Morphology Of Chloride Cells Research Articles

Changes in Na+,K+-ATPase activity and gill chloride cell morphology in the grouper Epinephelus coioides larvae and juveniles in response to salinity and temperature.

The activity of the enzyme Na+,K+-ATPase and morphological changes of gill chloride cells in grouper, Epinephelus coioides larvae and juveniles were determined 6–48 h after abrupt transfer from ambient rearing conditions (30–32 ppt, 26.5–30 °C) to different salinity (8, 18, 32, 40 ppt) and temperature (25, 30 °C) combinations. Na+,K+-ATPase activity in day 20 larvae did not change at salinities 8–32 ppt. Activity decreased significantly (P <0.01) after exposure to 40 ppt at 25–30 °C, which was accompanied by an increase (P <0.05) in density and fractional area of chloride cells. Enzyme activity in 40 ppt did not reach a stable level and larvae failed to recover from an osmotic imbalance that produced a low survival at 25 °C and death of all larvae at 30 °C. Enzyme activity and chloride cell morphology in day 40 groupers did not change in 8–40 ppt at 25 °C and 8–32 ppt at 30 °C. A significant decrease and a subsequent increase in Na+,K+-ATPase activity in 40 ppt at 30 °C was associated with the increase in chloride cell density resulting in an increased fractional area but a decreased cell size. Enzyme activity and chloride cells of day 60 grouper were unaffected by abrupt transfer to test salinities and temperatures. These results demonstrate that grouper larvae and juveniles are efficient osmoregulators over a wide range of salinities. Salinity adaptation showed an ontogenetic shift as the larvae grew and reached the juvenile stage. This development of tolerance limits may reflect their response to actual conditions existing in the natural environment.

The effect of highly alkaline water (pH 9.5) on the morphology and morphometry of chloride cells and pavement cells in the gills of the freshwater rainbow trout: relationship to ionic transport and ammonia excretion

The effect of highly alkaline water (pH 9.5) on the morphology and morphometry of chloride cells and pavement cells in the gills of the freshwater rainbow trout: relationship to ionic transport and ammonia excretion

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.

Acid Tolerance of Japanese Dace (a Cyprinid Teleost) in Lake Osorezan, a Remarkable Acid Lake

The Japanese dace (Tribolodon hakonensis) is the only teleost species that inhabits Lake Osorezan, a remarkable acid lake in Japan with the water pH of 3.4–3.8. In the present study, physiological changes following transfer of the dace acclimated to neutral stream water (pH 6.8–7.2) to acid lake water (pH 3.6–3.7) were examined with special reference to changes in gill chloride cell morphology. The dace survived direct transfer to acid lake water for 3 days. Plasma [Na ] and [Cl−] showed transient decreases at 1 hr after transfer; however, the decrease in [Na ] was greater than that in [Cl−]. The recovery of [Cl−] was more evident than that of [Na ]. The transient decreases of plasma [Na ] and [Cl−] were followed by acidosis. Blood pH was decreased by 0.13 unit at 6 hr, but partially restored by 24 hr. In the dace acclimated to neutral stream water, chloride cells were scattered in the gill filament. Following transfer to acid lake water, however, well-developed chloride cells were arranged in a radial fashion, forming follicular or gland-like structures in the gills. Each chloride cell was equipped with an apical pit, which faced a common lumen leading to the external environment. These findings provide morphological evidence for a significant role(s) of gill chloride cells in ion and acid-base regulation in the acid-tolerant dace.

Morphological alteration in two types of gill chloride cells in Japanese eels (Anguilla japonica) during catadromous migration

Gill chloride cell morphology and Na+,K+-ATPase activity were examined in cultured Japanese eels (Anguilla japonica) acclimated to fresh water (FW) or seawater (SW), and in yellow and silver eels caught in wild stocks. Gill Na+,K+-ATPase activity was higher in SW-acclimated cultured eels than in FW eels. Chloride cells were detected in both filament and lamellar epithelia by immunocytochemical staining using anti-Na+,K+-ATPase serum. The filament chloride cells were more abundant and larger in SW eels than in FW eels, whereas there was no apparent difference in lamellar chloride cells. In wild-caught eels, gill Na+,K+-ATPase activity increased as they developed from yellow to silver eels. Filament chloride cells in silver eels were more active than those in yellow eels. In contrast, lamellar chloride cells, which were frequently observed in yellow eels, had decreased in number or disappeared in silver eels. These findings suggest that chloride cells in the filament are responsible for excretion of excess salt in hyperosmotic environments, and that lamellar chloride cells may play a significant role in hypoosmotic environments, presumably acting as sites of ion uptake. The excellent euryhalinity of the Japanese eel may be due, at least in part, to the presence of functionally different types of chloride cells. The activation of filament chloride cells in silver eels in FW, together with the increase in Na+,K+-ATPase activity, could be interpreted as a preadaptive response to forthcoming entry into SW.

Effect of cadmium on the cytoskeleton and morphology of gill chloride cells in parr and smolt Atlantic salmon (Salmo salar)

This study documented the effect of cadmium on salmon parr and smolt gill morphology. Cadmium-induced changes in chloride cell (CC) cytoskeletal elements were investigated, as well as the modifications of CC surface area and density. In cadmium-treated parr (10 µg Cd l-1 for 2 days), immunofluorescent light microscopy revealed the appearance of an intense actin staining located in the CC apical part. Transmission electron microscopic observations revealed a change in the organization of the microfilaments at the CC apex, with the appearance of numerous aggregates of filamentous actin. Higher cadmium concentrations (30 and 50 µg l-1) and prolonged treatment times (7 to 14 days) did not modify such reorganisation. Microtubules were not significantly affected by similar treatments. Further, scanning electron microscopic analysis revealed that cadmium induces a significant increase of parr CC surface area as early as the second day of exposure. After 2 days, mature CC density had also increased. In smolt, a rise in CC surface area was observed, although CC density did not significantly increase.

Reduced hypoosmoregulatory ability and alteration in gill chloride cell distribution in mature chum salmon ( Oncorhynchus keta ) migrating upstream for spawning

Osmoregulatory ability of mature chum salmon (Oncorhynchus keta) during spawning migration was examined by following the changes in gill Na+, K+-ATPase activity and in the distribution and morphology of chloride cells. Mature chum salmon caught in Otsuchi Bay, northern Honshu Island, Japan, died within 5 d in seawater (SW) in association with a marked increase in plasma osmolality, whereas the fish transferred to fresh water (FW) maintained plasma osmolality efficiently. Gill Na+, K+-ATPase activity decreased in both SW-maintained and FW-transferred fish. Well-developed chloride cells, identified by immunocytochemical staining specific for Na+, K+-ATPase, were present mainly in the filament epithelium of immature fish caught in the ocean. In mature fish caught in the bay, however, additional chloride cells were also found in the lamellar epithelium. The number of filament chloride cells decreased markedly in the mature fish both in SW and in FW, whereas the number of lamellar chloride cells was maintained. These results suggest that the loss of hypoosmoregulatory ability in mature chum salmon may be attributable to the decrease in filament chloride cells and associated decrease in gill Na+, K+-ATPase activity, and also that appearance of lamellar chloride cells may be preparatory to the forthcoming upstream migration.

Morphometrical analysis chloride cell activity in the gill filaments and lamellae and changes in Na+, K+-ATPase activity during seawater adaptation in chum salmon fry

Changes in gill chloride cell morphology and Na+,K+-ATPase activity were examined following transfer of chum salmon (Oncorhynchus keta) fry from fresh water (FW) to seawater (SW). The gill Na+,K+-ATPase activity in chum salmon fry weighing 0.6-3 g had already been increased prior to SW entry, being twice as high as that in late alevin stages (0.3–0.4 g). The activity further increased gradually when the fry (2–3 g) were transferred to SW. Using immunocytochemical staining with an antiserum specific for the α-subunit of Na+,K+-ATPase, chloride cells were detected in both filament and lamellar epithelia in FW fry. The number of chloride cells in the filament was not affected by SW transfer, whereas lamellar chloride cells were significantly decreased. The size of filament chloride cells was 1.8-fold larger than that of FW fish 21 days after SW transfer, whereas there was no change in the size of lamellar chloride cells throughout the experiment. The intensity of immunoreaction gradually increased after transfer to SW in filament chloride cells. Electron-microscopic observations revealed that mitochondria-rich chloride cells were moderately developed in both filaments and lamellae of FW fish. In the SW-adapted fish, well-developed chloride cells were detected only in the filament. These results suggest that chloride cells in the lamella may be the site of ion uptake in FW chum salmon fry and that the well-developed chloride cells in the filament are responsible for salt secretion in SW. The rich population of filament chloride cells, concomitant with enhanced gill Na+,K+-ATPase activity, may explain the strong seawater adaptability in chum salmon fry prior to SW entry. © 1996 Wiley-Liss, Inc.

Relationship of the tubular system with other organelles in chlorride cells of the channel catfish, Ictalurus punctatus.

The morphology of chloride cells in the channel catfish, Ictalurus punctatus, has been studied by transmission electron microscopy. The chloride cell possesses abundant tubules, mitochondria, and granules. The employment of a special membrane stain in conjunction with a two- or tridimensional analysis reveals a complex interjoining and interlocking ring system of tubules. Tubular sides constituting the complex rings frequently lack granules. The tubular rings join with tubulous mitochondrial profiles and other cytoplasmic components. © 1995 Wiley-Liss, Inc.

Can sodium regulation be used to predict the relative acid-sensitivity of various life-stages and different species of aquatic fauna?

Fishes, Zooplankton, insect larvae, and benthic invertebrates differ in their sensitivity to acidic waters. Some species are able to survive and complete their life-cycle below pH 3.5 while others are eliminated once the pH drops below 5.5. Generally, acid-sensitive fauna are highly water permeable and have difficulty regulating osmotically essential ions, such as sodium and chloride, at low pH. Increased permeability during certain stages of a life-cycle (post-molt crayfish, for example) are often associated with increased acid-sensitivity. Special adaptations, including enlarged anal papillae in Chironomids and the number and morphology of chloride cells in such diverse organisms as crustaceans, insect larvae, and fishes, may enhance acid-tolerance. To test the hypothesis that Na+ regulation can be used to predict relative acid-sensitivity of aquatic fauna we will need a mechanism for standardizing Na+ regulatory capability.

Effects of lead loads on branchial osmoregulatory mechanisms in the rainbow trout Oncorhynchus mykiss.

Exposure of rainbow trout to lead chloride (PbCl2, 1 ppm) in fresh water killed all animals within 16 days. Exposure to this lethal dose for 6 days only showed a significant increase in the haematocrit. Calcium, sodium and chloride concentrations in plasma were not notably modified. Both the influx and the net flux of sodium fluctuated much less than the diffusional water fluxes through secondary lamellae in gills. Branchial Na,K-ATPase, Ca-ATPase and HCO3-ATPase activities were not sensitive to lead toxicity. Lead caused a cellular 'wave-shaped' degeneration and renewal with modification in the number and morphology of chloride cells. Results are discussed in relation to the hydromineral balance of the trout.

Physiological and morphological regulation of acid–base status during hypercapnia in rainbow trout (Oncorhynchus mykiss)

A kinetic analysis (Michaelis constant (Km) and maximal flux (Jmax)) of the branchial Na+ and Cl− influx mechanisms, along with measurements of blood total CO2 content [Formula: see text], net acidic–basic equivalent fluxes, and gill chloride cell morphology, was performed using rainbow trout (Oncorhynchus mykiss) before, during, and after 96 h exposure to environmental hypercapnia (water [Formula: see text]; 1 torr = 133.3 kPa). Exposure to hypercapnia caused (i) a net acidic equivalent loss (negative [Formula: see text]) that was accounted for entirely by reductions in titratable alkalinity flux (JTA), (ii) an increase in [Formula: see text] from 8.4 ± 0.5 to 20.7 ± 0.4 mmol/L, and (iii) no alteration either in [Formula: see text], [Formula: see text], or [Formula: see text]; [Formula: see text] increased (affinity was reduced). Chloride cell fractional area was reduced by 40% from 174 250 ± 15 650 μm2/mm2 under control conditions to 104 329 ± 17 991 μm2/mm2 after 96 h of hypercapnia. In the posthypercapnic period, there was (i) a net acidic equivalent gain (positive [Formula: see text]) that was accounted for entirely by an elevation in JTA, (ii) a rapid reduction of blood [Formula: see text], (iii) an increase of chloride cell fractional area to control values (179 105 ± 35 233μm2/mm2), and (iv) increases and decreases in [Formula: see text] (564 ± 50 versus 224 ± 21 μmol∙kg−1∙h−1 in the prehypercapnic period) and [Formula: see text] (381 ± 85 versus 585 ± 92 μmol∙kg−1∙h−1), respectively. The results suggest that morphological alteration of the gill chloride cell fractional area is an important response to acid–base disturbances. The results are discussed with respect to the relative roles of morphological alteration of gill chloride cell fractional area and variation in internal substrate (HCO3−) in modifying branchial Cl−/HCO3− exchange for acid–base regulation.

Interrelationships between gill chloride cell morphology and calcium uptake in freshwater teleosts

The involvement of the freshwater fish gill chloride cells (CCs) in trans-branchial calcium uptake (JinCa(2+)) was investigated. This was accomplished by assessing the interspecific relationships between the apical surface area of CCs exposed to the external environment and JinCa(2+). Three species of freshwater teleosts, the rainbow trout (Oncorhynchus mykiss), the American eel (Anguilla rostrata) and the brown bullhead catfish (Ictalurus nebulosus), were used. Chronic (ten-day) treatment with cortisol in each species was used as a tool to evoke variations in both JinCa(2+) and gill CC morphology in order to assess intraspecific relationships between CC surface area and JinCa(2+). The results of quantitative morphometry, based on analysis of scanning electron micrographs, demonstrated that catfish possessed the lowest fractional area of exposed CC (CCFA) on the gill filament epithelium (12,744 ± 2248 μm(2)/mm(2)) and was followed, in increasing order, by American eel (21,355 ± 981 μm(2)/mm(2)) and rainbow trout (149,928 ± 26,545 μm(2)/mm(2)). With the exception of catfish, chronic treatment with cortisol caused significant increases in CCFA owing to proliferation of CCs and/or enlargement of individual CCs (eel only). The rates of JinCa(2+) closely reflected the CC fractional area in each species. The results of correlation analysis revealed significant correlations between CC fractional area and JinCa(2+) in trout and eel. Owing to the absence of an effect of cortisol treatment, there was no significant correlation in catfish because of insufficient variation in CC fractional area in this species. CC fractional area was significantly correlated with JinCa(2+) among the three species examined. These results suggest that CC is involved in calcium uptake in freshwater teleosts and that both intra- and interspecific differences in the rates of calcium uptake can be accounted for by variability in the surface area of exposed CCs on the gill epithelia.

The interrelationships between gill chloride cell morphology and ionic uptake in four freshwater teleosts

We have investigated the role of the gill chloride cell in transbranchial Na+ and Cl− uptake in four species of freshwater teleost maintained in water of identical ionic composition. The basic experimental protocol was to determine whether interspecific variability in the rates of whole body Na+ or Cl− uptake could be accounted for by similar interspecific variability in the fractional area of branchial chloride cells exposed to the external environment. To investigate the underlying cause(s) of intraspecific variability, chronic (10 day) treatment with cortisol in each species was used as a tool to evoke variations in both the rates of ionic uptake and chloride cell morphology. Examination of transmission and scanning electron micrographs revealed distinctive chloride cell and pavement cell morphology in each species. The results of quantitative morphometry, based on analysis of scanning electron micrographs, demonstrated that European eel (Anguilla anguilla) possessed the lowest chloride cell fractional area on the filament epithelium (11 288 ± 2133 μm2/mm2) followed, in increasing order, by brown bullhead catfish (Ictalurus nebulosus; 48 341 ± 7694 μm2/mm2), tilapia (Oreochromis mossambicus; 85 194 ± 10 326 μm2/mm2), and rainbow trout (Oncorhynchus mykiss; 146 333 ± 31 356 μm2/mm2). With the exception of rainbow trout, chronic treatment with cortisol caused significant increases in the chloride cell fractional area of filament epithelium owing to enlargement of the surface area of individual chloride cells and (or) proliferation of chloride cells. Both the inter- and intra-specific differences in chloride cell fractional area were reflected by similar differences in whole body Cl− and Na+ uptake. The results of correlation analysis revealed (with the exception of whole body Na+ uptake in A. anguilla) significant correlations between chloride cell fractional area and the rates of ionic uptake within and among the four species that were examined. These data suggest that the chloride cell is a significant site of ionic uptake in freshwater teleosts and that both inter- and intra-specific differences in the rates of ionic uptake can be explained by variability in the surface area of chloride cells on the gill epithelia.

Effects of chronic exposure to soft, acidic water on gill development and chloride cell numbers in embryo-larval brook trout, Salvelinus fontinalis

Recruitment failure is considered to be a major factor contributing to the decline of fish populations in soft, acidic waters; direct mortality of embryo-larval fishes has been postulated as a major cause of the decline. Little is understood of the physiological consequences to embryo-larval fishes of prolonged exposure to soft, acidic waters; however, dysfunction of respiratory and ionoregulatory processes is suspected. In order to evaluate the effects of acid exposure on the respiratory and ionoregulatory systems of developing brook trout, Salvelinus fontinalis, differences in gill morphology and numbers of chloride cells were compared between groups cf developing embryo-larval fish continuously exposed to moderately hard well water (130.0 mg.l−1 as CaCO3, pH 7.94) or to reconstituted soft, acidic water (4.4 mg.l−1 as CaCO3, pH 5.25) designed to mimic acidic waters of northern Wisconsin acidified lakes. Exposures were maintained for up to 48 days (82 days after fertilization) during critical periods of growth and differentiation of branchial structures. The second right gill arch of each fish was examined for changes in the development of filaments and lamellae and for differences in numbers of chloride cells. Gills of fish that developed in soft, acidic water contained greater numbers of normal and degenerating chloride cells, exhibited hyperplasia of primary epithelium and multiple fusions of adjacent filaments and lamellar epithelium than the gills of control fish. Filament and lamellar lengths and numbers of lamellae per filament were significantly less (P

Effects of prolactin on chloride cells in opercular membrane of seawater-adapted tilapia

Effects of prolactin on morphology and numbers of chloride cells in the opercular membrane of seawater-adapted tilapia ( Oreochromis mossambicus) have been examined. Following five daily injections of ovine prolactin at a dose of 10 μg · g body wt −1, blood samples were taken and opercular membranes were removed and stained with a fluorescent mitochondrial dye (dimethylaminostyrylethylpyridiniumiodine), a fluorescent derivative of ouabain (anthroylouabain), and a histological stain specific for the extensive tubular system of chloride cells (zinc-osmium-iodine). Mean plasma osmolarity and sodium increased 23–24% following prolactin injection. An increase in the relative frequency of chloride cells between 20 and 180 μm 2 in cross-sectional area and a decrease in the relative frequency of chloride cells greater than 180 μm 2 were observed following prolactin injections. Average cell size decreased 46–70% and cell height decreased 26–38% following prolactin injections. There was no significant change in cell density. Anthroylouabain staining was observed in both prolactin- and saline-injected fish, and no significant effect on Na +,K +-adenosinetri-phosphatase activity was seen in either opercular membrane or gill tissue. The results demonstrate an effect of prolactin on chloride cell size and provide a morphological correlate for decreased secretory activity of chloride cells following prolactin injections.

Effects of cortisol on gill chloride cell morphology and ionic uptake in the freshwater trout,Salmo gairdneri

Daily intramuscular injection of cortisol (4 mg kg−1 body weight) in rainbow trout,Salmo gairdneri, for 10 days caused significant increases in the number and individual apical surface area of gill chloride cells per mm2 of filament epithelium. Concomitantly, whole body influxes of sodium (Na+) and chloride (Cl−) increased. Acute (3 h) intra-arterial infusion of cortisol did not affect whole body Na+ or Cl− influx. A significant correlation was observed between both Na+ and Cl− influxes and the fractional apical surface area of filament chloride cells in control, sham (saline-injected) and experimental (cortisol-injected) fish. The chloride cells displayed similar ultrastructural modifications in trout undergoing cortisol treatment as in trout transferred to ion-deficient water. These findings suggest the existence of structure/function relationships in which branchial chloride cell morphology is an important determinant of Na+ and Cl− transport capacity. We conclude that chronic cortisol treatment enhances whole body Na+ and Cl− influxes by promoting proliferation of branchial chloride cells. The results of correlation analysis indicate that the chloride cell is an important site of NaCl uptake in freshwater rainbow trout.

Adaptational Responses of Rainbow Trout to Lowered External Nacl Concentration: Contribution of the Branchial Chloride Cell

ABSTRACT Whole-body ionic fluxes and gill chloride cell (CC) morphology were monitored in rainbow trout (Salmo gairdneri) exposed acutely or chronically to natural fresh water (NFW; [Na+]=0·120 mmol l−1; [Cl−]=0·164 mmol l−1) or artificially prepared fresh water with reduced [NaCl] (AFW; [Na+]=0·017 mmoll−1; [Cl−]=0·014mmoll−1). Net fluxes of Na+ and Cl− became extremely negative (indicating net NaCl loss to the environment) upon immediate exposure to AFW exclusively as a result of reduced NaCl influx ( and ) and were gradually restored to control rates during prolonged (30 days) exposure to AFW. The restoration of in AFW was due both to increased and to reduced Cl− efflux , whereas the primary response contributing to the restoration of was an increase of . The total apical surface area of branchial CCs exposed to the external environment increased markedly after 24 h in AFW and remained elevated for 1 month as a consequence of enlargement of individual CCs and, to a lesser extent, increased CC density. and were correlated significantly with total CC apical surface area. Plasma cortisol levels rose transiently in fish exposed to AFW. Treatment of NFW-adapted fish with cortisol for 10 days (a protocol known to cause CC proliferation) caused pronounced increases in and as measured in both NFW and AFW. These results suggest that an important adaptational response of rainbow trout to low environmental [NaCl] is cortisol-mediated enlargement of branchial epithelial CCs which, in turn, enhances the NaCl-transporting capacity of the gill as a result of the proliferation of Na+ and Cl− transport sites.

Multicellular complex of chloride cells in the gills of freshwater teleosts.

Morphology of branchial chloride cells in the freshwater teleosts Plecoglossus altivelis, Cyprinus carpio, and Oreochromis mossambicus was studied by light and transmission electron microscopy. The chloride cell has an apical membrane directly in contact with the outer medium. Generally, two or more neighboring chloride cells share an apical pit, forming a multicellular complex. The chloride cells form a multicellular complex in which cells differ in cytoplasmic electron density, development of tubular system, and in cell size. Chloride cells are linked by junctions which are shallower than the tight junctions that occur between neighboring pavement cells or between pavement and chloride cells. Multicellular complexes of chloride cells create additional paracellular pathways marked apically by the shallower junctions. Since junctional structure affects transepithelial permeability, development of multicellular complexes of chloride cells in freshwater fishes may be related to the transport of some substances as in the gills of marine fishes.

Presence of chloride cells in gill filaments and lamellae of the skate Torpedo marmorata

The morphology of chloride cells in the gill of the skate Torpedo marmorata is described from a light and an electron microscopic study. Chloride cells have been regularly observed in both the gill filament (or primary epithelium) and the lamellae (or secondary epithelium). Chloride cells located in the filament usually display convex apical regions with microvilli protruding amongst microridges of neighbouring pavement cells, whereas chloride cells in the lamellae are located between the two epithelial layers, and contact with the external milieu is via a narrow apical opening. Present observations are discussed in relation to data on the presence of chloride cells in the lamellae of marine teleost fish, and it is suggested that the occurrence of chloride cells all along the lamellae might be in some way inversely related to fish activity.