Xanthophore

Chromatophores are pigment-containing and light-reflecting 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.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.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. Chromatophores are pigment-containing and light-reflecting 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 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). 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. Aristotle mentioned the ability of the octopus to change colour for both camouflage and signalling in his Historia animalium (ca 400 BC): 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): 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, creating different colours when seen from different directions.