Effects of salinity on chloride cells in the euryhaline cyprinodontid fish Rivulus marmoratus

Summary The ultrastructure and density of chloride cells in the gill, opercular epithelium, and opercular skin of the euryhaline self-fertilizing fish Rivulus marmoratus (Cyprinodontidae) were studied with electron and fluorescence microscopy. R. marmoratus raised from birth in 1, 50, 100, and 200% seawater were compared. Chloride cells from fish raised in each of the four salinities exhibited an invaginated pit structure at the apical crypt. Multicellular complexes were present in the 1% seawater group and in those fish raised in higher salinities where elaborate interdigitations were seen between cells. Chloride cells from gills of fish raised in 200% seawater had a significantly higher percentage of their cytoplasmic volume composed of mitochondria than did those from fish raised in 1% seawater (69.9% vs 37.4%). The opercular skin and opercular epithelium had the same density of chloride cells (4.2×10⁴-4.5×10⁴ chloride cells/cm²), and this number did not vary significantly with increased salinity. The opercular skin thus appears far more responsive to environmental salinity than the opercular epithelium. Chloride cells from the opercular epithelium of fish raised in 200% seawater were found to be 39% larger than those from fish raised in 1% seawater, whereas the chloride cells from the opercular skin of the 200% seawater group were 107% larger than those from the 1% seawater group.     

Degeneration and death,by apoptosis and necrosis,of the pavement and chloride cells in the gills of the teleost Oreochromis mossambicus

Summary Degeneration and death of branchial epithelial cells were studied in an African cichlid fish. In both freshwater and seawater fish the superficially located pavement cells are sloughed off at the end of their lifecycle. This process is preceded by degeneration via a process of cytoplasmic shrinkage and condensation related to apoptotic (physiologically controlled) cell death. The chloride cells are pleomorphic, i.e., accessory, mature, and degenerating cells. Degeneration of chloride cells mainly occurs by apoptosis. Degenerating cells show shrinkage and densification of cytoplasm and nuclei, and swelling of the tubular system; these cells are then separated from the ambient water by pavement cells. They are finally phagocytosed and digested by macrophages. Apoptosis of chloride cells, but not of pavement cells, is greatly stimulated when the fish are in seawater; this reflects an increase in cellular turnover of the chloride cells. Accidental cell death (necrosis) of pavement cells or chloride cells is rarely observed in fully adapted freshwater and seawater fish. Its incidence increases in the first few days following transfer of fish from fresh water to seawater.     

Ultrastructure and distribution dynamics of chloride cells in tilapia larvae in fresh water and sea water

Integumental and branchial chloride cells of tilapia larvae (Oreochromis mossambicus) were studied at the light-microscopical and ultrastructural level. Total numbers and distribution of chloride cells were quantified after immunostaining of cross sections of the entire larvae with an antibody against the alpha-subunit of Na+/K+-ATPase. The majority (66%) of Na+/K+-ATPase-immunoreactive (ir) cells, i.e. chloride cells, of freshwater tilapia larvae were located extrabranchially up to 48 h after hatching. Five days after hatching, the majority (80%) of chloride cells were found in the buccal cavity. Transfer of 24-h-old larvae to 20% sea water speeded up this process; 24 h after transfer (i.e. 48 h after hatching), the majority (59%) of chloride cells were located in the buccal cavity. The branchial chloride cell population of 24-h- and 120-h-old larvae consisted of immature, mature, apoptotic and necrotic chloride cells. However, relatively more immature chloride cells were observed in freshwater larvae (42-63%) than in (previously studied) freshwater adults (21%), illustrating the developmental state of the gills. After transfer to sea water, the incidence of degenerative chloride cells did not change. Furthermore, the incidence of immature cells had decreased and a new subtype of chloride cells, the "mitochondria-poor" cells, appeared more frequently. These mitochondria-poor chloride cells were characterised by an abundant tubular system and relatively few mitochondria, which were aligned at the border or concentrated in one part of the cytoplasm. Most of these cells did not contact the water. The function of their enhanced appearance after seawater transfer is unknown.     

Effects of spironolactone and RU486 on gene expression and cell proliferation after freshwater transfer in the euryhaline killifish

We have explored the possible mechanisms by which mineralocorticoid (MR) and glucocorticoid (GR) receptors regulate the response to freshwater transfer in the gills of the euryhaline killifish Fundulus heteroclitus. Killifish were implanted with RU486 (GR antagonist) or spironolactone (MR antagonist) at doses of 0.1–1.0 mg g⁻¹, and subsequently transferred from 10‰ brackish water to freshwater. Compared to brackish water sham fish, mRNA expression of CFTR and NKCC1 decreased in the gills of sham fish transferred to freshwater, whereas Na⁺,K⁺–ATPase α_1a mRNA expression and α protein abundance, as well as cell proliferation (detected using BrdU) increased. Spironolactone inhibited the normal increase in cell proliferation and Na⁺,K⁺-ATPase expression after freshwater transfer. RU486 increased plasma cortisol levels and may have slightly inhibited Na⁺,K⁺–ATPase activity, but did not change α _1a expression. RU486 had no effect on cell proliferation in the non-lamellar region of the gills, but increased proliferation in the lamellar region. Neither antagonist inhibited the suppression of CFTR or NKCC1 expression after freshwater transfer. Glucocorticoid receptor expression was reduced in all sham and antagonist treatments compared to untreated controls, but no other consistent differences were observed. The effects of spironolactone suggest that MR is important for regulating ion transport in killifish gills after freshwater transfer.     

Changes in the levels of chloride cells and (Na+ + K+)-dependent ATPase in the gills of yellow and silver eels adapting to seawater.

Changes were measured in the numbers of chloride cells and the levels of (Na+ + K+)-DEPENDENT ATPase in the gills of immature, yellow eels and mature, silver eels during adaptation from freshwater to seawater. The percentage of chloride cells in yellow eels more than doubled after six days in seawater; at this time the specific activity and concentration of (Na+ + K+)-dependent ATPase in gills start to increase in parallel to reach maxima after two weeks that are 2.5 times the starting values. It is concluded that adaptation of yellow eels to seawater involves an increase in the numbers of chloride cells in gills as well as an increased amount of (Na+ + K+)-dependent ATPase per chloride cell. Mature silver eels in freshwater had essentially the same numbers of chloride cells and the same specific activity of the enzyme in the gills as yellow eels fully adapted to seawater. Transferring silver eels to seawater did not alter the percentage of chloride cells in gills although the level of (Na+ + K+)-dependent ATPase and its specific activity increased slightly. Thus, although the silver eel is better prepared for life in seawater than the yellow eel, it still has to attain an increased level of (Na+ + K+)-dependent ATPase in its chloride cells to be fully adapted to seawater.     

Changes in the ultrastructure of the gills of the river lamprey,Lampetra fluviatilis (L.), during the anadromous spawning migration

Summary Two types of mitochondria-rich cells were found in the interplatelet areas of the gills of the migrating river lamprey. Both cell types are thought to be responsible for ion-transport across the gills. In the fresh-run migrant the gills are dominated by large, flask-shaped cells which show some ultra-structural similarities with the teleost chloride cell and have been tentatively referred to as ion-excretory cells. During the spawning migration the ion-excretory cells are replaced by smaller, mitochondria-rich cells which are similar in structure to the presumed ion-transporting cells in the ammocoete gill. They lack the tubular, smooth-membraned endoplasmic reticulum so characteristic of the lamprey ion-excretory cell and the teleost chloride cell and have been referred to as ion-uptake cells. The ion-uptake cells are found during the stenohaline, freshwater phases of the lamprey's life history. Ion-excretory cells are present during the periods of the life cycle when the animal is euryhaline. The possibility that the ion-excretory cells are also responsible for ion-uptake in fresh water is discussed.     

Mitochondria-rich cells in gills and skin of an African lungfish, Protopterus annectens

We used scanning electron microscopy, the vital dye DASPEI and an antibody to the inner mitochondrial membrane to study the presence and localisation of mitochondria-rich cells in the gills and skin (opercular, dorsal and ventral) of the lungfish Protopterus annectens in its free-swimming conditions and at the beginning of aestivation. In the free-swimming period, the gills were short and thick and the pavement cells were extremely large (30-40 microns). The mitochondria-rich cells, which were distributed in the secondary and primary epithelium, occurred as two morphologically different types, i.e. elongated and oval, similar to the alpha and beta chloride cells of fresh water teleosts. In the skin, only one type of mitochondria-rich cells was found, resembling the alpha chloride cells. All the mitochondria-rich cells distributed in the gills and skin were labelled with anti Ca(2+)-ATPase serum indicating the possible uptake of Ca2+ at freshwater chloride cell level. At the start of aestivation, the skin and gills were covered by a thick layer of mucus and the epithelium of the gills was reduced. The mitochondria-rich cells were almost completely covered by the pavement cells.     

Appearance of cuboidal cells in relation to salinity in gills of Fundulus heteroclitus, a species exhibiting branchial Na+ but not Cl− uptake in freshwater

Fundulus heteroclitus (killifish) is a model organism for ionoregulatory studies, particularly because of its opercular epithelium, although the gills are the major sites of ion exchange. Whereas Na⁺ and Cl⁻ are excreted through the gills in seawater (SW), the killifish is unusual in taking up only Na⁺ and not Cl⁻ at the gills in freshwater (FW). We describe morphological changes in the branchial epithelium following transfer from an acclimation medium of 10% SW to 100% SW or FW. In 10% SW, mitochondria-rich cells resemble typical seawater chloride cells (SWCCs) with accessory cells. After transfer to 100% SW, no change occurs in pavement cell (PVC) morphology or mitotic rate (measured by bromo-deoxyuridine technique), although the density of SWCC apertures increases several fold because of the uncovering of buried SWCCs by PVCs, in accord with increased rates of Na⁺ and Cl⁻ efflux. After transfer to FW, PVC morphology remains unchanged, but SWCCs and accessory cells are quickly covered by PVCs, with many undergoing apoptosis or necrosis. The mitotic rate doubles by 10–14 h but typical freshwater chloride cells (FWCCs) do not appear. Instead, a wedge-shaped cell type that is moderately rich in apically oriented mitochondria, with a large ovoid nucleus, thin cytoplasmic layer, paucity of vesicular-tubular network, and variably villous surface rapidly (by 3 h) and progressively appears in the filament epithelium, by both uncovering and mitosis. This cell type is similar to that recently identified as the site of Na⁺ uptake in the FW trout gill. We propose the new term “cuboidal cell” for this cell, based on its morphology, to avoid confusion with traditional terminology (of PVC). We hypothesize that the cuboidal cells are the sites of active Na⁺ uptake in FW F. heteroclitus and suggest that the lack of Cl⁻ uptake is attributable to the absence of typical FWCCs previously described in teleosts.This work was supported by NSERC Discovery grants (to C.M.W.) and by an NSERC International Fellowship (to P.L.). C.M.W. is supported by the Canada Research Chair Program.     

Changes in osmotic water permeability of the eel gills during seawater and freshwater adaptation

Summary Changes in osmotic water permeability of the isolated gills of the Japanese eel,Anguilla japonica were studied during transfer to seawater or to fresh water. The water permeability increased gradually during the course of seawater transfer and attained a maximal level after 2 weeks. The water permeability of the freshwater eel gills was reduced when calcium ions were added to the incubation medium at a concentration of 1 mM, where-as no effect of the ion was observed on the gills of the seawater-adapted eel even at a higher concentration (10 mM). In contrast to seawater transfer, the water permeability decreased to a low level almost immediately (3 h) after transfer from seawater to fresh water. The acute reduction of the water permeability was also seen in the gills of the hypophysectomized eel after transfer to fresh water.The gradual increase in the gill water permeability during seawater transfer is correlated with an increase in the number of chloride cells. In scanning electron microscopy, chloride cells of seawater-adapted eel gills exhibit a pit-like structure, which was larger than in the freshwater eel. On transfer from seawater to fresh water, the pit diameter became smaller within 6 h. Hypophysectomy did not affect the change in gill surface structures during transfer to fresh water. The junctions between the chloride cells of seawater eel gills are reported to be of the leaky type. The parallel change in osmotic water permeability and in pit size of the chloride cells during seawater or freshwater transfer or after hypophysectomy suggests that these cells could provide a major route of water as well as ion movement.This paper is a portion of a thesis presented to Hokkaido University by t. Ogasawara in partial fulfilment of the requirements for Doctor of Fisheries     

Structure and Function of the Chloride Cell of Fundulus heteroclitus and Other Teleosts

Teleosts, the bony fishes, inhabit both freshwater and seawaterenvironments. Some euryhaline fish, such as Fundulus heteroclitus,alternate between the two milieux several times daily. Regardlessof adaptation, the gills of these animals possess a highly specializedcell type called the chloride cell. This cell contains numerousmitochondria and exhibits a greatly amplified basolateral cellsurface richly endowed with Na,K-ATPase. Recent studies on isolatedopercular epithelia containing chloride cells have demonstratedactive chloride secretion and passive transepithelial sodiummovements, and have established the chloride secretory roleof this cell type in seawater-adapted teleosts. Current modelssuggest that chloride transport occurs via a transcellular route.Seawater chloride cells exist in multicellular units and sharesimple, shallow tight junctions which are thought to be theroute for passive sodium movement. Freshwater chloride cells,whose function remains to be elucidated, are generally describedas existing in a unicellular configuration. However, recentobservations in Fundulus heteroclitus adapted to salinitiesas low as 1% sea water reveal that chloride cells persist inmulticellular complexes with apical crypts. Strikingly, tightjunctions between chloride cells in this freshwater environmentare deep     

Fluorescent labelling of Na+,K+-ATPase in intact cells by use of a fluorescent derivative of ouabain: Salinity and teleost chloride cells

Summary Anthroylouabain, a fluorescent derivative of ouabain, was used to localize Na⁺,K⁺-ATPase in transport epithelia of two species of teleosts. Exposure of the opercular membrane of seawater-adapted tilapia (Oreochromis mossambicus) and the jaw skin of the long-jawed mudsucker (Gillichthys mirabilis) to a 2 M anthroylouabain solution resulted in the appearance of cells stained bright blue. These were deemed to be chloride cells by their large size, distinct morphology and co-localization of DASPEI fluorescence, a mitochondrial stain. Addition of ouabain (1 mM final concentration) greatly decreased anthroylouabain fluorescent staining of chloride cells of seawater-adapted fish. Exposure of opercular membranes from freshwater tilapia to 2 M anthroylouabain did not result in significant staining. Anthroylouabain is therefore a useful vital stain for localizing Na⁺,K⁺-ATPase in chloride cells of seawater-adapted teleosts, and may be useful for fluorescent labelling of ouabain-sensitive Na⁺,K⁺-ATPase in other tissues and species.     

Response of PAS-positive cells of the pituitary pars intermedia in the teleost Carassius auratus to acid water

Summary The PAS-positive or PIPAS cells in the pars intermedia of goldfish are activated after reduction of the pH of the ambient freshwater from 7.5 to 3.5. The cells increase in number and exhibit a five-fold increase in cell volume. Granular endoplasmic reticulum occupies most of the cytoplasm. Goldfish PIPAS cells (also termed calciumsensitive cells) are thought to have a hypercalcemic function. Therefore, their activation in acid water may be caused by the severe drop in plasma calcium concentration following exposure of the fish to low water pH. However, activation of the PIPAS cells in response to acidification of the water is not prevented when the calcium concentration of the water is increased to levels that result in hypercalcemia instead of hypocalcemia. Activation of the PIPAS cells occurs also in fish exposed to acidified freshwater enriched with NaCl to an osmolarity similar to that of the blood. This prevents the reduction in plasma osmolarity and Na⁺ and Cl^- concentrations that follow exposure of goldfish to acidified normal freshwater. Our observations do not support the hypothesis that the PIPAS cells in goldfish produce a hypercalcemic hormone, or indeed any hormone involved in calcium metabolism or osmoregulation. The cells may be implicated in acid-base regulation (a characteristic of many types of fish when exposed to acidified water) but the evidence is indirect.     

Ultrastructural features of chloride cells in the gill epithelium of the Atlantic salmon, Salmo salar, and their modifications during smoltification

To elucidate the ultrastructural modifications of the gill epithelium during smoltification, gills of the Atlantic salmon (Salmo salar) were examined by electron microscopy at three stages of this process, which were defined as follows: "parrs" were freshwater fish that had not yet started their transformation; "freshwater smolts" were freshwater fish that were ready to enter seawater; and "seawater smolts" were smolts that had been transferred from fresh water and maintained for 4 days in seawater (35%). In the gill epithelium of parrs, there were two types of chloride cells. The large chloride cells contained deeply stained mitochondria and numerous apical, irregular, dense, membrane-bound bodies that formed 77% of the chloride cell population and were distinguished easily from small chloride cells that have distinctly paler mitochondria and no dense bodies in their apical cytoplasm. In freshwater smolts, the large chloride cells formed 95% of the chloride-cell population. In contrast to the small chloride cells that were not modified, they almost doubled in size. Their tubular system developed extensively to form a tight network with regular meshes significantly smaller than those observed in parr chloride cells. Forty percent of the large chloride cells were associated with a new type of cell, the accessory cell, to which they were bound by shallow apical junctions. Half of these accessory cells were not seen to be in contact with the external medium. In seawater smolts, 80% of the large chloride cells were associated with accessory cells. Most accessory cells reached the external medium and sent numerous cytoplasmic interdigitations within the apical portion of the adjacent chloride cells. As a result, a section through the apical portion of the chloride cells and their associated accessory cells revealed a mosaic of interlocked cell processes bound together by an extended, shallow apical junction. It was concluded that the Atlantic salmon develops in fresh water most of the ultrastructural modifications of the gill epithelium which in most euryhaline fish are triggered by exposure to seawater. The effective transfer into seawater would act only as a final stimulus to achieve some adequacy between the freshwater smolt and its new environment.     

Recruitment and degeneration of mitochondrion-rich cells in the gills of Mozambique tilapia Oreochromis mossambicus during adaptation to a hyperosmotic environment

Cellular recruitment and degeneration of branchial mitochondrion-rich (MR) cells were examined in Mozambique tilapia transferred from hypoosmotic to hyperosmotic water. To examine apoptosis in the gills associated with salinity change, tilapia were directly transferred from freshwater to 70% seawater. The TUNEL assay showed that apoptotic cells in the gills were significantly increased at 1 day after transfer, which was supported by an electron-microscopic observation that gill MR cells underwent morphological changes characteristic of apoptosis such as an irregularly shaped electron-dense nucleus and distension of the tubular system. To further examine MR-cell recruitment, freshwater-acclimated tilapia were transferred either to freshwater or to 70% seawater after BrdU injection. Immunohistochemical detection of BrdU-labeled nuclei and Na(+)/K(+)-ATPase-rich MR cells allowed us to classify BrdU-labeled MR cells into two subtypes: a single MR cell and an MR-cell complex. Although newly generated single MR cells were observed similarly in both freshwater and 70% seawater-transferred fish, the density of MR-cell complexes was much higher in 70% seawater than in freshwater. Our findings indicated that transfer from hypoosmotic to hyperosmotic water enhanced apoptosis of freshwater-type MR cells, resulting in reduction in hyperosmoregulatory ability for freshwater adaptation, and stimulated the recruitment of MR-cell complexes to develop hypoosmoregulatory ability for seawater adaptation.     

The effects of aluminum and acid on the gill morphology in rainbow trout,Salmo gairdneii

Synopsis The objective was to determine the effects of acid and aluminum in acidified hard and soft water on the histology and morphometry of rainbow trout gills, and to determine relevant toxicity indicators within the gill tissue. Acid and aluminum promoted measurable primary epithelial hyperplasia which proved to be a reliable biological indicator of acid and aluminum contamination and possibly of some predictive value. Low levels of aluminum and acid resulted in hypertrophied chloride cells, suggesting a role in adapting to the contaminants. High concentrations of aluminum (>10 molI^-1) caused chloride cell necrosis and consequently a decline in cell numbers over time. Aluminum precipitates accumulating within the chloride cell cytoplasm probably lead to impaired function prior to cell degeneration. The morphological alterations resulted in a decrease in water space between secondary lamellae (up to 40% within 14 d) which may reduce the efficiency of gas exchange. Twice the aluminum was required in hard water to elicit a similar soft water tissue response. Pathological changes were more severe with aluminum at pH 5.2 than at pH 4.7; results of aluminum speciation suggest that both labile and non-labile fractions are responsible for the induction of gill lesions. Low levels of aluminum may protect fish from the effects of high hydrogen ion concentration.     

Epithelial remodeling and claudin mRNA abundance in the gill and kidney of puffer fish (<Emphasis Type="Italic">Tetraodon biocellatus</Emphasis>) acclimated to altered environmental ion levels

In water of varying ion content, the gills and kidney of fishes contribute significantly to the maintenance of salt and water balance. However, little is known about the molecular architecture of the tight junction (TJ) complex and the regulation of paracellular permeability characteristics in these tissues. In the current studies, puffer fish (Tetraodon biocellatus) were acclimated to freshwater (FW), seawater (SW) or ion-poor freshwater (IPW) conditions. Following acclimation, alterations in systemic endpoints of hydromineral status were examined in conjunction with changes in gill and kidney epithelia morphology/morphometrics, as well as claudin TJ protein mRNA abundance. T. biocellatus were able to maintain endpoints of hydromineral status within relatively tight limits across the broad range of water ion content examined. Both gill and kidney tissue exhibited substantial alterations in morphology as well as claudin TJ protein mRNA abundance. These responses were particularly pronounced when comparing fish acclimated to SW versus those acclimated to IPW. TEM observations of IPW-acclimated fish gills revealed the presence of cells that exhibited the typical characteristics of gill mitochondria-rich cells (e.g. voluminous, Na⁺-K⁺-ATPase-immunoreactive, exposed to the external environment at the apical surface), but were not mitochondria-rich. To our knowledge, this type of cell has not previously been described in hyperosmoregulating fish gills. Furthermore, modifications in the morphometrics and claudin mRNA abundance of kidney tissue support the notion that spatial alterations in claudin TJ proteins along the nephron of fishes will likely play an important role in the regulation of salt and water balance in these organisms.     

Mechanisms of Salinity Control in Sea Bass

Sea bass can regulate the concentration of Na⁺, K⁺, and Cl^-, among other ions, in their blood, skin, gills, and kidney. Therefore, the salinity of the water does not have a great influence on their metabolism, and sea bass can live in both sea and freshwater in accordance with the salt concentration. Most salinity control occurs in the gills, primarily through the control of chloride cells present there. The concentration of ions in the blood is controlled by the cotransporter Na⁺ / K⁺ / 2Cl^- (NKCC) in the chloride cell, and the subunits of Na⁺ / K⁺ ATPase (NKA) function to maintain homeostasis. The expression of NKA is regulated by subunits of the protein FXYD, allowing the sea bass to survive in compliance with the salinity. In this way, it is possible for sea bass to live in sea and freshwater by controlling the salinity of its body using functions of various channels, proteins, and genes present in the chloride cells of sea bass. In this study, we investigated recent studies of salt control mechanisms in sea bass and their application.     

Chloride transport activation by plasma osmolarity during rapid adaptation to high salinity of Fundulus heteroclitus

Transition from low salt water to sea water of the euryhaline fish, Fundulus heteroclitus, involves a rapid signal that induces salt secretion by the gill chloride cells. An increase of 65 mOsm in plasma osmolarity was found during the transition. The isolated, chloridecell-rich opercular epithelium of sea-water-adapted Fundulus exposed to 50 mOsm mannitol on the basolateral side showed a 100% increase in chloride secretion, which was inhibited by bumetanide 10^–4 m and 10^–4 m DPC (N-Phenylanthranilic acid). No effect of these drugs was found on apical side exposure. A Na⁺/H⁺ exchanger, demonstrated by NH₄Cl exposure, was inhibited by amiloride and its analogues and stimulated by IBMX, phorbol esters, and epithelial growth factor (EGF). Inhibition of the Na⁺/H⁺ exchanger blocks the chloride secretion increase due to basolateral hypertonicity. A Cl^–/HCO ₃ ^– exchanger was also found in the chloride cells, inhibited by 10^–4 m DIDS but not involved in the hyperosmotic response. Ca²⁺ concentration in the medium was critical for the stimulation of Cl^– secretion to occur. Chloride cell volume shrinks in response to hypertonicity of the basolateral side in sea-water-adapted operculi; no effect was found on the apical side. Freshwater-adapted fish chloride cells show increased water permeability of the apical side. It is concluded that the rapid signal for adaptation to higher salinities is an increased tonicity of the plasma that induces chloride cell shrinkage, increased chloride secretion with activation of the Na⁺K⁺2Cl^– cotransporter, the Na⁺/H⁺ exchanger and opening of Cl^– channels.The work was supported by the National Institutes of Health, Research Grant EYO1340 to J.A.Z. Part of this research was performed while Dr. Zadunaisky was a Scholar In Residence at the Fogarty International Center of The National Institutes of Health in Bethesda, Maryland. Ms. Dawn Roberts was a fellow of the Grass Foundation and Pew Foundation during this work. Grants from the National Science Foundation and the National Institutes of Health to the Mount Desert Island Biological Laboratory also provided assistance for this research.     

Morphofunctional changes of the interrenal gland and of ultrastructure of chloride cells in intact and hypophysectomized juveniles of starred sturgeon <Emphasis Type="Italic">Acipenser stellatus</Emphasis> (Acipenseridae) in the course of adaptation to sea water

The impact of hypophysectomy on the state of the interrenal gland and ultrastructure of chloride cells of gills is investigated in 18-month old juveniles of starred sturgeon Acipenser stellatus in the process of its adaptation to artificial sea water (14.6‰). Hypophysectomized juveniles, similarly to intact juveniles, are able to support a relative stability of osmolarity of blood serum in the course of adaptation to sea water by transition from hyperosmotic to hypoosmotic type of osmoregulation. Changes in the investigated parameters of cells of the interrenal gland (volume of nuclei, areas of cells and of lipophilic vacuoles) occurring in the hypophysectomized and in intact specimens in the process of adaptation to sea water are generally similar, but have different dynamics. In contrast to many teleostean species, in acipenserids the hypophyectomy does not cause atrophy of the interrenal gland. The latter is incorporated in the process of regulation in the course of adaptation of fish to sea water. Hypophyecotmy results in partial destruction of organoids in some chloride cells of gills. However, when the fish are transferred to sea water, the structural changes occur in chloride cells characteristic of their transition to the excretory state. This may happen only at activation of the transport enzyme Na⁺/K⁺-ATPase of these cells by cortisol produced by the interrenal gland. In the absence of hypophysis, the functional connection of organs of the axis hypothalamus (ACTH-immunopositive cells of tuberal nucleus) → the interrenal gland → chloride cells is realized in the fish.