Fig. 3. Transmission electron micrographs of branchial chloride cells (cc) of killifish adapted to defined freshwater with low- (A—C), mid- (D—F) and high- (G—I) NaCl. (B,E,H) Magnified views of apical regions of mitochondria-rich (MR) cells. In low- and mid-NaCl groups, the apical membrane of MR cells is flat or slightly projecting, and equipped with microvilli (asterisks) (B,E). In the high-NaCl group, on the other hand, MR cells form an apical pit (ap) (H). (C,F,I) Magnified views of the cytoplasm of MR cells. Note numerous mitochondria (m) and an extensive tubular system (t) in all experimental groups. pv, pavement cell; ac, accessory cell. Scale bars, 5 µm (A,D,G); 1 µm (B,C,E,F,H,I).