Cytomatrix in chromatophores

Although intracellular transport is a fascinating and vital function shared by diverse cell systems, our understanding of the processes involved remains incomplete, despite the enormous amount of attention it has received over the past 40 years (46). Recent work on pigment granule transport in chromatophores (e .g ., erythrophores, melanophores, and xanthophores), however, has begun to yield new information on three important aspects of the phenomenon : (a) the organization of the cytomatrix components involved in directed organelle transport; (b) the identification ofproteins powering organelle transport ; and (c) the precise determination of the physiological requirements (ions, nucleotides, pH) of transport . The progress made in these studies is largely due to the development of methods for culturing chromatophores and is based on the striking behavior of chromatophore pigment granules . Because of the relatively flat, discoid shape of the chromatophore and because of the size, color, and rapid rate of transport of its pigment granules, pigment granule motion is easily followed in living cells . Normally, chromatophores are observed to cyclically aggregate and disperse their numerous pigment granules in a radial pattern from the cell center. These are two radically different types of motion : aggregation is rapid (20 pm/s) and uniform (not unlike chromosome transport), and dispersion is slow (5 pm/s), nonuniform, and saltatory (like cytoplasmic flow and organelle transport in nerve cells) . These phenomena are temporally distinct, and chromatophores can be independently induced to aggregate (with epinephrine) or to disperse (with caffeine) their pigment, so that the factors involved in each ofthese processes can be separately analyzed . It is therefore possible to watch the response ofpigment granules in living cells and either directly relate this behavior to changes in the physiological conditions or to fix the cells and directly relate the observations made to the organizational properties of chromatophores . In this review I will emphasize recent discoveries from studies ofintracellular transport in chromatophores, especially with regard to their functional significance. A more historical perspective and detailed description of chromatophores is given in recent reviews by Porter et al . (57) and Luby-Phelps and Schliwa (37) .