Mitochondria-rich cells (MRCs) in the yolk-sac membrane of tilapia (Oreochromis mossambicus) larvae were examined by Na+/K +-ATPase immunocytochemistry and vital staining for glycoproteins following acclimation to high (7.5-7.9 mmol l-1), normal (0.48-0.52 mmol l-1) or low (0.002-0.007 mmol l-1) ambient Cl - levels. With a combination of concanavalin-A (Con-A)-Texas-Red conjugate staining (larvae exposed to the dye in vivo in the water) and a monoclonal antibody raised against Na+/K+-ATPase, MRCs were easily recognized and presumed to be active when Con-A-positive (i.e. with their apical membrane in contact with the water) or inactive when Con-A-negative. The proportion of active cells gradually increased during a 48-h acclimation to low-Cl- medium but decreased during acclimation to high-Cl- medium. Total densities of MRCs did not change when ambient chloride levels were altered. Furthermore, in live larvae exposed to changes in ambient Cl-, yolk-sac MRCs, vitally stained with DASPEI and subsequently traced in time, did not significantly alter turnover. The polymorphism of the apical membrane compartment of the MRCs represents structural modification of the active MRCs. Yolk-sac pavement cells labeled with the membrane marker FM1-43 (fluorescent lipophilic tracer) were shown to cover active MRCs in larvae transferred from normal to high ambient Cl- levels, thereby inactivating the MRCs.
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