Fig. 1. Image of trout hepatocytes – using laser-scanning microscopy (LSM 510, Zeis) – pre-incubated with 5 µmol l^–1 acridine orange (AO) for 10 min showing two distinct emission spectra: (1) green fluorescence corresponding to the monomeric AO emission, weak in the cytoplasm and bright in the nucleus, and (2) red fluorescence from separated or aggregated compartments in the cytoplasm, corresponding to aggregated AO emission. The red fluorescence indicates an accumulation of AO in the form of dimers and/or polymers due to the acidic pH inside the compartmental lumen. The LSM is equipped with a x32 oil-immersion objective; 488 nm line of an argon laser was used for excitation; fluorescence emissions were simultaneously recorded in green (505–530 nm) and red (>650 nm) channels.