Poikilothennic vertebrates have the ability to chromatically adapt to different colored backgrounds and conditions of overhead illumination. These physiological color changes are based upon the translocation of pigment granules within integumental pigment cells (chromatophores). Color changes depend mainly on melanophores (melanocytes), pigment cells containing melanin granules (melanosomes), although some brightly colored animals may depend on yellow-, orange-, or red-colored pigment cells as well as reflecting cells to achieve chromatic adaptation. Melanosome dispersion (centrifugal organelle migration) within melanophores leads to skin darkening, whereas the aggregation (centripetal movement) of melanosomes within the pigment cells leads to a lightening of the skin. Patterns and pattern changes are the result of differential distributions of the various chromatophores in skin and differential responses of the cells to specific melanotropic honnones or neurotransmitters.