Melanophores: Smooth Muscle Cells in Disguise

Melanophores are specialized cells derived from the neural crest that contain membrane bound vesicles called melanosomes. Melanosomes are filled with melanin, a dark, nonfluorescent pigment that plays a principal role in physiological color adaptation of animals. Melanophores regulate melanosome trafficking on cytoskeletal filaments to generate a range of striking chromatic patterns. The mechanism of physiological color change by these melanophores encompasses both physical and biochemical aspects of melanosome dynamics. Melanophores aggregate or disperse their melanosomes when the host requires changing its color in response to the environmental cues (e.g., social interactions or camouflage). Interestingly, morphological, embryological and physiological evidence has revealed that melanophores of fish, amphibian and reptiles are functionally modified smooth muscle cells [1]. Moreover, during contraction of melanophore there is an aggregation of melanin granules which is to be considered the visible counterpart of an aggregation of colloidal particles that occur during smooth muscle contraction. The biochemical events underlying melanosome dispersion is analogous to those for smooth muscle relaxation: both processes result from increase in cAMP levels and require the presence of extracellular Ca2+ for their action.