Distribution of chloride cells in teleost larvae

Retention of larval skin traits in adult amphibious killifishes: a cross-species investigation

The gills are the primary site of exchange in fishes. However, during early life-stages or in amphibious fishes, ionoregulation and gas-exchange may be primarily cutaneous. Given the similarities between larval and amphibious fishes, we hypothesized that cutaneous larval traits are continuously expressed in amphibious fishes across all life-stages to enable the skin to be a major site of exchange on land. Alternatively, we hypothesized that cutaneous larval traits disappear in juvenile stages and are re-expressed in amphibious species in later life-stages. We surveyed six species spanning a range of amphibiousness and characterized cutaneous ionocytes and neuroepithelial cells (NECs) as representative larval skin traits at up to five stages of development. We found that skin ionocyte density remained lower and constant in exclusively water-breathing, relative to amphibious species across development, whereas in amphibious species ionocyte density generally increased. Additionally, adults of the most amphibious species had the highest cutaneous ionocyte densities. Surprisingly, cutaneous NECs were only identified in the skin of one amphibious species (Kryptolebias marmoratus), suggesting that cutaneous NECs are not a ubiquitous larval or amphibious skin trait, at least among the species we studied. Our data broadly supports the continuous-expression hypothesis, as three of four amphibious experimental species expressed cutaneous ionocytes in all examined life-stages. Further, the increasing density of cutaneous ionocytes across development in amphibious species probably facilitates the prolonged occupation of terrestrial habitats.

Quantification of Cutaneous Ionocytes in Small Aquatic Organisms

Aquatic organisms have specialized cells called ionocytes that regulate the ionic composition, osmolarity, and acid/base status of internal fluids. In small aquatic organisms such as fishes in their early life stages, ionocytes are typically found on the cutaneous surface and their abundance can change to help cope with various metabolic and environmental factors. Ionocytes profusely express ATPase enzymes, most notably Na⁺/K⁺ ATPase, which can be identified by immunohistochemistry. However, quantification of cutaneous ionocytes is not trivial due to the limited camera's focal plane and the microscope's field-of-view. This protocol describes a technique to consistently and reliably identify, image, and measure the relative surface area covered by cutaneous ionocytes through software-mediated focus-stacking and photo-stitching-thereby allowing the quantification of cutaneous ionocyte area as a proxy for ion transporting capacity across the skin. Because ionocytes are essential for regulating ionic composition, osmolarity, and acid/base status of internal fluids, this technique is useful for studying physiological mechanisms used by fish larvae and other small aquatic organisms during development and in response to environmental stress.

Structural and functional maturation of skin during metamorphosis in the Atlantic halibut (Hippoglossus hippoglossus)

To establish if the developmental changes in the primary barrier and osmoregulatory capacity of Atlantic halibut skin are modified during metamorphosis, histological, histochemical, gene expression and electrophysiological measurements were made. The morphology of the ocular and abocular skin started to diverge during the metamorphic climax and ocular skin appeared thicker and more stratified. Neutral mucins were the main glycoproteins produced by the goblet cells in skin during metamorphosis. Moreover, the number of goblet cells producing neutral mucins increased during metamorphosis and asymmetry in their abundance was observed between ocular and abocular skin. The increase in goblet cell number and their asymmetric abundance in skin was concomitant with the period that thyroid hormones (THs) increase and suggests that they may be under the control of these hormones. Several mucin transcripts were identified in metamorphosing halibut transcriptomes and Muc18 and Muc5AC were characteristic of the body skin. Na⁺, K⁺-ATPase positive (NKA) cells were observed in skin of all metamorphic stages but their number significantly decreased with the onset of metamorphosis. No asymmetry was observed between ocular and abocular skin in NKA cells. The morphological changes observed were linked to modified skin barrier function as revealed by modifications in its electrophysiological properties. However, the maturation of the skin functional characteristics preceded structural maturation and occurred at stage 8 prior to the metamorphic climax. Treatment of Atlantic halibut with the THs disrupter methimazole (MMI) affected the number of goblet cells producing neutral mucins and the NKA cells. The present study reveals that the asymmetric development of the skin in Atlantic halibut is TH sensitive and is associated with metamorphosis and that this barrier's functional properties mature earlier and are independent of metamorphosis.

Impact of ultraviolet-B radiation on planktonic fish larvae: alteration of the osmoregulatory function

Coastal marine ecosystems are submitted to variations of several abiotic and biotic parameters, some of them related to global change. Among them the ultraviolet-B (UV-B) radiation (UVBR: 280-320 nm) may strongly impact planktonic fish larvae. The consequences of an increase of UVBR on the osmoregulatory function of Dicentrarchus labrax larvae have been investigated in this study. In young larvae of D. labrax, as in other teleosts, osmoregulation depends on tegumentary ion transporting cells, or ionocytes, mainly located on the skin of the trunk and of the yolk sac. As early D. labrax larvae passively drift in the top water column, ionocytes are exposed to solar radiation. The effect of UVBR on larval osmoregulation in seawater was evaluated through nanoosmometric measurements of the blood osmolality after exposure to different UV-B treatments. A loss of osmoregulatory capability occured in larvae after 2 days of low (50 μWcm(-2): 4 h L/20 h D) and medium (80 μWcm(-2): 4 h L/20 h D) UVBR exposure. Compared to control larvae kept in the darkness, a significant increase in blood osmolality, abnormal behavior and high mortalities were detected in larvae exposed to UVBR from 2 days on. At the cellular level, an important decrease in abundance of tegumentary ionocytes and mucous cells was observed after 2 days of exposure to UVBR. In the ionocytes, two major osmoeffectors were immunolocalized, the Na+/K(+)-ATPase and the Na+/K+/2Cl- cotransporter. Compared to controls, the fluorescent immunostaining was lower in UVBR-exposed larvae. We hypothesize that the impaired osmoregulation in UVBR-exposed larvae originates from the lower number of tegumentary ionocytes and mucous cells. This alteration of the osmoregulatory function could negatively impact the survival of young larvae at the surface water exposed to UVBR.

Ontogenetic changes in location and morphology of chloride cells during early life stages of the Nile tilapia Oreochromis niloticus adapted to fresh and brackish water

Ontogenetic changes in the location, size, density and morphology of chloride cells in the Nile tilapia Oreochromis niloticus adapted to fresh and brackish water are described using Na(+) /K(+) -ATPase immunohistochemistry, light microscopy (LM), scanning electron microscopy (SEM) and confocal scanning laser microscopy (CSLM). The pattern of chloride cell distribution changed during development under both treatments, with chloride cell density decreasing significantly from hatch to 7 days post-hatch, but appearing on the inner opercular area at 3 days post-hatch and increasing significantly thereafter (P < 0·05). Chloride cells were always denser in fresh- than in brackish-water larvae. In both treatments, chloride cells located on the outer operculum and tail showed a marked increase in size with age, but cells located on the abdominal epithelium of the yolk sac and the inner operculum showed a significant decrease in size (P < 0·05). Chloride cells from brackish-water adapted larvae from 1 day post-hatch onwards were always significantly larger (P < 0·05) than those from freshwater-adapted larvae. SEM revealed structural differences in chloride cell apical morphology according to environmental conditions. There appears to be clearly defined temporal staging of the appearance of adaptive mechanisms that confer an ability to cope with varying environmental conditions during early development.

Influence of salinity on the localization and expression of the CFTR chloride channel in the ionocytes of Dicentrarchus labrax during ontogeny

The expression and localization of the cystic fibrosis transmembrane conductance regulator (CFTR) were determined in four osmoregulatory tissues during the ontogeny of the sea-bass Dicentrarchus labrax acclimated to fresh water and sea water. At hatch in sea water, immunolocalization showed an apical CFTR in the digestive tract and integumental ionocytes. During the ontogeny, although CFTR was consistently detected in the digestive tract, it shifted from the integument to the gills. In fresh water, CFTR was not present in the integument and the gills, suggesting the absence of chloride secretion. In the kidney, the CFTR expression was brief from D4 to D35, prior to the larva-juvenile transition. CFTR was apical in the renal tubules, suggesting a chloride secretion at both salinities, and it was basolateral only in sea water in the collecting ducts, suggesting chloride absorption. In the posterior intestine, CFTR was located differently from D4 depending on salinity. In sea water, the basolateral CFTR may facilitate ionic absorption, perhaps in relation to water uptake. In fresh water, CFTR was apical in the gut, suggesting chloride secretion. Increased osmoregulatory ability was acquired just before metamorphosis, which is followed by the sea-lagoon migration.

Epithelial Ca(2+) channel expression and Ca(2+) uptake in developing zebrafish

The purpose of the present work was to study the possible role of the epithelial Ca(2+) channel (ECaC) in the Ca(2+) uptake mechanism in developing zebrafish (Danio rerio). With rapid amplification of cDNA ends, full-length cDNA encoding the ECaC of zebrafish (zECaC) was cloned and sequenced. The cloned zECaC was 2,578 bp in length and encoded a protein of 709 amino acids that showed up to 73% identity with previously described vertebrate ECaCs. The zECaC was found to be expressed in all tissues examined and began to be expressed in the skin covering the yolk sac of embryos at 24 h postfertilization (hpf). zECaC-expressing cells expanded to cover the skin of the entire yolk sac after embryonic development and began to occur in the gill filaments at 96 hpf, and thereafter zECaC-expressing cells rapidly increased in both gills and yolk sac skin. Corresponding to ECaC expression profile, the Ca(2+) influx and content began to increase at 36-72 hpf. Incubating zebrafish embryos in low-Ca(2+) (0.02 mM) freshwater caused upregulation of the whole body Ca(2+) influx and zECaC expression in both gills and skin. Colocalization of zECaC mRNA and the Na(+)-K(+)-ATPase alpha-subunit (a marker for mitochondria-rich cells) indicated that only a portion of the mitochondria-rich cells expressed zECaC mRNA. These results suggest that the zECaC plays a key role in Ca(2+) absorption in developing zebrafish.

Ontogeny of osmoregulation in postembryonic fish: a review

Salinity and its variations are among the key factors that affect survival, metabolism and distribution during the fish development. The successful establishment of a fish species in a given habitat depends on the ability of each developmental stage to cope with salinity through osmoregulation. It is well established that adult teleosts maintain their blood osmolality close to 300 mosM kg(-1) due to ion and water regulation effected at several sites: tegument, gut, branchial chambers, urinary organs. But fewer data are available in developing fish. We propose a review on the ontogeny of osmoregulation based on studies conducted in different species. Most teleost prelarvae are able to osmoregulate at hatch, and their ability increases in later stages. Before the occurrence of gills, the prelarval tegument where a high density of ionocytes (displaying high contents of Na+/K+-ATPase) is located appears temporarily as the main osmoregulatory site. Gills develop gradually during the prelarval stage along with the numerous ionocytes they support. The tegument and gill Na+/K+-ATPase activity varies ontogenetically. During the larval phase, the osmoregulatory function shifts from the skin to the gills, which become the main osmoregulatory site. The drinking rate normalized to body weight tends to decrease throughout development. The kidney and urinary bladder develop progressively during ontogeny and the capacity to produce hypotonic urine at low salinity increases accordingly. The development of the osmoregulatory functions is hormonally controlled. These events are inter-related and are correlated with changes in salinity tolerance, which often increases markedly at the metamorphic transition from larva to juvenile. In summary, the ability of ontogenetical stages of fish to tolerate salinity through osmoregulation relies on integumental ionocytes, then digestive tract development and drinking rate, developing branchial chambers and urinary organs. The physiological changes leading to variations in salinity tolerance are one of the main basis of the ontogenetical migrations or movements between habitats of different salinity regimes.

Gills are needed for ionoregulation before they are needed for O2 uptake in developing zebrafish,Danio rerio

A variation on the classic ablation method was used to determine whether O2 uptake or ionoregulation is the first to shift from the skin to the gills in developing zebrafish, Danio rerio. Zebrafish larvae,ranging in age from 3 to 21 days postfertilization, were prevented from ventilating their gills and forced to rely on cutaneous processes by exposing them to one of two anaesthetics (tricaine methanesulphonate or phenoxyethanol)or by embedding their gills in agar. They were then placed in solutions designed to compensate selectively for impaired O2 uptake (42%O2), impaired ionoregulatory capacity (50% physiological saline) or impairment of both functions (42% O2+50% physiological saline). Survival under these conditions was compared with that in normoxic (21%O2) fresh water. Neither hyperoxia nor 50% physiological saline had any significant effect on the survival of newly hatched larvae (3 days postfertilization), suggesting that at this stage cutaneous exchange was sufficient to satisfy both ionoregulatory and respiratory requirements. At 7 days postfertilization, the skin still appeared capable of satisfying the O2 requirements of larvae but not their ionoregulatory requirements. Physiological saline significantly improved survival at 7 days postfertilization; hyperoxia did not. At 14 days postfertilization, both hyperoxia and 50% saline significantly improved survival, indicating that at this stage gills were required for O2 uptake as well as for ionoregulation. At 21 days postfertilization, only hyperoxia significantly improved survival. By this stage, larvae apparently are so dependent on gills for O2 uptake that they suffocate before the effects of ionoregulatory impairment become apparent. Thus, it would appear that in zebrafish it is the ionoregulatory capacity of the skin not its ability to take up O2 that first becomes limiting. This raises the possibility that ionoregulatory pressures may play a more important role in gill development than is generally appreciated.

Developmental and environmental regulation of chloride cells in young American shad, Alosa sapidissima

Location, abundance, and morphology of gill chloride cells were quantified during changes in osmoregulatory physiology accompanying early development in American shad, Alosa sapidissima. During the larval-juvenile transition of shad, gill chloride cells increased 3.5-fold in abundance coincident with gill formation, increased seawater tolerance, and increased Na(+),K(+)-ATPase activity. Chloride cells were found on both the primary filament and secondary lamellae in pre-migratory juveniles. Chloride cells on both the primary filament and secondary lamellae increased in abundance (1.5- to 2-fold) and size (2- to 2.5-fold) in juveniles held in fresh water from August 31 to December 1 (the period of downstream migration) under declining temperature. This proliferation of chloride cells was correlated with physiological changes associated with migration (decreased hyperosmoregulatory ability and increased gill Na(+),K(+)-ATPase activity). Increases in chloride cell size and number of fish in fresh water were delayed and of a lower magnitude when shad were maintained at constant temperature (24 degrees C). When juveniles were acclimated to seawater, chloride cell abundance on the primary filament did not (though size increased 1.5- to 2-fold), but cells on the secondary lamellae disappeared. Na(+),K(+)-ATPase was immunolocalized to chloride cells in both fresh water and seawater acclimated fish. The disappearance of chloride cells on the secondary lamellae upon seawater acclimation is evidence that their role is confined to fresh water. The proliferation of chloride cells in fresh water during the migratory-associated loss of hyperosmoregulatory ability is likely to be a compensatory mechanism for increasing ion uptake. J. Exp. Zool. 290:73-87, 2001.

Modification of morphology and function of integument mitochondria-rich cells in tilapia larvae (Oreochromis mossambicus) acclimated to ambient chloride levels

Similar to those of the gills of adults, three types of mitochondria-rich (MR) cells with different morphologies of apical surfaces (wavy convex, shallow basin, and deep hole) were identified on the integument of freshwater-acclimated tilapia larvae (Oreochromis mossambicus). The object of this study is to test the hypothesis that these subtype cells may represent MR cells equipped with variable efficiencies in Cl(-) uptake. Larvae acclimated to low-Cl(-) =0.001-0.007 mM) water developed higher densities of MR cells than those acclimated to high-Cl(-) =7.3-7.9 mM) water. The percentage of wavy-convex-type cells in total MR cells was higher in low-Cl(-)-acclimated larvae than in high-Cl(-)-acclimated larvae, which displayed only deep-hole type. In addition, Cl(-) influx rates of whole larva measured with (36)Cl(-) showed a coincident correlation with MR cell densities, that is, low-Cl(-) larvae displayed higher Cl(-) influx rates than did high-Cl(-) larva, suggesting that tilapia larvae develop a higher density of MR cells with larger apical surfaces (wavy-convex type) to boost Cl(-) uptake in Cl(-)-deficient water. The distinct types of apical surfaces may represent different phases of MR cells that possess different efficiencies of Cl(-) uptake. Increased apical membrane surface areas of MR cells may provide larvae with rapid regulation of Cl(-) before new MR cells differentiate.

Metamorphosis and early larval development of the flatfishes (Pleuronectiformes): an osmoregulatory perspective

Flatfish (Pleuronectiformes) distribution in the environment is influenced by salinity, and varies among species and with developmental stage. Osmoregulatory ability likely plays an important role in defining species and developmental stage-specific distribution. Although the mechanisms of osmoregulation in adult and juvenile teleosts have been widely addressed, far less is known about their larval osmoregulatory physiology. Much of our current understanding of larval fish ion-regulation stems from studies using flatfishes, and this article reviews advances in this field, primarily from the point of view of the developing flatfishes. Addressed here are the ontogeny of salinity tolerance, the development of several important osmoregulatory tissues (the skin, gut, and gill), and the influence of the endocrine system on osmoregulation during early larval development and metamorphosis.

Metamorphosis in the summer flounder, Paralichthys dentatus: thyroidal status influences gill mitochondria-rich cells

Metamorphosis in the summer flounder (Paralichthys dentatus) is mediated by thyroid hormones (TH) and is accompanied by changes in gill mitochondria-rich cells (MRCs) and in salinity tolerance. Altered thyroid status during larval development and metamorphosis in this species influences salinity tolerance, though the influence of any hormone on MRCs of larval marine teleosts is not known. This study characterized the effect of altered thyroid status on MRC intracellular membranes, mitochondria size and ultrastructure, immunoreactive (ir)-Na(+),K(+)-ATPase, and cell size and density during metamorphosis in summer flounder. Inhibition of metamorphosis with thiourea (30 ppm) (TU, an inhibitor of TH synthesis) inhibited changes in MRCs, producing large "larval" type MRCs with weak reactivity to osmium; large, electron-lucent mitochondria; and weak ir-Na(+),K(+)-ATPase. Replacement of TH with TU + thyroxine-Na salt (100 ppb) rescued the fish from developmental inhibition, producing smaller "juvenile" type MRCs with strong reactivity to osmium; smaller, electron-opaque mitochondria; and strong ir-Na(+), K(+)-ATPase. The findings suggest that TH are necessary for MRCs to change from larval to juvenile form during metamorphosis.

In vivo sequential changes in chloride cell morphology in the yolk-sac membrane of mozambique tilapia (Oreochromis mossambicus) embryos and larvae during seawater adaptation

Changes in chloride cell morphology were examined in the yolk-sac membrane of Mozambique tilapia (Oreochromis mossambicus) embryos and larvae transferred from fresh water to sea water. By labelling chloride cells with DASPEI, a fluorescent probe specific for mitochondria, we observed in vivo sequential changes in individual chloride cells by confocal laser scanning microscopy. In embryos transferred from fresh water to sea water 3 days after fertilization, 75 % of chloride cells survived for 96 h, and cells showed a remarkable increase in size. In contrast, the cell size did not change in embryos and larvae kept in fresh water. The same rate of chloride cell turnover was observed in both fresh water and sea water. Using differential interference contrast (DIC) optics and whole-mount immunocytochemistry with anti-Na(+)/K(+)-ATPase, we classified chloride cells into three developmental stages: a single chloride cell without an apical pit, a single chloride cell with an apical pit, and a multicellular complex of chloride and accessory cells with an apical pit. DIC and immunofluorescence microscopy revealed that single chloride cells enlarged and were frequently indented by newly differentiated accessory cells to form multicellular complexes during seawater adaptation. These results indicate that freshwater-type single chloride cells are transformed into seawater-type multicellular complexes during seawater adaptation, suggesting plasticity in the ion-transporting functions of chloride cells in the yolk-sac membrane of tilapia embryos and larvae.

Metamorphosis in the summer flounder Paralichthys dentatus: changes in gill mitochondria-rich cells

Salinity tolerance changes during larval development and metamorphosis in the summer flounder (Paralichthys dentatus) and other teleosts. The physiological mechanisms responsible for osmoregulation during these early stages of development are not well understood. This study characterized changes in ultrastructure, intracellular membranes and immunoreactive Na(+)/K(+)-ATPase of mitochondria-rich cells (MRCs) in the gills of summer flounder during metamorphosis. Gill ultrastructure at the start of metamorphosis revealed only one type of MRC, which had weak reactivity to osmium and lacked a well-defined apical pit. In juveniles, two types of MRCs were observed: light-staining MRCs (LMRCs) with weak reactivity to osmium, and dark-staining MRCs (DMRCs) with strong reactivity to osmium and positioned adjacent to LMRCs. Compared with MRCs at the start of metamorphosis, the mitochondria of juvenile MRCs appeared smaller, with more transverse cristae and electron-dense matrices. Changes in MRCs during metamorphosis were also accompanied by increased immunoreactive Na(+)/K(+)-ATPase. These findings suggest that gill MRCs develop during the metamorphosis of summer flounder as the gill takes on an increasingly important osmoregulatory role.

Metamorphosis in the summer flounder, Paralichthys dentatus: thyroidal status influences salinity tolerance

Metamorphosis in the summer flounder (Paralichthys dentatus) is controlled by thyroid hormones (TH) and takes place as the larvae move from a salinity of about 35 parts per thousand (ppt) in the ocean to salinity ranging from 0-35 ppt in estuaries. Historically, the role of TH in juvenile and adult teleost osmoregulation has been ambiguous, and it is not known if TH influences larval teleost osmoregulatory development. This study addresses the influence of thyroxine (T4) on the development of tolerance to low (5 ppt) and high salinity (45 and 50 ppt) as determined by salinity tolerance tests. In untreated larvae, tolerance to both low and high salinity was high during early premetamorphosis (early pre-M) and decreased or was very low from late prometamorphosis (late pro-M) through mid-metamorphic climax (mid-MC). Salinity tolerance increased 2-3-fold during late MC when whole-animal T4 levels are highest, and reached maximum tolerance at the juvenile stage. The early induction of metamorphosis by exposing larvae in pre-M to exogenous T4 reduced tolerance to low salinity during early and mid-MC, though tolerance of fish that had developed into juveniles was not impaired. In contrast, T4 increased high salinity tolerance during early and mid-MC, and the juvenile stage. This T4-induced heterochrony in salinity tolerance with regards to developmental stage suggests that the effects of T4 on salinity tolerance may be uncoupled from accelerated metamorphosis. Treatment of larvae with thiourea (TU, an inhibitor of T4 synthesis) inhibited metamorphosis and reduced tolerance to high salinity, but did not affect tolerance to low salinity. Reduced tolerance to high salinity by TU was only partially counteracted by T4 treatment, suggesting that TU also affects hypoosmoregulatory activity by an extrathyroidal mechanism. Our findings suggest that in the summer flounder T4 plays a more important role in the development of hypoosmoregulatory ability than hyperosmoregulatory ability. J. Exp. Zool. 284:414-424, 1999.

Effect of low ambient mineral concentrations on the accumulation of calcium, magnesium and phosphorus by early life stages of the air-breathing armoured catfish Megalechis personata (Siluriformes: Callichthyidae)

The accumulation of calcium, magnesium and phosphorus was measured during an 8-week period in the early life stages of the air-breathing armoured catfish Megalechis personata acclimated to low-mineral fresh water (0.073 mmol l-1 calcium, 0.015 mmol l-1 magnesium, &lt;0.001 mmol l-1 phosphate) and high-mineral fresh water (0.59 mmol l-1 calcium, 1.94 mmol l-1 magnesium, &lt;0.001 mmol l-1 phosphate). The fish accumulated calcium twice as fast and phosphorus 1.5 times as fast in low-mineral fresh water (LMF) as in high-mineral fresh water (HMF), while the rate of accumulation of magnesium did not differ in LMF and HMF. The difference in the rates of accumulation of calcium and phosphorus between LMF and HMF was independent of the growth performance (food intake) in LMF and HMF. The mineral content of young M. personata from natural swamps and rainforest creeks in Suriname followed the LMF accumulation curves. The transition from aquatic respiration to bimodal respiration in the third week after hatching did not affect rates of mineral accumulation. The high rates of accumulation of calcium and magnesium of M. personata in LMF of 654 and 58 micromol h-1 kg-1, respectively, exceed the rates of uptake of calcium and magnesium of teleosts reported in the literature. The high rates of mineral accumulation in the early life stages of M. personata reflect the exponential growth during the first 8 weeks after hatching and the requirements of the juveniles while building their dermal armour. M. personata is well-adapted to neotropical fresh waters with an extremely low mineral content. The accumulation of calcium and phosphorus is discussed in relation to the function of the bony armour of M. personata.

Presence of Na-K-ATPase in mitochondria-rich cells in the yolk-sac epithelium of larvae of the teleost Oreochromis mossambicus

The purpose of this study is to provide biochemical evidence for the functions of the mitochondria-rich cell (MR cell) in the yolk-sac epithelium of the developing larvae of tilapia Oreochromis mossambicus. Western blotting with the antibody (6F) raised against avian Na-K-ATPase alpha1 subunit demonstrated the presence of Na-K-ATPase in yolk-sac epithelium of tilapia larvae and about 1. 46-fold more of the enzyme in seawater larvae than in freshwater ones. The yolk-sac MR cells were immunoreacted to the antibody (alpha5) against the alpha subunit of avian Na-K-ATPase and were double-labeled with anthroylouabain and dimethylaminostyrylethyl-pyridiniumiodine, suggesting the existence and activity of Na-K-ATPase in these cells. Binding of 3H-ouabain in the yolk sac of seawater larvae was much higher than in that of freshwater larvae (4.183+/-0.143 pmol/mg protein versus 1.610+/-0. 060 pmol/mg protein or 0.0508+/-0.0053 pmol/yolk sac versus 0. 0188+/-0.0073 pmol/yolk sac). These biochemical results are further evidence that yolk-sac MR cells are responsible for a major role in the osmoregulatory mechanism of early developmental stages before the function of gills is fully developed.

Partial cloning of the hormone-binding domain of the cortisol receptor in tilapia, Oreochromis mossambicus, and changes in the mRNA level during embryonic development

Cortisol is one of the central hormones in osmoregulation in fish, especially in seawater adaptation. A cDNA of 453 bp was cloned from liver mRNA of freshwater-reared tilapia (Oreochromis mossambicus), by reverse transcription polymerase chain reaction (RT-PCR) with primers designed for the hormone-binding domain of glucocorticoid receptors (GRs) in mammals and rainbow trout. The sequence of PCR product has 83% homology to the trout GR at the nucleotide level and 92% at the amino acid level. The PCR product of tilapia showed highest homology (74% at the amino acid level) to GR among human steroid hormone receptors, including mineralocorticoid receptor. The length of the receptor mRNA of tilapia was about 6.5 kb as determined by Northern blot hybridization. The mRNA concentration in the gills was relatively higher among various organs, the highest concentration being observed in blood cells. Signal intensity of the receptor message in the gills was stronger in fish reared in freshwater than in those reared in seawater or in concentrated (160%) seawater. During early development of tilapia, the highest concentration of receptor mRNA in the total RNA extracted from the whole egg was found just after fertilization, and its concentration decreased steadily toward hatching. The absolute amount of receptor mRNA per egg increased gradually before the initiation of cortisol production by the embryo. When embryos were transferred from fresh water to seawater 2 days before hatching, no difference was observed in the signal intensity of the receptor mRNA among embryos after 1, 2 (the day of hatching), 4, and 7 days.

Mitochondria-rich cells in the branchial epithelium of the teleost,Oreochromis mossambicus, acclimated to various hypotonic environments

Branchial mitochondria-rich (MR) cells were examined on the afferent side of gill filaments in tilapia (Oreochromis mossambicus) acclimated to different hypotonic environments, local fresh water (LFW), hard fresh water (HFW) and 5‰ salt water (SW). Scanning electron micrographs (SEM) identified three types of apical surfaces of the MR cells, wavy convex, shallow basin and deep hole. In spite of the different types of apical surfaces, light microscopic (LM) and transmission electron microscopic (TEM) studies suggested that these cells were MR cells. The relative abundance of these 3 types of branchial MR cells varied with external hypotonic milieus. Wavy-convexed MR cells were dominant in the gills of fish adapted to HFW, whereas shallow-basined MR cells were evident in LFW-adapted fish. In SW-adapted fish, most of the MR cells were deep holes. Experiments on adaptation to various hypotonic milieus revealed that the changes of the branchial MR cells were reversible and occurred within 24 hours following transfer. The morphological alterations of the MR cells correlated with ionic changes in different milieus, indicating that these distinct types of MR cells may play key roles for osmoregulation in hypotonic media.

Calcium balance in embryos and larvae of the freshwater-adapted teleost, Oreochromis mossambicus

Changes in Ca(2+) content and flux, and the development of skin chloride cells in embryos and larvae of tilapia, Oreochromis mossambicus, were studied. Tilapia embryos hatched within 96h at an ambient temperature of 26-28°C. Total body Ca(2+) content was maintained at a constant level, about 4-8 nmol per individual, during embryonic development. However, a rapid increase in body Ca(2+) level was observed after hatching, 12.8 to 575.3 nmol per individual from day 1 to day 10 after hatching. A significant influx and efflux of Ca(2+) occurred during development, with the average influx rate for Ca(2+) increasing from 5.9 pmol mg(-1) h(-1) at 48h postfertilization to 47.8 pmol mg(-1) h(-1) at 1 day posthatching. The skin was proposed as the main site for Ca(2+) influx before the development of gills, and the increased Ca(2+) influx may be ascribed to gradual differentiation of skin surface and chloride cells during embryonic development. Ca(2+) efflux was 16-56 pmol mg(-1) h(-1) in 1-day-old larvae. The resulting net influx of Ca(2+), 10-12 pmol mg(-1) h(-1), accounted for the increased Ca(2+) content after hatching. When comparing the measured and estimated ratios of efflux and influx, active transport was suggested to be involved in the uptake of Ca(2+). Chloride cells, which may be responsible for the active uptake of Ca(2+), started to differentiate in the skin of embryos 48h after fertilization, and the density of chloride cells increased following the development. A possibility of active transport for Ca(2+) in early developmental stages of tilapia is suggested.

Hypoosmoregulation of larvae of the turbot, Scophthalamus maximus: drinking and gut function in relation to environmental salinity

Measurement of blood osmolarity of pre-metamorphic turbot larvae demonstrated that hypoosmoregulation is well established in larvae 6 days post-hatch (121 degree-days) and older. Blood osmolarity of 121-420 degree-day larvae reared in 100% seawater was significantly greater than blood osmolarity of larvae reared in 50% seawater. Hypoosmoregulation involved drinking, but instantaneous drinking rates of 340 degree-day larvae reared in 100% seawater were only slightly more than those of similarly aged larvae reared in 50% seawater. Adaptation to environmental salinity involved changes in gut water absorption; 65-74% water absorption occurred in larvae reared in seawater compared to 30-35% water absorption in larvae reared in 50% seawater. Gastrointestinal water absorption occurred prior to the rectum. In seawater this occurred alongside a decrease in gut fluid osmolarity but desalting was apparently less significant than in adult fish. Absolute water absorption by the gut of 340 degree-day larvae reared in seawater was about 2-fold that of larvae held in 50% seawater, while the osmotic gradient between internal body fluids and environmental media differed by 4-fold, which implies changes the in water permeability of skin and/or developing gills.