Chromatophores--models for studying cytomatrix translocations

Teleost chromatophores have proven valuable in the study of the cytomatrix and its involvement in intracellular transport. These cells, which move their pigment rapidly, can be isolated from the scales of fish and maintained in culture for several days. Their transport activity can be followed with the light microscope and recorded by cinematography . Individual pigment granules can be seen moving radially toward the cell center in aggregation, then back toward the cortices during dispersion . These two motions are different . In aggregation, the granules move smoothly and rapidly (5-10 Am/s), reminding one of the behavior of chromosomes during anaphase . In dispersion, which takes twice as long as aggregation, the granules move haltingly, first out (orthograde) and then back (retrograde) . This saltatory motion continues throughout dispersion but ceases suddenly at the onset of the next aggregation . The direction of pigment motion is easily controlled by the addition of epinephrine to the medium to stimulate aggregation or of caffeine (a phosphodiesterase inhibitor) to stimulate dispersion . This relative ease in temporally separating two types of movement distinct in direction, rate, and character enhances the value of chromatophores for experimental studies on the mechanisms of intracellular transport . All ofthe fish chromatophores we have studied are capable of rapid pigment transport and possess a similar internal organization (5, 18, 32) . A highly structural centrosome complex consisting of a centriole pair and surrounding stratified layers of dense material occupies the cell center (31, 42) . Thousands of microtubules nucleate from this structure and extend outward, organizing the pigment into linear columns . There are few, if any, intermediate filaments in chromatophores (none in erythrophores) and only small amounts of filamentous actin, which is subcortical in distribution (3, 43) . A smooth endoplasmic reticulum (SER)'-like membrane network encompasses the centrosome and extends outward along microtubule bundles to the plasma membrane (25, 38) . The cell constituents described above are clearly demon-