Functional Diversity of Chloride Cells in Iono-and Osmo-Regulation in Fish

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1. To clarify the involvement of cortisol in functional differentiation of branchial chloride cells, cellular gene expression and localization of cortisol receptor were examined in chum salmon fry in fresh water (FW) and those adapted to seawater (SW) by jn situ hybridization and immunocytochemical staining. The receptor gene expression examined by in situ hybridization and specific immunostaining with anti-cortisol receptor serum were found in two types of chloride cells distributed in the gill filaments and lamellae, indicating that cortisol may be one of the important factors regulating chloride cell functions. Gene expression of cortisol receptor in filament chloride cells, which were highly activated in SW-adapted fry, was significantly greater in the fry adapted to SW than in FW-adapted fry. Cortisol receptors were also present in undifferentiated cells in the interlamellar regions, also suggesting the involvement of cortisol in the functional differentiation of chloride cells.2. Changes in chloride cell morphology were examined in the yolk-sac membrane of Mozambique tilapia embryos and larvae transferred from FW to seawater SW. Labeling chloride cells with DASPEI, a fluorescent probe specific for mitochondria, we observed jn vivo sequential changes in individual chloride cells by confocal laser scanning microscopy. In embryos transferred from FW to SW 3 days after fertilization, 75% of chloride cells survived 96 h later, and each cell showed a remarkable increase in size. In contrast, the cell size did not change in embryos and larvae kept in FW. The same rate of chloride cell turnover was observed in both FW and SW.Immunofluorescence observations revealed that single chloride cells enlarged and were frequently indented by newly-differentiated accessory cells to form multicellular complexes during SW adaptation. These results indicate that FW-type single chloride cells are transformed into SW-type multicellular complexes during SW adaptation.