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First published online March 12, 2009
Journal of Experimental Biology 212, 1003-1010 (2009)
Published by The Company of Biologists 2009
doi: 10.1242/jeb.025957

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Morphological and functional classification of ion-absorbing mitochondria-rich cells in the gills of Mozambique tilapia

Mayu Inokuchi^*,1, Junya Hiroi², Soichi Watanabe¹, Pung-Pung Hwang³ and Toyoji Kaneko¹

¹ Department of Aquatic Bioscience, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Bunkyo, Tokyo 113-8657, Japan
² Department of Anatomy, St Marianna University School of Medicine, Kawasaki, Kanagawa 216-8511, Japan
³ Institute of Cellular and Organismic Biology, Academia Sinica, Nankang, Taipei 11529, Taiwan

^* Author for correspondence (e-mail: niida{at}marine.fs.a.u-tokyo.ac.jp)

Accepted 19 January 2009

To clarify ion-absorbing functions and molecular mechanisms of mitochondria-rich (MR) cells, Mozambique tilapia (Oreochromis mossambicus) were acclimated to artificial freshwaters with normal or lowered Na⁺ and/or Cl^– concentration: (1) normal Na⁺/normal Cl^– (control); (2) normal Na⁺/low Cl^–; (3) low Na⁺/normal Cl^–; and (4) low Na⁺/low Cl^–. Scanning electron microscopy (SEM) revealed that concave and convex apical surfaces of MR cells predominantly developed in low Na⁺ and low Cl^– waters, respectively, whereas small apical pits predominated in control conditions. Expression of Na⁺/H⁺ exchanger-3 (NHE3) mRNA in the gills was increased in low Na⁺ waters (low Na⁺/normal Cl^– and low Na⁺/low Cl^–), whereas that of Na⁺/Cl^– cotransporter (NCC) expression was upregulated in low Cl^–, but not in low Na⁺/low Cl^–. Immunofluorescence staining showed that enlarged NHE3-immunoreactive apical regions were concave or flat in low Na⁺ waters, whereas NCC-immunoreactive regions were enlarged convexly in low Cl^– waters. Using SEM immunocytochemistry the distribution of NHE3/NCC was compared with SEM images obtained simultaneously, it was further demonstrated that NHE3 and NCC were confined to concave and convex apical surfaces, respectively. These results indicated that small apical pits developed into concave apical surfaces to facilitate Na⁺ uptake through NHE3, and into convex apical surfaces to enhance Na⁺/Cl^– uptake through NCC. Our findings integrated morphological and functional classifications of ion-absorbing MR cells in Mozambique tilapia.

Key words: Na⁺/H⁺ exchanger-3, Na⁺/Cl^– cotransporter, mitochondria-rich cell, Mozambique tilapia, gill morphology

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