Functional morphology of chloride cells in killifish Fundulus heteroclitus, a euryhaline teleost with seawater preference

Recent advances in the functional morphology of chloride cells in killifish Fundulus heteroclitus, a euryhaline teleost with seawater (SW) preference, were reviewed. Immunocytochemical detection of chloride cells with anti-Na/K+-ATPase revealed transitional processes of the chloride cell distribution during early life stages. Chloride cells first appear in the yolk-sac membrane at an early embryonic stage, followed by their appearance in the body skin in the later embryonic stages. The principal site for the chloride cell distribution then shifts from the yolk-sac membrane and body skin during embryonic stages to the gills and opercular membrane in larval and later developmental stages. Morphologically distinct SW- and freshwater (FW)-type chloride cells were identified in adult killifish adapted to SW and FW, respectively. Both types of chloride cells are equally active in the two environments, but exert different ion-transporting functions. Following direct transfer of killifish from SW to FW, SW-type chloride cells were transformed into FW-type cells as a short-term response, followed by the promotion of chloride cell replacement as a long-term response. In killifish acclimated to low NaCl (0.1 mM) FW, intense immunoreactivity for vacuolar-type proton pump (V-ATPase) was detected in the basolateral membrane of FW-type chloride cells, whereas the immunoreactivity was much weaker in fish acclimated to FW with higher NaCl concentrations (1.0 and 10.0 mM). These results suggest the occurrence of active ion absorption in FW-type chloride cells and the involvement of V-ATPase in ion-absorbing mechanisms. In view of recent advances in this field, future chloride cell research should be considered in relation to the functional diversity of chloride cells.