooth muscle regulatory proteins [calponin] and caldesmon. Chromatophore_sentence_94

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. Chromatophore_sentence_95

The exception to this is the melanophores of the retinal pigmented epithelium of the eye. Chromatophore_sentence_96

These are not derived from the neural crest. Chromatophore_sentence_97

Instead, an outpouching of the neural tube generates the optic cup, which, in turn, forms the retina. Chromatophore_sentence_98

When and how multipotent chromatophore precursor cells (called chromatoblasts) develop into their daughter subtypes is an area of ongoing research. Chromatophore_sentence_99

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. Chromatophore_sentence_100

Studies using mutant fish have demonstrated that transcription factors such as kit, sox10, and mitf are important in controlling chromatophore differentiation. Chromatophore_sentence_101

If these proteins are defective, chromatophores may be regionally or entirely absent, resulting in a leucistic disorder. Chromatophore_sentence_102

Practical applications Chromatophore_section_9

Chromatophores are sometimes used in applied research. Chromatophore_sentence_103

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. Chromatophore_sentence_104

This is seen as a useful model system for understanding patterning in the evolutionary developmental biology field. Chromatophore_sentence_105

Chromatophore biology has also been used to model human condition or disease, including melanoma and albinism. Chromatophore_sentence_106

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. Chromatophore_sentence_107

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. Chromatophore_sentence_108

Human homologues of receptors that mediate pigment translocation in melanophores are thought to be involved in processes such as appetite suppression and tanning, making them attractive targets for drugs. Chromatophore_sentence_109

Therefore, pharmaceutical companies have developed a biological assay for rapidly identifying potential bioactive compounds using melanophores from the African clawed frog. Chromatophore_sentence_110

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). Chromatophore_sentence_111

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. Chromatophore_sentence_112

Cephalopod chromatophores Chromatophore_section_10

Coleoid cephalopods (including octopuses, squids and cuttlefish) have complex multicellular organs that they use to change colour rapidly, producing a wide variety of bright colours and patterns. Chromatophore_sentence_113

Each chromatophore unit is composed of a single chromatophore cell and numerous muscle, nerve, glial, and sheath cells. Chromatophore_sentence_114

Inside the chromatophore cell, pigment granules are enclosed in an elastic sac, called the cytoelastic sacculus. Chromatophore_sentence_115

To change colour the animal distorts the sacculus form or size by muscular contraction, changing its translucency, reflectivity, or opacity. Chromatophore_sentence_116

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. Chromatophore_sentence_117

However, a similar effect is achieved. Chromatophore_sentence_118

Octopuses and most cuttlefish can operate chromatophores in complex, undulating chromatic displays, resulting in a variety of rapidly changing colour schemata. Chromatophore_sentence_119

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. Chromatophore_sentence_120

This means the pattern of colour change functionally matches the pattern of neuronal activation. Chromatophore_sentence_121

This may explain why, as the neurons are activated in iterative signal cascade, one may observe waves of colour changing. Chromatophore_sentence_122

Like chameleons, cephalopods use physiological colour change for social interaction. Chromatophore_sentence_123

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. Chromatophore_sentence_124

See also Chromatophore_section_11


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