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Fig. 16. Transmission electron micrographs of MR cells (cc) in gill filaments of killifish adapted to seawater (SW: A­C) and freshwater (FW: D­F). In SW-adapted fish, the apical membrane of MR cells forms an apical pit (ap). In FW-adapted fish, on the other hand, the apical membrane is flat or shows projections, being equipped with numerous microvilli (asterisk). B and E are magnified views of the apical region of MR cells. In SW-adapted fish, the MR cells interdigitate with accessory cells (ac) and the junctions are shallow and leaky (B). In FW-adapted fish, the junctions between pavement cells (pv) and MR cells are deep and tight. Subapical vesicles (arrowheads) are characteristic of FW-adapted fish (E). C and F are magnified views of the cytoplasm of MR cells in SW- and FW-adapted fish. Note the numerous mitochondria (m) in both SW- and FW-adapted fish, and a better developed tubular system (t) in SW- than in FW-adapted fish. Bars: A, D, 1 μm; B, C, E, F, 0.5 μm. Reprinted with permission from Can. J. Zool., 79, Katoh et al., Distinct seawater and freshwater types of chloride cells in killifish, Fundulus heteroclitus, 822­829, 2001, NRC Research Press.

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