Characterization of a novel filament system in goldfish xanthophores

Morphological studies on the mechanisms of pigmentary organelle transport in fish xanthophores and melanophores

Pigmentary organelle translocations within fish chromatophores undergo physiological color changes when exposed to external signals. Chromatophores can be isolated in high yields, and their pigmentary organelles can be tracked readily by microscopy. The combined efforts of morphology and biomolecular chemistry have led to the identification of and determination of the interrelationships between cytoskeletal elements and accessory proteins, motor molecules, cytomatrix, and pigmentary organelles of various sizes. Fish chromatophores have been classified as fast, intermediate, and slow translocators, based on the relative numbers of microtubules. Studies on cultured goldfish (Carassius auratus L.) xanthophores for over 20 years have demonstrated that in this slow translocator, tubulovesicular structures of the smooth endoplasmic reticular (SER) cisternae are involved in the disperson and aggregation of associated carotenoid droplets (CD) with some involvement of cytoskeletal elements. Killifish (Fundulus heteroclitus L.) melanophore, a fast translocator, was also examined. Recent work demonstrates a bright fluorescent "starburst"-like spot that we call an actin filament-organizing center (AFOC) with radiating microfilaments, akin to the microtubule-organizing center (MTOC) with radiating microtubules. Melanosomes translocate single-file on microtubules and are not associated with SER cisternae. Slower CD dispersion or aggregation in goldfish xanthophores seems to be predominantly microfilament-based transport, or microfilament- and microtubule-based transport, respectively. Faster melanosome translocations in killifish melanophores are based on microtubules, with our evidence indicating microfilament involvement. Neural crest-derived chromatophores are models for vesicular transport in axons, and immunocytochemical and imaging technologies may help to elucidate the cellular transport mechanisms.

The effect of temperature on the light-induced pigment movement in fish corneal chromatophores

Corneal chromatophores of unusual morphology were used for studies on the influence of temperature on the intracellular pigment movement in two species of marine fish from different temperature zones: the tropical puffer, Canthigaster cinctus, and boreal whitespotted greenling, Hexagrammos stelleri. It was shown that both dispersion under bright illumination and aggregation at darkening are slower or decrease at lower temperatures when examined in the range of 12-27 degrees C. The mean speed of the pigment translocations in the individual cell process was 0.38 micron/s at the highest temperature examined, with a range of 0.17-1.0 micron/s. Near the middle of the temperature range, the dynamic characteristics of cell pigment movement in tropical and boreal species were rather close, suggesting that there would be little divergent adaptations with respect to the mechanisms of the pigment transport. Corneal chromatophores are considered as a new promising model for cell motility studies.

Association of actin filaments with axonal microtubule tracts

Axoplasmic organelles move on actin as well as microtubules in vitro and axons contain a large amount of actin, but little is known about the organization and distribution of actin filaments within the axon. Here we undertake to define the relationship of the microtubule bundles typically found in axons to actin filaments by applying three microscopic techniques: laser-scanning confocal microscopy of immuno-labeled squid axoplasm; electronmicroscopy of conventionally prepared thin sections; and electronmicroscopy of touch preparations-a thin layer of axoplasm transferred to a specimen grid and negatively stained. Light microscopy shows that longitudinal actin filaments are abundant and usually coincide with longitudinal microtubule bundles. Electron microscopy shows that microfilaments are interwoven with the longitudinal bundles of microtubules. These bundles maintain their integrity when neurofilaments are extracted. Some, though not all microfilaments decorate with the S1 fragment of myosin, and some also act as nucleation sites for polymerization of exogenous actin, and hence are definitively identified as actin filaments. These actin filaments range in minimum length from 0.5 to 1.5 microm with some at least as long as 3.5 microm. We conclude that the microtubule-based tracks for fast organelle transport also include actin filaments. These actin filaments are sufficiently long and abundant to be ancillary or supportive of fast transport along microtubules within bundles, or to extend transport outside of the bundle. These actin filaments could also be essential for maintaining the structural integrity of the microtubule bundles.