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SUMMARY 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.

This study characterized the local effects of extracellular osmolality and prolactin (PRL) on branchial ionoregulatory function of a euryhaline teleost, Mozambique tilapia (Oreochromis mossambicus). First, gill filaments were dissected from freshwater (FW)-acclimated tilapia and incubated in four different osmolalities, 280, 330, 380, and 450 mosmol/kg H2O. The mRNA expression of Na(+)/K(+)-ATPase α1a (NKA α1a) and Na(+)/Cl(-) cotransporter (NCC) showed higher expression with decreasing media osmolalities, while Na(+)/K(+)/2Cl(-) cotransporter 1a (NKCC1a) and PRL receptor 2 (PRLR2) mRNA levels were upregulated by increases in media osmolality. We then incubated gill filaments in media containing ovine PRL (oPRL) and native tilapia PRLs (tPRL177 and tPRL188). oPRL and the two native tPRLs showed concentration-dependent effects on NCC, NKAα1a, and PRLR1 expression; Na(+)/H(+) exchanger 3 (NHE3) expression was increased by 24 h of incubation with tPRLs. Immunohistochemical observation showed that oPRL and both tPRLs maintained a high density of NCC- and NKA-immunoreactive ionocytes in cultured filaments. Furthermore, we found that tPRL177 and tPRL188 differentially induce expression of these ion transporters, according to incubation time. Together, these results provide evidence that ionocytes of Mozambique tilapia may function as osmoreceptors, as well as directly respond to PRL to modulate branchial ionoregulatory functions.

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Spatiotemporal changes in branchial ionocyte distribution were investigated following transfer from seawater (SW) to freshwater (FW) in Japanese seabass. The mRNA expression levels of cystic fibrosis transmembrane conductance regulator (CFTR) and Na/K/2Cl cotransporter 1a (NKCC1a) in the gills rapidly decreased after transfer to FW, whereas Na/H exchanger 3 (NHE3) and Na/Cl cotransporter 2 (NCC2) expression were upregulated following the transfer. Using quadruple-color whole-mount immunofluorescence staining with anti-Na/K-ATPase, anti-NHE3, anti-CFTR and T4 (anti-NKCC1a/NCC2) antibodies, we classified ionocytes into one SW type and two FW types: NHE3 cell and NCC2 cell. Time course observation after transfer revealed an intermediate type between SW-type and FW-type NHE3 ionocytes, suggesting functional plasticity of ionocytes. Finally, on the basis of the ionocyte classification of Japanese seabass, we observed the location of ionocyte subtypes on frozen sections of the gill filaments stained by triple-color immunofluorescence staining. Our observation indicated that SW-type ionocytes transformed into FW-type NHE3 ionocytes and at the same time shifted their distribution from filaments to lamellae. However, FW-specific NCC2 ionocytes appeared mainly in the filaments. Taken together, these findings indicate that ionocytes originated from undifferentiated cells in the filaments and expanded their distribution to the lamellae during FW acclimation.

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SignificanceViviparity is a type of reproductive system in which the embryo utilizes a maternal nutrient supply until birth. Viviparous species occur in many taxa including bony fish. The mother fish raises her offspring in the ovarian lumen or oviduct during gestation. Embryos of the viviparous bony fish Xenotoca eiseni (family Goodeidae) utilize nutrients secreted into the ovarian lumen. However, the source of the maternal nutrients and their mother-to-embryo transport have not been experimentally demonstrated. In this study, we focus on the yolk nutrient protein vitellogenin (Vtg) as a matrotrophic factor. Our results are of fundamental importance to the investigation of viviparous systems in teleosts and in other vertebrates and invertebrates.