Zoological Studies

Morphological Modification of Mitochondria-Rich Cells of the Opercular Epithelium of Freshwater Tilapia, Oreochromis mossambicus, Acclimated to Low Chloride Levels

Yi-Er Shieh, Ru-Shiow Tsai and Pung-Pung Hwang (2003) The present study attempted to elucidate the effect of Cl^- concentration on the morphology of the opercular epithelium of freshwater tilapia, Oreochromis mossambicus. Similar to the situation in gills, 3 types of mitochondria-rich (MR) cells with different morphologies of apical crypts (wavy-convex, shallow-basin, and deep-hole) were identified in the opercular epithelium of freshwater tilapia. Most of the MR cells were located in the upper central area of the opercular epithelium, and the shallow-basin type dominated. When freshwater tilapia were acclimated to 3 artificial fresh waters, low-Na⁺ low-Cl^- (L-Na-LCl), high-Na⁺ low-Cl^- (H-Na-L-Cl), and high-Na⁺ high-Cl^- (H-Na-H-Cl), for 1 wk, cell densities of MR cells were higher in L-Na-L-Cl and H-Na-L-Cl media than in the H-Na-H-Cl medium. The wavy-convex MR cells appeared in the low-Cl- media only and were never observed in the H-Na-H-Cl medium. In an acute-exposure experiment, tilapia preacclimated to H-Na-H-Cl were transferred to a low-Cl^- environment, and the numbers of MR cells dramatically increased within 6 h (significantly different from that at 0 h, p < 0.05, t-test), and increased linearly for 96 h. The wavy-convex MR cells were first seen at 24 h and increased until 96 h. On the contrary, deep-hole MR cells began to decrease after 6 h of exposure to the low-Cl^- environment. The increased cell densities and morphological changes in MR cells suggest that the expanded apical surfaces of MR cells are associated with the stimulated Cl^- uptake by tilapia in a low-Cl^- environment.