| Research Abstract |
Mozambique tilapia Oreochromis mossambicus embryos were transferred from freshwater to seawater and vice versa, and short-term changes in the localization of three major ion transport proteins, Na^+/K^+-ATPase, Na^+/K^+/2Cl^- cotransporter (NKCC) and cystic fibrosis transmembrane conductance regulator (CFTR) were examined within mitochondrion-rich cells (MRCs) in the embryonic yolk-sac membrane. Triple-color immunofluorescence staining allowed us to classify MRCs into four types: type I, showing only basolateral Na^+/K^+-ATPase staining; type II, basolateral Na^+/K^+-ATPase and apical NKCC ; type III, basolateral Na^+/K^+-ATPase and basolateral NKCC ; type IV, basolateral Na^+/K^+-ATPase, basolateral NKCC and apical CFTR, In freshwater, three types of MRCs, types I, II and III were observed. Following transfer from freshwater to seawater, type-IV cells appeared at 12 h and showed a remarkable increase in number between 24 h and 48 h, whereas type-III cells disappeared. When transferred from seawater back to freshwater, type-IV cells decreased and disappeared at 48 h, type-III cells increased, and type-II cells which were not found in seawater appeared at 12 h and increased in number thereafter. Type-I cells existed consistently irrespective of salinity changes. These results suggest that type I is an immature MRC, type II is a freshwater-type ion absorptive cell, type III is a dormant type-IV cell and/or an ion absorptive cell (with a different mechanism from type II), and type IV is a seawater-type ion secretory cell. The intracellular localization of the three ion transport proteins in type-IV cells is completely consistent with a widely accepted model for ion secretion by MRCs. A new model for ion absorption was established based on type-II cells possessing apical NKCC.
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