Endothelin receptors in teleost fishes: cardiovascular effects and branchial distribution

By observing gill blood flow using epi-illuminating microscopy, in parallel with cardiovascular recordings and immunohistochemistry, we have tried to identify the receptor mediating endothelin (ET) type 1 (ET1)-induced pillar cell contraction in the lamellae of the Atlantic cod (Gadus morhua). Intra-arterial injection of the specific ET(B) receptor agonist BQ-3020 induced dose-dependent increases in ventral aortic blood pressure, gill vascular resistance, and pillar cell area (indicating contraction). The specific ET(A) receptor antagonist BQ-610 did not prevent either pillar cell contraction or increased gill vascular resistance induced by ET-1 injection. The cardiovascular responses were corroborated by the detection of ET(B) receptor-like immunoreactivity (IR) associated with pillar cells in the lamellar region and in neuroendocrine cells. ET(B) receptor-like IR was also found lining the muscle layer of lamellar arterioles and filament arteries. In contrast, strong ET(A) receptor-like IR was found on branchial nerves throughout the filaments. In addition, ET-like IR was concentrated in neuroendocrine cells in the filament and lamellae. We also present data suggesting that ET-mediated pillar cell contraction is widespread among teleost fish, including Atlantic cod, rainbow trout (Oncorhynchus mykiss), sculpin (Myoxocephalus scorpius), and mackerel (Scomber scombrus). Taken together, our results suggest that an ET(B)-like receptor mediates pillar cell contraction in fishes, whereas ET(A)-like receptors may serve another function in the gill, inasmuch as ET(A) receptor-like IR is found on branchial nerves.     

Cadmium-induced changes in gill morphology of zebrafish, Brachydanio rerio (Hamilton–Buchanan), and rainbow trout, Salmo gairdneri Richardson

Freshwater zebrafish and brackish water rainbow trout were exposed to different concentrations of cadmium for up to 6 weeks. The gill morphology was examined by light and electron microscopy, and a morphometric analysis was performed. The morphometric study of the secondary lamellae revealed an increase in the portion exterior to the basal lamina, resulting in an increased diffusion distance, after exposure to cadmium concentrations of 10μg l⁻¹ and above. In both species this was due to an increase in volume of the non-tissue spaces of the secondary lamellar epithelium. Furthermore, the water space between neighbouring secondary lamellae was clearly reduced. Morphological examination revealed some gross alterations compared with control fish. These initially consisted in curling of the secondary lamellae and finally resulted in local teleangiectasia. Partial lifting of the secondary lamellar epithelium from the pillar cells resulted in large non-tissue spaces which were invaded by leucocytes. The first sign of degeneration was observed in the chloride cells, which were characterized by a dispersed cytoplasm and a smooth apical plasma membrane.     

A morphological study on posterior gills of the mangrove crab Ucides cordatus

Morphological and histological studies on posterior gills of the mangrove crab Ucides cordatus showed that the 5th gill (of 7) has a larger surface area and a greater number of lamellae compared to the 6th gill. Regular separation of gill lamellae, important when the gill is in air, is maintained by enlargements of the marginal canals. Conical, spine-like structures along the efferent vessel of both 5th and 6th gills were also observed. In addition, pillar cells, a discontinuous lamellar septum and a hypobranchial artery were observed. The presence of valve-like structures near the efferent vessel was also indicated. These structures, together with the pillar cells, may have a role in directing the hemolymph flow towards certain gills during particular physiological states. Localization of osmoregulatory epithelia in the lamellae of both gills was inferred from dimethylaminostyrylethylpyridiniumiodine staining. Apparently gills 5 and 6 have osmoregulatory epithelial cell patches of similar area, corresponding to 43% and 38% of the total lamellae area, respectively. However, their localization is quite different. Gill number 5 osmoregulatory patches seem to be restricted to the afferent region of the lamella whereas in gill number 6, they are more dispersed over the entire lamella. These differences may be related to the particular functional characteristics of these gills.     

Fine structure of the branchial epithelium in the teleost Oreochromis niloticus

We have studied the gill epithelium of Oreochromis niloticus using transmission electron microscopy with the particular interested relationship between cell morphology and osmotic, immunoregulatory, or other non‐regulatory functions of the gill. Pavement cells covered the filament epithelium and lamellae of gills, with filament pavement cells showing distinct features from lamellar pavement cells. The superficial layer of the filament epithelium was formed by osmoregulatory elements, the columnar mitochondria‐rich, mucous and support cells, as well as by their precursors. Light mitochondria‐rich cells were located next to lamellae. They exhibited an apical crypt with microvilli and horizontal small dense rod‐like vesicles, sealed by tight junctions to pavement cells. Dark mitochondria‐rich cells had long dense rod‐like vesicles and a small apical opening sealed by tight junctions to pavement cells. The deep layer of the filament epithelium was formed by a network of undifferentiated cells, containing neuroepithelial and myoepithelial cells, macrophage and eosinophil‐like cells and their precursors, as well as precursors of mucous cells. The lateral‐basal surface was coated by myoepithelial cells and a basal lamina. The lamellar blood lacunae was lined by pillar cells and surrounded by a basal lamina and pericytes. The data presented here support the existence of two distinct types of pavement cells, mitochondria‐rich cells, and mitochondria‐rich cells precursors, a structural role for support cells, a common origin for pavement cells and support cells, a paracrine function for neuroepithelial cells in the superficial layer, and the control of the lamellar capillary base by endocrine and contractile cells. Data further suggest that the filament superficial layer is involved in gill osmoregulation, that may interact, through pale mitochondria‐rich cells, with the deep layer and lamellae, whereas the deep layer, through immune and neuroendocrine systems, acts in the regeneration and defense of the tissue. J. Morphol. 2010. © 2010 Wiley‐Liss, Inc.     

Localization of angiotensin-converting enzyme in the trout gill

Angiotensin-converting enzyme (ACE) was localized in perfused trout gills by measuring gill extraction of two radiolabeled ACE inhibitors, 125I-351A (an iodinated derivative of lisinopril) and 3H-RAC-X-65, and by autoradiography of gills perfused with 125I-351A. A 125I-351A pulse was preferentially extracted by the arterio-arterial (AA) pathway (61.7% +/- 1.8% extraction; mean +/- SE, N = 4); the arteriovenous (AV) pathway extracted an additional 10%. Extraction by either pathway was reduced by simultaneous perfusion with 10(-5) M unlabeled lisinopril. AA extraction of RAC-X-65 during continuous perfusion was maximal (75% +/- 5%, N = 6) during the first few minutes of perfusion and decreased steadily to 38% +/- 9% by 20 min and to less than 10% by 40 min. AV extraction of RAC-X-65 was negligible. Autoradiography of gills continuously perfused with 125I-351A showed that the radiolabel was concentrated in the respiratory lamellae. The highest grain density was associated with the pillar cells nearest the medial (inner) lamellar margin. Afferent filamental arteries and afferent lamellar arterioles were labeled to a lesser extent. Relatively little label was found on the efferent lamellar arterioles or efferent filamental arteries. 125I-351A binding was not evident in AV vessels. These findings support the hypothesis that the gill is an important site for formation of plasma angiotensin II and they suggest that enzymes associated with mammalian endothelial cells are also common to gill pillar cells.     

The source of lamellar mitochondria-rich cells in the air-breathing fish, Trichogaster leeri

Pavement cells and the mitochondria-rich cells (MRCs) are two of the main types of cells in fish gill epithelia. The pavement cells are generally responsible for gas exchange and MRCs for ion regulation. MRCs are found especially in the trailing edge and the interlamellar region of gill filament. In some species, MRCs are also observed in the gill lamellae. A previous study reported the likelihood of having lamellar MRCs in air-breathing fishes. Nevertheless, the source of lamellar MRCs is unclear. We used the air-breathing fish, Trichogaster leeri, to investigate the source of proliferated cells on the lamellae when 5-bromo-2-deoxyuridine (BrdU) was injected at different times before fish were sampled from deionized water. There were two major findings in this study. First, undifferentiated cells were found in the lamellae, as well as in the filaments. And, within 12-24 hr, a proliferated cell, identified as BrdU cell, could differentiate to an MRC in the gill lamellae. Second, the filaments and the lamellae in T. leeri responded to ionic stress differently but the proportion of the proliferated MRCs to the BrdU cells remained constant. Our results suggested that the lamellar MRCs were mainly differentiated from the cells that proliferated earlier from the lamellae.     

Scanning electron microscopy of the gills of a freshwater catfish,Rita Rita

Variations in the gross morphology and surface architecture of the gill filaments and secondary lamellae of a freshwater catfish (Rita rita) have been investigated using scanning electron microscopy. Heterogeneity of the gill has been correlated with the distribution of lamellar water-flow at different regions of a gill filament. Higher lamellar water flow (cc/pore/cmH₂O/sec) was estimated for the middle region of the filaments. The filaments are covered with epithelial cells whose surface is provided with well-developed microridges. The lamellae are generally covered with microvillous epithelial cells. The variations in surface architecture of the gill filaments and secondary lamellae have been correlated with their probable functions.     

FHL5, a novel actin-binding protein, is highly expressed in eel gill pillar cells and responds to wall tension

Supporting evidence for the contractile nature of fish branchial pillar cells was provided by demonstrating the presence of actin fibers and a novel four-and-a-half LIM (FHL) protein in which expression is specific for contractile tissues and sensitive to the tension applied to the pillar cell. When eel gill sections were stained with rhodamine-phalloidin, a selective fluorescent probe for fibrous actin, a strong bundle-like staining was observed around collagen columns in pillar cells, suggesting the presence of abundant actin fibers. A cDNA clone encoding a novel member of the actin-binding FHL family, FHL5, was isolated from a subtracted cDNA library of eel gill. Northern analysis revealed that FHL5 mRNA is highly expressed only in gills, heart, and skeletal muscle. In gills, FHL5 was found to be confined to pillar cells by immunohistochemistry. Confocal fluorescence microscopy showed that FHL5 is present in both cytosol and nucleus; within the cytosol, a large portion of FHL5 is colocalized with the phalloidin-positive actin bundles. Furthermore, transfection of myogenic C2C12 cells with FHL5 cDNA demonstrated, in addition to its interaction with actin stress fibers, a nuclear shuttling activity of FHL5. The mRNA and protein levels were found to be elevated on 1) transfer of eels from seawater to freshwater, 2) volume expansion by infusion of isotonic dextran-saline, and 3) constriction of gill vasculature by bolus injection of endothelin-1. These results suggest contractile nature of pillar cells and a role of FHL5 in maintaining the integrity and regulating the dynamics of pillar cells.     

Fish gill morphology: inside out

In this short review of fish gill morphology we cover some basic gross anatomy as well as in some more detail the microscopic anatomy of the branchial epithelia from representatives of the major extant groups of fishes (Agnathans, Elasmobranchs, and Teleosts). The agnathan hagfishes have primitive gill pouches, while the lampreys have arch-like gills similar to the higher fishes. In the lampreys and elasmobranchs, the gill filaments are supported by a complete interbranchial septum and water exits via external branchial slits or pores. In contrast, the teleost interbranchial septum is much reduced, leaving the ends of the filaments unattached, and the multiple gill openings are replaced by the single caudal opening of the operculum. The basic functional unit of the gill is the filament, which supports rows of plate-like lamellae. The lamellae are designed for gas exchange with a large surface area and a thin epithelium surrounding a well-vascularized core of pillar cell capillaries. The lamellae are positioned for the blood flow to be counter-current to the water flow over the gills. Despite marked differences in the gross anatomy of the gill among the various groups, the cellular constituents of the epithelium are remarkably similar. The lamellar gas-exchange surface is covered by squamous pavement cells, while large, mitochondria-rich, ionocytes and mucocytes are found in greatest frequency in the filament epithelium. Demands for ionoregulation can often upset this balance. There has been much study of the structure and function of the branchial mitochondria-rich cells. These cells are generally characterized by a high mitochondrial density and an amplification of the basolateral membrane through folding or the presence of an intracellular tubular system. Morphological subtypes of MRCs as well as some methods of MRC detection are discussed.     

RING finger, B-box, and coiled-coil (RBCC) protein expression in branchial epithelial cells of Japanese eel, Anguilla japonica.

An RBCC (RING finger, B-box, and coiled-coil) protein was identified that belongs to the superfamily of zinc-binding proteins and is specifically expressed in the gill of eel, Anguilla japonica. Euryhaline fishes such as eels can migrate between freshwater and seawater, which is considered to be accomplished by efficient remodeling of the architecture and function of the gill, a major osmoregulatory organ. To identify molecules involved in such adaptive changes, we performed differential display using mRNA preparations from freshwater and seawater eel gills and obtained an RBCC clone among several differentially expressed clones. The clone encoded a protein of 514 amino acid residues with structural features characteristic of the RBCC protein; we therefore named it eRBCC (e for eel). eRBCC mRNA was specifically expressed in the gills with a greater extent in the gills of freshwater eels. Immunohistochemistry revealed that the expression of eRBCC is confined to particular epithelial cells of the gills including freshwater-specific lamellar chloride cells. The RING finger of eRBCC was found to have a ubiquitin ligase activity, suggesting an important regulatory role of eRBCC in the remodeling of branchial cells.     

Vascular anatomy of the gills in a high energy demand teleost,the skipjack tuna (Katsuwonus pelamis)

Tunas (family: Scombridae, Tribe: Thunnini) exhibit anatomical, physiological, and biochemical adaptations that dramatically increase the ability of their cardiorespiratory systems to transfer oxygen from the water to the tissues. In the present study the vascular anatomy of the skipjack tuna, Katsuwonus pelamis, gill was examined by light and scanning electron microscopic analysis of methyl methacrylate vascular corrosion replicas prepared under physiological pressure. The gill filament contains three distinct blood pathways, respiratory, interlamellar, and nutrient. The respiratory, or arterio-arterial (AA) pathway, is the site of gas exchange and consists of the afferent and efferent filamental arteries (AFA and EFA) and arterioles (ALA and ELA) and the lamellae. Each ALA in the basal filament supplies ten or more lamellae and they anastomose with their neighbor to form a continuous vascular arcade. Four modifications in the lamellar circulation appear to enhance gas exchange efficiency. 1) The ALA deliver blood directly to the outer margin of the lamellae where unstirred boundary layer effects are predicted to be minimal and water PO2 highest. 2) Pillar cells are closely aligned along the outer boundary of the inlet side and the inner boundary of the outlet side of the lamellae to form multiple distributing and receiving blood channels. 3) Elsewhere in the lamella, pillar cells are aligned to form diagonal channels that direct blood from the outer to the inner lamellar margins, thereby reducing vascular resistance. 4) The lamellar sinusoid is especially widened near the efferent end to augment oxygen saturation of blood flowing through the inner margin. These adaptations, plus the presence of a bow-shaped interlamellar septum, and a thinned filament core appear to decrease gill vascular resistance and maximize gas-exchange efficiency. The interlamellar (IL) and nutrient systems originate from post-lamellar vessels and are arterio-venous (AV) pathways. IL vessels form an extensive ladder-like lattice on both sides of the filamental cartilage and are supplied in part by narrow-bore vessels from the medial wall of the EFA. Their function is unknown. Nutrient vessels are formed from the confluence of a myriad of tortuous, narrow-bore vessels arising from the basal region of the EFA and from efferent branchial arteries. They re-enter the filament and eventually drain into the IL system or filamental veins. As these AV pathways are retained despite considerable reduction in filamental tissue, it is evident that they are integral components of other non-respiratory homeostatic activities of the gill.     

Morphology of the gills of larval and parasitic adult sea lamprey,Petromyzon marinus L.

The general morphology of the gills is similar in larval (ammocoetes) and parasitic adult sea lampreys, Petromyzon marinus, despite different methods of ventilation necessitated by their feeding habits. The gill lamellae are supported by randomly-distributed pillar cells which enclose blood spaces and collagen columns. The distribution of these cells in lampreys is different from that of higher fishes and it may be inefficient for respiratory exchange. The presence of cytoplasmic microfilaments suggests that these cells have the ability to reduce the lamellar blood spaces through contraction. Marginal channels at the tips of the lamellae are lined only by endothelial cells. The thickness of the water-blood pathway in lampreys falls within the range described for higher fishes, with the most efficient gas exchange likely occurring at the lamellar tips where only a single layer of epithelial cells is present. The abrupt increase in height of the epithelium near the lamellar bases in adults, compared to the gradual transition in height along the lamellae in ammocoetes, is perhaps reflective of higher oxygen requirements during the parasitic stage. The consistent appearance of wide, lateral intercellular spaces within the respiratory epithelium of lampreys indicates possible involvement of these spaces in transport. Mucous secretion appears to be an important function of the superficial platelet cells in ammocoetes. “Mitochondria-rich” and “mitochondria-poor” superficial cells are observed in both ammocoetes and adults, with the mitochondria-rich cells more prevalent toward the lamellar bases. The possibility that at least some of these cells may be involved in absorption is discussed. Mitochondria-rich cells in the interlamellar region are morphologically different in ammocoetes and adults but all possess an abundance of smooth endoplasmic reticulum and hence resemble “chloride cells” of higher fishes. The similarity of these cells in the parasitic adult lamprey to chloride cells of marine fishes may reflect the potential of the adult lamprey to osmoregulate in salt water. A scarcity of these cells in ammocoetes and their resemblance to chloride cells in freshwater fishes may reflect the restriction of larval lampreys to a freshwater habitat.     

Ultrastructural study of the early development and localization of Loma salmonae in the gills of experimentally infected rainbow trout

The early ultrastructural stages of Loma salmonae were studied in the gills of experimentally infected rainbow trout. No parasitic stages were identified during the first 2 wk of the infection. By week 3 postexposure (PE), uninucleate and binucleate meronts were recognized within host cells (no xenomas) associated with the capillary channels of secondary lamellae and lamellar arteries. An inflammatory reaction was absent. In secondary lamellae, infected cells were isolated from the capillary lumen, and some were recognized as pillar cells. In lamellar arteries, infected cells were localized beneath the endothelium and not in the lumen. Inflammatory reaction and destruction of parasites inside blood cells in the lumen of secondary lamellae were observed by week 4 PE. Three hypotheses, i.e., isolation, internalization, and evasion, are proposed to explain the localization of the infected cells in the gills. It is concluded that meronts are the earliest parasitic stage observed by week 3 PE, pillar cells are secondarily infected by phagocytosis of infected cells in the blood, endothelial cells of gills are not infected, and inflammatory response to the parasite starts by week 4 PE.     

斑马鱼鳃的光镜和透射电镜观察

应用光学显微镜和透射电镜对斑马鱼(Danio rerio)鳃的组织结构及鳃丝、鳃小片超微结构进行了观察。结果表明,斑马鱼有4对全鳃,鳃耙呈长锥状,鳃丝呈梳状排列在鳃弓上,鳃小片均匀排列在鳃丝两侧。鳃小片由上皮细胞、柱细胞、内皮细胞和毛细血管网组成,鳃小片基部和血管周围分布有泌氯细胞,胞内有丰富的线粒体和排泄小泡,根据线粒体形态特征和细胞质电子密度可将其分为两个亚型。黏液细胞通常与泌氯细胞对生存在,并且有通外的开口。斑马鱼鳃组织结构与其他硬骨鱼鳃结构相似,其结构和功能有密切的关系。     

Pillar cell and erythrocyte localization of fugu ET(A) receptor and its implication

Endothelin, a vasoconstrictor peptide, plays important roles not only in the mammalian circulatory system but also in non-mammalian systems, such as the gill lamellar vascular network with complex structural characteristics. Here, we show that (i) the contraction of pillar cells that delimit the lamellar vasculature is controlled by endothelin through the type A endothelin receptor (ET(A)) linked to the intracellular calcium signaling system and (ii) ET(A) receptor is also highly expressed on fugu erythrocytes, a hitherto unexpected finding. Database mining revealed the presence of five endothelin receptor (ETR) sequences in the fugu genome. By Northern blotting, cDNA cloning, and fura-2 monitoring, the branchial ETR subtype was shown to be ET(A) able to induce a Ca(2+) transit. Immunohistochemistry revealed its pillar cell and erythrocyte localization. These results suggest an endothelin/ET(A)-mediated coordinated regulation of the pillar cell shape and erythrocyte membrane flexibility.     

Cellular types in the gill arch of the teleost Cyprinus carpio (Cyprinidae, Pisces). A histochemical and ultrastructural study

On adult specimens of the common carp (Cyprinus carpio), we have carried out a histochemical and ultrastructural study of the epithelia which form the gill arch. Secondary lamellae have two cellular types, granular and mucous, which produce neutral carbohydrates and proteins rich in tryptophane, and another mucous cell type which contains glycosaminoglycans. In gill rakers, three cellular types show different histochemical behaviour: 1) granular cells elaborate neutral mucosubstances; 2) a second cell type contains glycosaminoglycans,, and 3) a third cell type secretes neutral and acid carbohydrates. We discuss the possible role of these cells according to their secretion. We describe pillar and chloride cells in secondary lamellae, and chloride and neuroendocrine-like cells in gill rakers.     

The structure of the gill of the trout,Salmo gairdneri (Richardson)

Summary A light and electron microscopic study was made of the structure of the gill arch, filament and secondary lamella of Salmo gairdneri R. Blood pathways through the gill were traced from serial histological sections, and from the examination of ink perfused tissue and perspex casts formed following resin injection of the circulatory system.The epithelium covering the gill consists of unspecialized, dark, chloride and mucous cells. The distribution of specialized cells appears to be related to gill function. The basement membrane underlying the epithelium consists of three layers, the inner collagen layer being continuous with the connective tissue core of the gills.Blood supply to the secondary lamellar respiratory surface is via branchial, filament and secondary lamellar arteries. Blood spaces of the secondary lamellae are delimited by pillar cells containing what appears to be contractile material. The marginal channel of each lamella is bounded distally by cells of endothelial origin. A network of lymph spaces within the filaments connects with efferent branchial arteries. Nutritionary capillaries within the filaments connect with afferent branchial arteries. No shunts between afferent and efferent filament arteries were found.Data from this study and previous physiological and histopathological studies suggest a mechanism for the control of blood flow to suit the respiratory requirements of the fish. This mechanism involves a system of recruitment of additional respiratory units and changes in overall blood flow patterns.This work formed part of a thesis submitted for the degree of Doctor of Philosophy in 1971 and for which M. M. was in receipt of a studentship from the Natural Environmental Research Council. The authors are grateful for the support given by research grants from the M.R.C (P.T.) and the N.E.R.C. (M.M.), and to Prof. G. M. Hughes in whose department the work was carried out.     

Mast cell responses to Ergasilus (Copepoda), a gill ectoparasite of sea bream

Immunocytochemical, light microscopy and ultrastructural studies were conducted on gill of sea bream, Sparus aurata L., naturally parasitized with the important parasitic copepod Ergasilus sp. to assess pathology and cellular responses. Thirty-seven S. aurata were examined from a fish farm; 26 (70%) were parasitized, with infection intensity ranging from 3 to 55 parasites per fish. Hosts were divided into two groups, lightly infected fish (<15 parasites per fish) and heavily infected fish (>15 parasites per fish). In histological sections, the copepod encircled gill lamellae with its second antennae, compressed the epithelium, provoked hyperplasia and hemorrhage, occluded arteries and often caused lamellar disruption. Fusion of the secondary lamellae due to epithelial hyperplasia was common in all infected fish; heavily infected fish showed more intense branchial inflammation. In both healthy and infected fish, mast cells (MCs) were free within the connective tissue inside and outside the blood vessels of the primary lamellae and made close contact with vascular endothelial cells, mucous cells and rodlet cells (RCs). MCs were irregular in shape with a cytoplasm filled by numerous electron-dense, membrane-bound granules. Immunostaining of primary and secondary gill filaments with an antibody against the antimicrobial peptide (AMP) piscidin 3 (anti-piscidin 3 antibody, anti-HAGR) revealed a subpopulation of MCs that were positive. These MCs were more abundant in gills of heavily infected fish than in either lightly infected or uninfected fish (ANOVA, P<0.05). Our report documents the response of gill to ectoparasite infection and provides further evidence that mast cells and their AMPs may play a role in responding to branchial ectoparasite infections.     

Assessment of Waterborne Amine-Coated Silver Nanoparticle (Ag-NP)-Induced Toxicity in <Emphasis Type="Italic">Labeo rohita</Emphasis> by Histological and Hematological Profiles

Silver nanoparticles (Ag-NPs) have wide applications in the medical field; however, the toxicological effects are still poorly studied. The study was aimed to determine the effects of 15.78 nm spherical and amine-coated Ag-NPs on hematology and histology of gills and liver tissues in 28 days treated Labeo rohita (L. rohita). It was found that Ag-NPs induced alterations in the hematological parameters in a dose dependent manner. The Ag-NPs also induced histological alterations in a dose-dependent manner. In gill tissues, it induced fusion of secondary lamellae, separation of gill epithelium, fusion and necrosis of lamellar cells, hyperplasia, deformed cartilaginous skeleton, separation and lifting of epithelium, and curling of lamellae in a dose dependent manner. In the liver, Ag-NPs produced abnormalities in hepatic tissues by reducing the size of hepatocytes and nuclei, and stimulated the production of necrotic and apoptotic bodies. It was concluded that Ag-NPs are toxic to aquatic organisms and induce hematotoxicity and histopathological conditions in exposed fish.     

Epithelial and pillar cell replacement in gills of juvenile trout, Salmo gairdneri Richardson

Young rainbow trout, Salmo gairdneri (Richardson) were injected intraperitoneally with tritiated thymidine, and killed at intervals between 2 hr and 16 days after inoculation. Labelled epithelial cells were first detected autoradiographically along the base of gill lamellae. Epithelial cells proliferated here and then migrated toward the tips of the lamellae. Uniform labelling along the length of the filaments at the base of lamellae indicated that cells were dividing at a constant rate. Transverse sections of filaments showed that epithelial proliferation was also uniform across the base of the lamellae. The interior of the lamellae often had labelled pillar cells, indicating that these cells also divide. The high intensity of the label in animals killed 16 days after inoculation with tritiated thymidine suggests that division probably occurs slowly, less than once every 16 days.