Apoptosis in skin pigment cells of the medaka, Oryzias latipes (Teleostei), during long-term chromatic adaptation: The role of sympathetic innervation

Many teleost fish can adapt their body color to a background color by changing the morphology and density of their skin pigment cells. Melanophore density in fish skin decreases during long-term adaptation to a white background. Although cell death, especially apoptosis, is thought to be involved in these morphological changes, there are no data clearly supporting this mechanism. Using medaka fish, Oryzias latipes, we observed that, on a white background, melanophore size was reduced first and this was followed by a decrease in melanophore density caused by gradual cell death. The process of cell death included loss of cell activity, cell fragmentation, phagocytosis of the fragments, and clearance via the epidermis. Apoptosis was assessed by the appearance of phosphatidylserine on the cell surface of melanophores that had lost motile activity, and DNA fragments involved in cell fragmentation were detected by the TUNEL (TDT-mediated dUTP-biotin nick end-labeling) assay. However, when chemically denervated fish were used, although melanophore size was reduced as expected, cell death was suppressed even on a white background. In skin tissue culture, apoptosis in melanophores was stimulated significantly by norepinephrine, but not by melanin-concentrating hormone. These results indicate that melanophore density decreases by apoptosis, and suggest that sympathetic innervation has an important role in the regulation of apoptosis in melanophores. In analogous fashion, leucophores showed a significant decrease in density with an increase of cell death on a black background. We suggest that apoptosis regulates the balance of pigment cells in the skin of medaka fish to adapt their body color to a particular background.