Functional Morphology of Mitochondrion-Rich Cells in Euryhaline and Stenohaline Teleosts

In teleosts, mitochondrion-rich (MR) cells, or chloride cells, are importantly involved in freshwater and seawater adaptation and acid-base regulation. In this paper, we reviewed recent advances in functional morphology of MR cells in relation to environmental adaptation in euryhaline and stenohaline teleost species with different life histories and adaptability to osmotic and ionic environments. MR cells already existed as early as during the embryonic stages before the formation of the gills. The principal site for MR-cell distribution shifts from the yolk-sac membrane and body skin during the embryonic stages to the gills in larval and later developmental stages, although the timing of the shift differs in different species. In euryhaline teleosts, MR cells alter their morphology and ion-transporting functions to meet unexpected environmental salinity changes, whereas alteration of MR-cell functions takes place as a preadaptive response for the forthcoming upstream and downstream migrations in diadromous teleosts. In stenohaline teleosts inhabiting either freshwater or seawater, on the other hand, MR cells do not possess the functional plasticity to switch the direction of ion transport, but merely adjust the degree of unidirectional ion transport. Thus, euryhalinity or stenohalinity of teleosts is primarily determined by their ability of functional alteration and plasticity of MR cells.