Acute responses of gill mitochondria-rich cells in Mozambique tilapia Oreochromis mossambicus following transfer from normal freshwater to deionized freshwater

Acute responses of gill mitochondria-rich (MR) cells to direct transfer from freshwater to deionized freshwater were examined in Mozambique tilapia. Scanning electron microscopic observations showed that apical openings of MR cells were morphologically classified into a small apical pit, a convex apical surface, and a concave apical surface. Following transfer from normal freshwater to deionized freshwater, the sizes of convex and concave apical surfaces were significantly increased at 24 and 6 h, respectively. Meanwhile, whole-mount immunocytochemistry revealed that freshwater-type, ion-absorbing MR cells were functionally classified into MR cells with apical Na+/Cl- cotransporter (NCC) and those with apical Na+/H+ exchanger 3 (NHE3). On transfer to deionized freshwater, the intensity and size of apical-NHE3 immunoreaction were increased at 6 h, whereas the apical-NCC immunoreaction became enlarged and intense at 24 h. Considering that convex and concave MR cells are activated forms of apical-NCC cells and apical-NHE3 cells, respectively, our findings indicate that preexisting concave MR cells with apical NHE3 enlarge their apical surfaces as an acute response to exposure to deionized freshwater, followed by activation of convex MR cells with apical NCC.