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The structure of the kidney and the localization of Na(+), K(+)-ATPase (NKA) immunopositive cells were examined throughout the postembryonic development of the Persian sturgeon, Acipenser persicus, from newly hatched prelarvae (10mm) to 20 days post hatch (20 DPH) larvae (31mm). Investigations were conducted through histology and immunohistochemistry by using the light and immunofluorescence microscopy. The pronephros was observed in newly hatched prelarvae. The cells lining the distal pronephric tubules and their collecting ducts showed laterally expressed NKA immunofluorescence that later extended throughout the whole cytoplasm. Mesonephrogenous placodes and pre-glomeruli were distinguished at 2 DPH along the collecting ducts posteriorly. Their tubules were formed and present in kidney mesenchyma, differentiated into neck, proximal, distal and collecting segments at 7 DPH when NKA immunopositive cells were observed. Their distal and collecting tubules showed an increasing immunofluorescence throughout their cytoplasm while the glomeruli remained unstained. From D 9 to D 17, the epithelial layer of pronephric collecting duct changed along the mesonephros to form ureters. Ureters, possessing isolated strong NKA immunopositive cells, appeared as two sac-like structures hanging under the trunk kidney. Since NKA immunopositive cells were not observed on the tegument or along the digestive tract of newly hatched prelarva, and also the gills are not formed yet, the pronephros is the only osmoregulatory organ until 4 DPH. At the larval stage, the pronephros and mesonephros are functional osmoregulatory organs and actively reabsorb necessary ions from the filtrate., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

Three-spined sticklebacks ( Gasterosteus aculeatus L.) of the Camargue region (Rhone delta, northern Mediterranean Sea) occupy coastal mesohaline lagoons and freshwater canals. In this study, the acclimation capacity to different salinities of a population living in freshwater canals was investigated. Freshwater ( FW; 5 ‰) individuals were compared to fish acclimated for at least 2 weeks to brackish water ( BW; 15 ‰) and seawater ( SW; 30 ‰). Blood osmotic pressure and gill Na+/K+- ATPase ( NKA) gene expression of the α1 subunit and α1a and α1b isoforms were determined at these salinities. NKA protein expression in the gill ionocytes and the remodelling of these cells were also investigated through NKA immunolabelling and electron microscopy. Blood osmolalities of fish acclimated to FW, BW and SW were significantly different. Branchial NKA α1a and α1b expressions also differed, with a higher expression of NKA α1b in fish acclimated to SW. Gill ionocytes in fish acclimated to FW were located along the lamellae and at their base and appeared elongated, whereas these cells were restricted to gill filaments in fish acclimated to SW with a round shape. Finally, electron microscopy revealed three different types of apical structures for these ionocytes: a honeycomb-like structure and a dome shape in FW, and deeply encrypted in SW. This study shows that ionocyte cell type and morphology, and gill NKA expression are salinity-dependent. This branchial remodelling is directly linked to the physiological homeostatic status reached by the fish. It also highlights that sticklebacks of the Camargue region can rapidly acclimate to different salinities and can easily migrate between FW canals and mesohaline coastal lagoons. [ABSTRACT FROM AUTHOR]