chromatophore 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 in the skin of a squid chromatophores are largely responsible for generating skin and eye colour in ectothermic 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). 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 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. chromatophores are studied by scientists to understand human disease and as a tool in drug discovery. human discovery aristotle mentioned the ability of the octopus to change colour for both camouflage and signalling in his historia animalium (ca 400 bc): 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. charles darwin described the colour-changing abilities of the cuttlefish in the voyage of the beagle (1860): 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 a veiled chameleon, chamaeleo calyptratus. structural green and blue colours are generated by overlaying chromatophore types to reflect filtered light. 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. colour-producing molecules fall into two distinct classes: biochromes and structural colours or "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. 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. xanthophores and erythrophores chromatophores that contain large amounts of yellow pteridine pigments are named xanthophores; those with mainly red/orange carotenoids are termed erythrophores. 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. therefore, the distinction between these chromatophore types is not always clear. 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. iridophores and leucophores leucophore layer composition iridophores, sometimes also called guanophores, are chromatophores that reflect light using plates of crystalline chemochromes made from guanine. 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. 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, in particular in the tapetum lucidum. like iridophores, they utilize crystalline purines (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 metallic colours, whereas leucophores produce reflective white hues. melanophores at the bottom a mutant zebrafish larva that fails to synthesise melanin in its melanophores, at the top a non-mutant, wildtype larva