The changes in cell shape during pigment migration in melanophores of a teleost, Oryzias latipes

Characterization of Melanophore Morphology by Fractal Dimension Analysis

Fractal or focal dimension (FD) analysis is a valuable tool to identify physiologic stimuli at the cellular and tissue levels that allows for quantification of cell perimeter complexity. The FD analysis was determined on fluorescence images of caffeine- or epinephrine-treated (or untreated control) killifish Fundulus heteroclitus (Linneaus) melanophores in culture. Cell perimeters were indicated by rhodamine-phalloidin labeling of cortical microfilaments using box-counting FD analysis. Caffeine-treated melanophores displayed dispersed melanosomes in cells with less serrated edges and reduced FD and complexity. Complexity in epinephrine-treated cells was significantly higher than the caffeine-treated cells or in the control. Cytoarchitectural variability of the cell perimeter is expected because cells change shape when cued with agents. Epinephrine-treated melanophores demonstrated aggregated melanosomes in cells with more serrated edges, significantly higher FD and thus complexity. Melanophores not treated with caffeine or epinephrine produced variable distributions of melanosomes and resulted in cells with variably serrated edges and intermediate FD with a larger SE of the regression and greater range of complexity. Dispersion of melanosomes occurs with rearrangements of the cytoskeleton to accommodate centrifugal distribution of melanosomes throughout the cell and to the periphery. The loading of melanosomes onto cortical microfilaments may provide a less complex cell contour, with the even distribution of the cytoskeleton and melanosomes. Aggregation of melanosomes occurs with rearrangements of the cytoskeleton to accommodate centripetal distribution of melanosomes. The aggregation of melanosomes may contribute to centripetal retraction of the cytoskeleton and plasma membrane. The FD analysis is, therefore, a convenient method to measure contrasting morphologic changes within stimulated cells.

Morphological color changes in fish: Regulation of pigment cell density and morphology

Pigment cells enable fish to change their coloration. It has been recognized that fish color changes can be divided into two categories; one is a physiological color change, which is attributed to rapid motile responses of chromatophores, and the other is a morphological color change, which results from changes in the morphology and density of chromatophores. Long-term adaptation of fish to a certain background can be a general cue to morphological color changes, and has been studied from the beginning of the 19th century. Although the motile mechanism and its control in fish chromatophores are now being elucidated, it is not yet clear how chromatophores change their density and what controls morphological color changes. In recent years, chromatophores, especially melanophores, have been shown to differentiate and to die by apoptosis under the influence of factors that regulate motile responses. Those factors are likely to utilize common intracellular signaling pathways used in part to regulate both types of color changes. In this article, after briefly reviewing the history of early studies, recent findings are discussed relevant to increases or decreases in chromatophores, and changes in their morphology. Finally, morphological color changes are discussed as physiological phenomena involved in the balance between differentiation and apoptosis of chromatophores.

Actin microfilaments in melanophores of Fundulus heteroclitus. Their possible involvement in melanosome migration

In melanophores of Fundulus heteroclitus, hormone-stimulated melanosome aggregation is accompanied by cytoplasmic flow from the cellular processes to the perikaryon, and reversal of these events takes place upon hormone-induced melanosome dispersion. These cells contain parallel arrays of microtubules, the majority of which are located in the perikaryon and in cortical regions of the processes. Studies with heavy meromyosin binding demonstrated two types of actin filaments: 11 a decorated meshwork of filaments similar to those usually found in close association with plasma membranes, and 2) filaments decorated in a manner similar to that of stress fibers. There is an apparent increase in the amount of filaments during melanosome aggregation. These results are discussed in relation to intracellular movement.

The effects of lumicolchicine, colchicine and vinblastine on pigment migration in fish chromatophores

The effects of lumicolchicine, colchicine, vinblastine and cold temperature on the pigment migration in melanophores and xanthophores of Fundulus heteroclitus and Oryzias latipes were examined by light and electron microscopy. Xanthophores of both species which contain numerous microfilaments and a poorly developed microtubule system were extremely sensitive to the alkaloids. Lumicolchicine and colchicine induced irreversible dispersion while vinblastine caused permanent aggregation of the pigments. Treatment in lumicolchicine or colchicine at 5 mM for 60 minutes did not disrupt microtubules of melanophores to an appreciable degree, however, melanosome aggregation was partially inhibited by these drug in Oryzias. When melanophores were kept in the cold in the presence of colchicine at 1 mM, almost all microtubules were disrupted and their repolymerization at room temperature was nearly completely inhibited by colchicine. These melanophores lacking in microtubules responded to epinephrine with slow aggregation. Vinblastine at 0.1 mM induced partial aggregation of melanosomes and disruption of microtubules but most melanophores were still able to respond with pigment migration. Vinblastine at 1 mM made all melanophores punctate and immobile. Large vinblastine-induced crystals were frequently seen in the dendritic processes. The results of the present investigation suggest that cytoplasmic microtubules in fish melanophores facilitate melanosome migration only in directional orientation and appear not be responsible for the motive force.