Nonlesions, Misdiagnoses, Missed Diagnoses, and Other Interpretive Challenges in Fish Histopathology Studies

Differentiating salient histopathologic changes from normal anatomic features or tissue artifacts can be decidedly challenging, especially for the novice fish pathologist. As a consequence, findings of questionable accuracy may be reported inadvertently, and the potential negative impacts of publishing inaccurate histopathologic interpretations are not always fully appreciated. The objectives of this article are to illustrate a number of specific morphologic findings in commonly examined fish tissues (e.g., gills, liver, kidney, and gonads) that are frequently either misdiagnosed or underdiagnosed, and to address related issues involving the interpretation of histopathologic data. To enhance the utility of this article as a guide, photomicrographs of normal and abnormal specimens are presented. General recommendations for generating and publishing results from histopathology studies are additionally provided. It is hoped that the furnished information will be a useful resource for manuscript generation, by helping authors, reviewers, and readers to critically assess fish histopathologic data.

Optimization of whole-body zebrafish sectioning methods for mass spectrometry imaging

Mass spectrometry imaging (MSI) methods and protocols have become widely adapted to a variety of tissues and species. However, the MSI literature contains minimal information on whole-body cryosection preparation for the zebrafish (ZF; Danio rerio), a model organism routinely used in developmental, toxicity, and carcinogenicity studies. The optimal medium for embedding and cryosectioning a whole organism or soft-tissue specimen for histological examination is a synthetic polymer mixture that is incompatible with MSI as a result of ion suppression. We describe the optimal methods and results for embedding and cryosectioning whole-body ZF for MALDI-MSI. We evaluated 13 distinct embedding media formulations and found a supportive hydrogel with the consistency of cartilage to be the optimal embedding medium. The hydrogel medium does not interfere with MSI data collection, aids in tissue stability, is readily available for purchase, and is easy to prepare and handle during cryosectioning. Additionally, we decreased the matrix cluster interference commonly caused by α-cyano-4-hydroxycinnamic acid by adding ammonium phosphate to the solvent spray solution. The optimized methods developed in our laboratory produced high-quality cryosections, as well as high-quality mass spectral images of sectioned ZF.

Comparative sensitivity of six scleractinian corals to temperature and solar radiation

Scleractinian corals were exposed to 6 combinations of temperature and solar radiation to evaluate effects on coral bleaching, survival, and tissue surface area changes during and after exposure. A recirculating coral exposure system was coupled to a solar simulator to allow laboratory testing of 6 species of Caribbean corals (Diploria clivosa, Montastraea faveolata, Porites divaricata, Stephanocoenia intersepta, Siderastrea radians, and Siderastrea siderea). Significant bleaching occurred in all of the corals exposed to high irradiance except S. siderea. Elevated light levels resulted in a decrease in photochemical efficiency for all species during the exposure period, with S. siderea showing the smallest decrease. The most prominent reductions in photochemical efficiency occurred in M. faveolata and S. intersepta, and these species exhibited extensive tissue loss and the highest mortality. In contrast to high irradiance, high temperatures significantly decreased photochemical efficiency for only D. clivosa and did not lead to severe tissue loss for this species. These results demonstrate species-specific responses to solar radiation and temperatures, with M. faveolata and S. intersepta being the most susceptible to bleaching due to high irradiance.

Multigenerational exposure of the estuarine sheepshead minnow (Cyprinodon variegatus) to 17 β-estradiol. I. organism-level effects over three generations

A 280-d study examined the effects of 17β-estradiol (E2) on reproduction and development of the sheepshead minnow (Cyprinodon variegatus) exposed from the parental (F0) through three subsequent (F1, F2, and F3) generations and evaluated the need for multigenerational assessments of the risks of endocrine-disrupting chemicals. This first three-generation study exposed adult F0 and F1 fish to measured concentrations of 0.01, 0.04, 0.08, 0.2, and 0.3 μg E2/L; the F2 and F3 generations were exposed to 0.2 μg E2/L or less. The cumulative 21-d production of normal embryos was significantly reduced in the F0 generation at 0.3 μg E2/L and in the F1 and F2 generations at 0.08 μg E2/L or more. The daily reproductive rate was significantly reduced in all three generations at 0.08 μg E2/L or more during spawning days 8 to 14 and 15 to 21. The proportion of infertile eggs from F1 fish was significantly increased above that of the solvent controls at 0.04 and 0.2 μg E2/L and from F2 fish at 0.04 μg E2/L or more. Changes in liver, kidney, and gonadal tissues were seen in the F0 and F1 generations exposed to 0.2 μg E2/L or more. The female gonadosomatic index was significantly decreased at 0.3 μg E2/L in the F0 and F1 generations. Estradiol affected the hepatosomatic index only in female F1 fish, but not in a dose-dependent manner. All F1 fish in 0.3 μg E2/L appeared to be phenotypically female. Our results indicate that life-cycle exposure to E2 significantly decreased embryo production by F1 and F2 fish at concentrations lower than those affecting the F0 generation, and they emphasize the importance of evaluating the impact of an estrogenic chemical on reproduction through a minimum of two (F0 and F1) generations.

Chronic Mycobacterium marinum infection acts as a tumor promoter in Japanese Medaka (Oryzias latipes)

An accumulating body of research indicates there is an increased cancer risk associated with chronic infections. The genus Mycobacterium contains a number of species, including M. tuberculosis, which mount chronic infections and have been implicated in higher cancer risk. Several non-tuberculosis mycobacterial species, including M. marinum, are known to cause chronic infections in fish and like human tuberculosis, often go undetected. The elevated carcinogenic potential for fish colonies infected with Mycobacterium spp. could have far reaching implications because fish models are widely used to study human diseases. Japanese medaka (Oryzias latipes) is an established laboratory fish model for toxicology, mutagenesis, and carcinogenesis; and produces a chronic tuberculosis-like disease when infected by M. marinum. We examined the role that chronic mycobacterial infections play in cancer risk for medaka. Experimental M. marinum infections of medaka alone did not increase the mutational loads or proliferative lesion incidence in all tissues examined. However, we showed that chronic M. marinum infections increased hepatocellular proliferative lesions in fish also exposed to low doses of the mutagen benzo[a]pyrene. These results indicate that chronic mycobacterial infections of medaka are acting as tumor promoters and thereby suggest increased human risks for cancer promotion in human populations burdened with chronic tuberculosis infections.

Comparison of estrogen-responsive plasma protein biomarkers and reproductive endpoints in sheepshead minnows exposed to 17beta-trenbolone

Protein profiling can be used for detection of biomarkers that can be applied diagnostically to screen chemicals for endocrine modifying activity. In previous studies, mass spectral analysis revealed four peptides (2950.5, 2972.5, 3003.4, 3025.5m/z) in the plasma of estrogen agonist-treated male and gravid female sheepshead minnows (Cyprinodon variegatus, SHM), which served as distinct estrogenic biomarkers. In this study, a 21-day reproductive assay with adult SHM was conducted to investigate possible dose-related effects of the synthetic androgen, 17beta-trenbolone, on expression of these four estrogen-responsive peptides. In addition, the response of the peptide biomarkers were compared to traditional reproductive endpoints of fecundity, histopathology, secondary sex characteristics, length, weight, hepatosomatic index, female gonadosomatic index and plasma vitellogenin (VTG) levels. Fish were continuously exposed to 0.005, 0.05, and 5.0 microg/l, a solvent control (triethylene glycol, TEG), and a seawater control (SW) using an intermittent flow-through dosing system. Plasma was analyzed for the presence of the four peptide biomarkers by MALDI-TOF MS and VTG protein by quantitative ELISA. Male fish from the trenbolone treatments and controls showed no expression of the four peptide biomarkers or measurable levels of VTG. The estrogen-responsive biomarkers and plasma VTG were constitutively expressed in females from the SW, TEG, 0.005 and 0.05 microg/l exposures. All four peptide biomarkers were significantly reduced (p<0.0002 to p<0.005) at the 5.0 microg/l treatment level which corresponded with significant reductions in fecundity and changes in ovarian morphology. A distinct but non-significant reduction in VTG was also observed in female fish from the 5.0 microg/l treatment. Results of this study suggest application of these estrogen-responsive protein biomarkers may be a cost effective alternative to fecundity measures which are labor intensive and expensive to conduct.

Mercury concentrations in salmonids from western U.S. National Parks and relationships with age and macrophage aggregates

Mercury accumulation in aquatic foodwebs and its effects on aquatic biota are of growing concern both for the health of the fish and the piscivores that prey upon them. This is of particular concern for western U.S. National Parks because it is known that mountainous and Arctic areas are sinks for some contaminants. The Western Airborne Contaminants Assessment Project seeks, in part, to ascertain mercury concentrations and evaluate effects of contaminants on biota in 14 lakes from 8 National Parks or Preserves. In this paper we report that mercury has accumulated to concentrations in troutthat may negatively impact some piscivorous wildlife, indicating potential terrestrial ecosystem effects. Additionally, we show that mercury concentrations increase with age in 4 species of trout, providing evidence of bioaccumulation. Finally, we demonstrate that mercury is associated with tissue damage in the kidney and spleen, as indicated by increases in macrophage aggregates. This finding suggests that mercury, and possibly other contaminants, are Macrophage aggregates suggests that mercury, a negatively affecting the trout that inhabit these remote and protected ecosystems. Our results indicate that mercury is indeed a concern for the U.S. National Parks, from an organismic and potentially an ecosystem perspective.

Histopathological Evidence of Regeneration following Hepatotoxic Effects of the Cyanotoxin Microcystin-LR in the Hardhead Catfish and Gulf Killifish

The cyanobacterial toxin microcystin-LR (MC-LR) causes liver damage in several freshwater fish species. In the present study, two estuarine species, hardhead catfish Arius felis and gulf killifish Fundulus grandis, were injected intraperitoneally with MC-LR at 45-300 μg/kg and their livers examined histopathologically for up to 23 d postinjection (PI) in the catfish and 5 d PI in the killifish. The livers from both species exhibited extensive, diffuse hepatocellular necrosis by 6 h PI. The necrosis persisted, and by day 2 large numbers of basophilic cells had emerged throughout the liver parenchyma. These cells occurred individually or in small clusters. By 72 h, the basophilic cells appeared to be highly proliferative with numerous mitotic figures and were arranged in cords and tubules similar to mature hepatic parenchyma. Regeneration of the liver parenchyma was noted in gulf killifish at 5 d PI, as tracts of basophilic cells were still evident. In hardhead catfish, there were no signs of necrosis or proliferating basophilic cells by 9 d PI, and the hepatic parenchyma appeared normal except for the vacuolation of many hepatocytes and some areas of hepatic megalocytosis. The degenerative changes in the two species studied were similar to but more pronounced than those observed in freshwater fish species exposed to MC-LR. Evidence of regeneration from the hepatotoxic effects of MC-LR suggests that the toxin can be used to study regenerative processes in fish livers.

Diagnostic criteria for proliferative hepatic lesions in brown bullhead Ameiurus nebulosus

Brown bullhead Ameiurus nebulosus is used as indicator species for contaminant effects at areas of concern (AOC) in the Great Lakes and other areas. One of the beneficial use impairments at numerous AOC is 'fish tumors and other deformities'. An impairment occurs when the prevalence of fish tumors and other deformities exceeds those at unimpacted or control sites or when survey data confirm the presence of neoplastic or preneoplastic liver lesions in bullhead or white sucker Catostomus commersonii. Numerous surveys have been conducted over the years assessing neoplasia in these fishes, both liver and skin tumors. However, a major problem in comparing the results has been a lack of consistent criteria for evaluating histological changes in bullhead livers. As individual AOC develop and implement remedial action plans, realistic and attainable delisting targets need to be specified. For this to occur and be consistent from site to site there must be standardization of the criteria being used to evaluate specific impairments. In this report, specific diagnostic criteria are provided for both non-neoplastic and neoplastic proliferative hepatocellular and biliary lesions. These criteria should assist fish pathologists in describing and categorizing proliferative liver lesions from brown bullhead.

Quantitative evaluation of macrophage aggregates in brook trout Salvelinus fontinalis and rainbow trout Oncorhynchus mykiss

Macrophage aggregates (MAs) occur in various organs of fishes, especially the kidney, liver and spleen, and contain melanin, ceroid/lipofuscin and hemosiderin pigments. They have been used as indicators of a number of natural and anthropogenic stressors. Macrophage aggregates occur in salmonids but are poorly organized, irregularly shaped, and are generally smaller than those in derived teleosts. These features complicate quantification, and thus these fishes have seldom been used in studies correlating MAs with environmental stressors. To alleviate these complications, we developed color filtering algorithms for use with the software package ImagePro Plus (Media Cybernetics) that select and quantify pigmented area (i.e. colors ranging from gold to brown to black) in tissue sections. Image analysis results compared well with subjective scoring when tested on brook trout Salvelinus fontinalis and rainbow trout Oncorhynchus mykiss captured from high-elevation lakes or hatcheries. Macrophage aggregate pigments correlated positively with age and negatively with condition factor. Within individual fish, pigmentation correlated positively among organs, suggesting that the kidney, liver or spleen are suitable indicator organs. In age-matched fishes, MA pigments were not different between hatcheries and lakes in the organs examined. Between lakes, differences in pigments were observed in the kidney and spleen, but were not explained by age, condition factor, sex or maturation state. Our results indicate that quantification of the area occupied by MA pigments is an efficient and accurate means of evaluating MAs in salmonid organs and that organ pigmentation correlates with age and condition factor, as seen in studies with more derived fishes.

Diagnostic criteria for proliferative thyroid lesions in bony fishes

Thyroid proliferative lesions are rather common in bony fishes but disagreement exists in the fish pathology community concerning diagnostic criteria for hyperplastic versus neoplastic lesions. To simplify the diagnosis of proliferative thyroid lesions and to reduce confusion regarding lesion interpretation, we propose specific criteria for distinguishing hyperplastic from neoplastic lesions. Development of these criteria was based on the examination of a large series of proliferative lesions from Japanese medaka (Oryzias latipes), lesions from other small fish species, and a reexamination of the 97 cases of proliferative thyroid lesions from bony fishes deposited in the Registry of Tumors in Lower Animals. Specific diagnostic criteria are provided for all lesion categories including follicular cell hyperplasia (simple, nodular, or ectopic), adenoma (papillary or solid), and carcinoma (well- or poorly differentiated). These criteria should assist fish pathologists in describing and categorizing naturally occurring proliferative lesions from wild fishes, lesions that develop in laboratory fishes due to suboptimal culture practices or water quality, those in fishes used in toxicological assays, and captive aquarium fishes.

MX [3-chloro-4-(dichloromethyl)-5-hydroxy-2[5H]-furanone], a drinking-water carcinogen, does not induce mutations in the liver of cII transgenic medaka (Oryzias latipes)

Mutagenicity assays with Salmonella have shown that 3-chloro-4-(dichloromethyl)-5-hydroxy-2[5H]-furanone (MX), a drinking-water disinfection by-product, is a potent mutagen, accounting for about one-third of the mutagenic potency/potential of chlorinated drinking water. The ability of MX to induce mutations was investigated in the liver of medaka (Oryzias latipes), a small fish model, utilizing the cII transgenic medaka strain that allows detection of in vivo mutations. Methylazoxymethanol acetate (MAMAc), a carcinogen in medaka, served as a positive control. Fish were exposed to MX at 0, 1, 10, or 30 mg/L for 96 h, whereas the MAMAc exposures were for 2 h at 0, 0.1, 1, or 10 mg/L. Both exposures were conducted under static water conditions and with fasted medaka. Following exposure, fish were returned to regular culture conditions to allow mutation expression for 15 or 40 d for MX or for 15 or 32 d for MAMAc. Mutations were not induced in medaka exposed to MX for 96 h. However, a concentration- and time-dependent increase in mutations was observed from the livers of fish exposed to 1 and 10 mg/L MAMAc. In conclusion, mutation induction was not observed in the livers of cII medaka exposed to MX for 96 h; however, studies are planned to examine mutation induction in the gills and skin to explore the possibility that MX-induced DNA damage occurs primarily in the tissues of initial contact.

p-Nitrophenol and glutathione response in medaka (Oryzias latipes) exposed to MX, a drinking water carcinogen

When chlorine is introduced into public drinking water for disinfection, it can react with organic compounds in surface waters to form toxic by-products such as 3-chloro-4-(dichloromethyl)-5-hydroxy-2[5H]-furanone (MX). We investigated the effect of exposure to MX on cytochrome P450 2E1 (CYP2E1)-like activity and total glutathione (GSH) in the liver of the small fish model, medaka (Oryzias latipes). The multi-site carcinogen methylazoxymethanol acetate (MAMAc) was the positive control compound. Both medaka liver microsome preparations and S-9 fractions catalyzed the hydroxylation of p-nitrophenol (PNP), suggesting CYP2E1-like activity in the medaka. Male medaka exposed for 96 h to the CYP2E1 inducers ethanol and acetone under fasted conditions showed significant increases in PNP-hydroxylation activity. Furthermore, total reduced hepatic GSH was reduced in fish fasted for 96 h, indicating that normal feeding is a factor in maintaining xenobiotic defenses. Exposure to MX and MAMAc induced significant increases in hepatic CYP2E1-like activity, however MX exposure did not alter hepatic GSH levels. These data strengthen the role of the medaka as a suitable species for examining cytochrome P450 and GSH detoxification processes and the role these systems play in chemical carcinogenesis.

Use of the Japanese medaka (Oryzias latipes) and guppy (Poecilia reticulata) in carcinogenesis testing under national toxicology program protocols

A need exists for whole animal toxicity, mutagenesis, and carcinogenesis models that are alternative to the traditional rodent test models and that are economical, sensitive, and scientifically acceptable. Among small fish models, the Japanese medaka (Oryzias latipes) is preeminent for investigating effects of carcinogenic and/or toxic waterborne hazards to humans. The guppy (Poecilia reticulata), although less widely used, is valuable as a comparison species. Both species are easy to maintain and handle in the laboratory and there is a large body of background information on their responsiveness to a range of classes of carcinogens. There are considerable data on the occurrence of background diseases and on spontaneous neoplastic lesions, both of which occur relatively rarely. With few modifications, the medaka and guppy are amenable to carcinogenicity testing under the rigid standards established by the National Toxicology Program (NTP) for rodent tests. The advantages of the small fish in carcinogenesis studies are best realized in long-term studies that involve environmentally realistic exposures. Studies to identify chronic effects can be conducted in about 12 months, near the life span of medaka in our laboratory. Practically, 9-month studies are optimal but shorter study cycles and a variety of exposure/growout and initiation/promotion scenarios are available. Studies on 3 compounds tested in medaka under NTP protocols are under review and preliminary analysis indicates that chronic carcinogenicity bioassays with medaka, guppy, and potentially with other small fish species are feasible and scientifically valid.

Life cycle of Calyptospora funduli (Apicomplexa: Calyptosporidae)

The taxonomic status of the extraintestinal piscine coccidium Calyptospora funduli is based in part on its requirement of an intermediate host (the daggerblade grass shrimp Palaemonetes pugio). In the present study, grass shrimp fed livers of Gulf killifish (Fundulus grandis) infected with sporulated oocysts of C. funduli exhibited numerous sporozoites suspended in the intestinal contents when fresh squash preparations were examined by light microscopy. Using this method, sporozoites were not seen in intestinal epithelial cells of the grass shrimp or in any other cell types. Ultrastructural examination, however, revealed sporozoites in the cytoplasm of the gut basal cells. Cross-sections of 1-13 sporozoites were seen within a single cell, and those sporozoites each appeared to be situated in individual membrane-bound vesicles, rather than in a single parasitophorous vacuole. These ultrastructural observations indicate that in the grass shrimp intermediate host, sporozoites that develop into an infective stage probably undergo that development in gut mucosal basal cells. Prior studies revealed that these sporozoites modified their structure over 4-5 days and that before that time, they were not infective to the fish host. Following ingestion of an infected shrimp by a killifish, the infective sporozoites apparently reach the liver of their killifish definitive hosts through the bloodstream. Sporozoites were seen in blood smears from the longnose killifish, Fundulus similis, 4 hr after fish were fed experimentally infected grass shrimp. Additionally, coccidian trophozoites and early meronts were seen in hepatocytes from several longnose killifish at 48, 72, and 96 hr postinfection. This study, in conjunction with previous findings, clearly confirms that a true intermediate host is required in the life cycle of C. funduli, that a developmental period of about 5 days in grass shrimp is necessary for sporozoites to become infective to killifishes, and that sporozoites do occur intracellularly in gut basal cells of the grass shrimp.

Proliferative lesions in swimbladder of Japanese medaka Oryzias latipes and guppy Poecilia reticulata

Thirteen cases of proliferative lesions of the swimbladder were encountered in Japanese medaka Oryzias latipes and guppy Poecilia reticulata from about 10,000 medaka and 5000 guppies used in carcinogenicity tests and histologically examined. Two of the 4 cases from medaka and 8 of the 9 from guppies occurred in untreated control specimens. The lesions affected the gas gland epithelium and included hyperplasia, adenoma, and adenocarcinoma. One medaka had hyperplasia of the gas gland epithelium and in 1 guppy the gland was enlarged with an increase in the number of epithelial layers. Gas gland adenomas, 3 cases in medaka and 1 in the guppy, were typically larger than the hyperplastic lesions, formed expansive masses up to 1 mm in greatest dimension, and exhibited a solid or glandular growth pattern and mild cellular pleomorphism. Adenocarcinoma was the most advanced lesion and all 7 cases occurred in guppies. Adenocarcinomas sometimes filled the entire swimbladder and measured up to 2.5 mm in diameter. Cells of adenocarcinomas were highly pleomorphic, with atypical nuclei, and an elevated mitotic activity. Because most of these tumors occurred in fish from control groups or in tests with noncarcinogenic compounds, the lesions observed here are probably spontaneous rather than chemically induced. Their rare occurrence, however, makes swimbladder proliferative lesions in small-fish carcinogenesis models sensitive indicators of compounds that might target cells of the gas gland.

Evaluation of bleached kraft mill process water using Microtox(R), Ceriodaphnia dubia, and Menidia beryllina toxicity tests

To determine whether a 7- to 10-d embryo toxicity/teratogenicity test with the inland silverside fish, Menidia beryllina, is a sensitive indicator for evaluation of bleached kraft mill effluents, we compared this test with the Microtox(R) 15-min acute toxicity test and the Ceriodaphnia dubia 7-d chronic toxicity test. Water samples used in each test were collected from three areas in a bleached kraft pulp and paper mill using a 100% chlorine dioxide bleaching process: 1) river water prior to use in the mill; 2) the combined acid/base waste stream from the pulping process prior to biological treatment in the aerated stabilization basin (ASB); and 3) the effluent from the ASB with a retention time of approximately 11 d. Relative toxicity determined by the three tests for each water sampling location was compared. All three toxicity tests were predictive indicators of toxicity; however, the C. dubia and M. beryllina tests were the more similar and sensitive indicators of toxicity. Process water (ASB influent) prior to biological treatment in the ASB was toxic at all concentrations using the Microtox(R) and C. dubia tests. The fish embryo test showed no toxicity at 1% concentrations, slight toxicity at 10%, and acute toxicity at the 100% ASB influent concentration. Tests with biologically-treated ASB effluent indicated a substantial reduction in observed toxicity to Microtox(R) bacteria, C. dubia, and M. beryllina. No toxic responses were observed in any test at a 1% ASB effluent concentration which was the approximate effluent concentration in the receiving river following mixing. No relationship was found among any toxicological response and effluent levels of adsorbable organic halides, polychlorinated phenolic compounds, 2,3,7,8-tetrachlorodibenzo-p-dioxin, 2,3,7,8-tetrachlorodibenzofuran, total suspended solids, color, chemical oxygen demand, or total organic carbon.

Diagnostic criteria for degenerative, inflammatory, proliferative nonneoplastic and neoplastic liver lesions in medaka (Oryzias latipes): consensus of a National Toxicology Program Pathology Working Group

Diagnostic criteria are presented for degenerative, inflammatory, nonneoplastic proliferative, and neoplastic lesions in the liver of medaka (Oryzias latipes), a small fish species frequently used in carcinogenesis studies. The criteria are the consensus of a Pathology Working Group (PWG) convened by the National Toxicology Program. The material examined by the PWG was from Medaka exposed to N-nitrosodiethylamine for 28 days, removed to clean water, and sacrificed 4, 6, or 9 mo after initiation of exposure. Degenerative lesions included hepatocellular intracytoplasmic vacuolation, hepatocellular necrosis, spongiosis hepatis, hepatic cysts, and hepatocellular hyalinization. Inflammatory lesions consisted of granulomas, chronic inflammation, macrophage aggregates, and focal lymphocytic infiltration. Nonneoplastic proliferative lesions comprised foci of cellular alteration (basophilic focus, eosinophilic focus, vacuolated focus, and clear cell focus) and bile duct hyperplasia. Neoplastic lesions included hepatocellular adenoma, hepatocellular carcinoma, cholangioma, and cholangiocarcinoma. Two lesions composed mainly of spindle cells were noted, hemangiopericytoma and spindle cell proliferation. Rather than being an exhaustive treatment of medaka liver lesions, this report draws from the published literature on carcinogen-induced liver lesions in medaka and other fish species and attempts to consolidate lesion criteria into a simplified scheme that might be useful to pathologists and other researchers using medaka lesions for risk assessment or regulatory purposes.

Exocrine pancreatic neoplasms in the mummichog (Fundulus heteroclitus) from a creosote-contaminated site

A high prevalence of exocrine pancreatic neoplasms occurred in mummichog, Fundulus heteroclitus, from a creosote-contaminated site in the Elizabeth River, Virginia. A total of 20 neoplasms were found in a group of about 1,300 fish obtained at this site over a 2-yr period. Of 240 fish collected during October 1991, 3.3% had pancreatic neoplasms. Adjusted total lesion prevalence for large adult fish (Size Class III: total length = 75-85 mm; Size Class IV: total length > 85 mm) was 6.7%. Pancreatic neoplasms were not observed in 234 fish collected at this site during May 1991, nor were they found in 420 fish obtained during fall 1991 from 1 uncontaminated and 6 moderately contaminated localities. Lesions involved both mesenteric and intrahepatic exocrine pancreas and ranged from well-differentiated acinar cell adenomas to poorly differentiated acinar cell carcinomas. One fish had an atypical acinar cell focus. All specimens with pancreatic neoplasms also had hepatocellular lesions. This epizootic of exocrine pancreatic neoplasia is the first to be reported in a wild fish population. Based on chemical characterization of the site and limited experimental data on chemically induced pancreatic carcinogenesis in other small fish species, the neoplasms were probably caused by exposure of the mummichog to chemical carcinogens in their environment.

Ultrastructure of normal and neoplastic exocrine pancreas in the mummichog, Fundulus heteroclitus

The ultrastructure of exocrine pancreatic neoplasms occurring in the mummichog, Fundulus heteroclitus, from a creosote-contaminated site in the Elizabeth River, Virginia, is described and related to nonneoplastic exocrine pancreas. Normal mummichog pancreas was an anastomosing tubular gland, with parenchymal cells organized as branched, anastomosing tubules around a central ductular system. The pancreatic ductular system consisted sequentially of terminal canalicular lumens lined by acinar cells, pancreatic preductules formed by an acinar and a centroductular cell, pancreatic ductules lined by 2 centroductular cells, and pancreatic ducts lined by cuboidal or columnar epithelial cells resting on a basal lamina and stromal sheath. Acinar cell adenomas retained the normal tubular organization and relationship between acinar and centroductular cells. Tumor cells exhibited nuclear pleomorphism but contained a full complement of normal zymogen granules and rough endoplasmic reticulum (RER). Some adenomas exhibited necrosis and cellular degeneration. Acinar cell carcinomas ranged from well to poorly differentiated. They exhibited loss of cell polarity, moderate to severe nuclear pleomorphism, extensive variation in size, shape, and number of zymogen granules, variability in RER content, and cellular degeneration. Acinar cell neoplasms in the mummichog were similar to those induced chemically in other fishes and certain mammals, suggesting that this fish population has been exposed to potent chemical carcinogens and that the species may be an effective indicator of polluted estuarine environments.

Host specificity of Calyptospora funduli (Apicomplexa: Calyptosporidae) in atheriniform fishes

Calyptospora funduli has a broad host specificity, infecting at least 7 natural and 10 additional experimental definitive hosts, all atheriniform fishes within 5 families, but most in the genus Fundulus. Barriers, apparently innate ones, prevent any development of C. funduli in perciform fishes but allow incomplete or abnormal development of the parasite in a few unnatural atheriniform hosts. In the freshwater species Fundulus olivaceus and Fundulus notti, these abnormalities consisted of asynchronous development, degeneration of the parasite in early stages of development, and the formation of numerous macrophage aggregates. Rivulus marmoratus has the ability to eliminate infections with a granulomatous inflammatory response. Additional barriers that limit natural infections of C. funduli in other hosts include feeding behavior, environmental conditions, and geographic isolation.

Unusual eosinophilic granule cell proliferation in coho salmon (Oncorhynchus kisutch)

Whitish, opaque, coalescing masses were observed coating the visceral organs of numerous adult coho salmon when they were dissected for artificial spawning in the autumn of 1983 at the Puntledge River hatchery, British Columbia. One affected fish was observed in November 1991. Histological examination of 10 of these fish (nine in 1983 and one in 1991) revealed that the lesions consisted of unusual proliferations of eosinophilic granule cells (EGCs) with some characteristics of neoplasia. The proliferative lesions extended throughout the gastrointestinal tract and infiltrated through all layers of the gut. Pancreatic tissue and adipose tissues associated with the pyloric caeca were also infiltrated and replaced by the proliferating EGCs. A mild diffuse infiltration was also observed in the spleen, liver and kidney. Histochemical analysis and ultrastructural examination showed the proliferating cells to be essentially identical with the EGCs which are usually concentrated in the stratum granulosum of the intestine in normal salmonid fishes.

Adenocarcinoma of the retinal pigment epithelium in the guppy Poecilia reticulata Peters

A single case of adenocarcinoma of the retinal pigment epithelium occurred in a guppy, Poecilia reticulata, Peters. This is the first such tumour reported from fishes. The left eye of the affected fish was severely exophthalmic because of a large intraocular tumour mass. The tumour, which displaced normal retina anteriorly, consisted mainly of melanin-containing epithelial cells. Neoplastic cells were bilayered and arranged in a tubular pattern. The tumour was confined to the orbit. Although the specimen was from a group exposed to a mixture of halogenated organic compounds, the lesion was not considered to have been chemically induced because of its rare occurrence within the group as a whole.

Hepatic neoplasms in the mummichog Fundulus heteroclitus from a creosote-contaminated site

High prevalences of idiopathic hepatic lesions were found in mummichog, Fundulus heteroclitus, from a site in the southern branch of the Elizabeth River, VA, contaminated with polycyclic aromatic hydrocarbons. Grossly visible hepatic lesions occurred in a total of 93% of the individuals from this site and 33% of these fish had hepatocellular carcinomas. Hepatic lesions were not detected in fish from two less contaminated sites. Lesions included foci of cellular alteration, hepatocellular adenoma, early and advanced hepatocellular carcinomas, and cholangiocellular proliferative lesions. Advanced carcinomas exhibited several distinct cellular patterns and some livers contained multiple neoplasms occupying up to 80% of the hepatic parenchyma. Sediments from the contaminated site contained extremely high concentrations (2200 mg/kg dry sediment) of polycyclic aromatic hydrocarbons, which are believed to originate from an adjacent wood treatment facility that has used creosote. Concentrations were 730- and 35-fold higher than those at the two other sites. These findings indicate a strong positive association between exposure to creosote-contaminated sediments and the high prevalence of hepatic neoplasms in a feral population of mummichog and support the putative role of polycyclic aromatic hydrocarbons in fish hepatocarcinogenesis. Additionally, they suggest that the mummichog may be a useful indicator of exposure to carcinogens in aquatic environments.

Goussia girellae n. sp. (Apicomplexa: Eimeriorina) in the opaleye, Girella nigricans

Goussia girellae n. sp. is described from the opaleye fish, Girella nigricans. Merogonic stages were observed in the apices of intestinal epithelial cells, in the lamina propria, and in extra-intestinal sites including liver, gills, and spleen. Gamonts were observed in the intestinal epithelial cells. Only unsporulated oocysts were detected in the intestine, and sporulation occurred when feces containing oocysts were incubated for 48 h in seawater at 21 degrees C. Oocysts are elongated (24.8 x 14.7 micron) with a wall about 200 nm thick and have no residuum, micropyle, or polar granule. Sporocysts are ellipsoid (8.5 x 4.5 micron), have a thin two-layered wall approximately 30 nm thick, and consist of two valves joined by a suture. Although moribund opaleye were also infected with Gyrodactylus sp., Cryptobia sp., Cardicola sp., and epitheliocystis organisms (chlamydia), all fish were heavily infected with G. girellae and morbidity was thus attributed to the coccidium.

Exocrine pancreatic neoplasms induced by methylazoxymethanol acetate in the guppy Poecilia reticulata

Exocrine pancreatic neoplasms developed in the guppy Poecilia reticulata following a single brief exposure to methylazoxymethanol acetate [(MAM-Ac) CAS: 592-62-1]. Fish 6-10 days old were exposed to concentrations of MAM-Ac up to 100 mg/liter for 2 hours. Exposed specimens were transferred to carcinogen-free water and sampled periodically for tumor development. Pancreatic neoplasms occurred in approximately 9% of histologically examined individuals exposed to 10 mg MAM-Ac/liter or less. Neoplastic lesions were not found in 122 control specimens. The neoplasms included 6 cases diagnosed as adenoma, 7 cases diagnosed as acinar cell carcinoma, and 2 cases diagnosed as adenocarcinoma. Adenomas consisted mainly of well-differentiated acinar cells that were filled with zymogen granules. Two adenomas also contained foci of atypical, less-differentiated acinar cells possessing basophilic, fibrillar cytoplasm. Acinar cell carcinomas occurred in several cellular patterns that ranged from well-differentiated to more anaplastic lesions; however, none exhibited areas of ductular proliferation. Adenocarcinomas, on the other hand, exhibited a glandular growth pattern and contained numerous ductlike structures. Both types of carcinomas appear to arise from primary acinar cells. Thus lesions probably progress from adenomas to acinar cell carcinomas and adenocarcinomas. The findings of carcinogen-induced pancreatic neoplasms in guppies further strengthen the usefulness of small fish species in carcinogen testing and provide an additional model for pancreatic tumors.

Intraocular neoplasms induced by methylazoxymethanol acetate in Japanese medaka (Oryzias latipes)

Intraocular neoplasms developed in the Japanese medaka, a small fish species, following a single brief exposure to methylazoxymethanol acetate [(MAM-Ac) CAS: 592-62-1]. Specimens 6-10 days old were exposed to doses of MAM-Ac up to 100 mg/liter for 2 hours and then transferred to carcinogen-free water for "grow-out." Of 218 exposed fish examined, 98 (45%) had neoplastic lesions in various stages of development. Of those exposed to 30 mg/liter or more, 57% had the lesions. No lesions were found in eyes or other tissues of 95 control specimens. Early and advanced neoplastic lesions were recognized. Early lesions were characterized by complexes of neoplastic retinal epithelium and tubes that consisted of cells of the sensory retina. Areas of mitotically active, heterogeneous cells associated with such complexes gave rise to advanced neoplasms. We considered the advanced neoplasms to be medulloepitheliomas, which differentiated into three principal cellular patterns: 1) solid masses of unpigmented cells, which frequently showed photoreceptor differentiation as well as ductular formation; 2) heavily pigmented cuboidal to columnar cells resembling retinal epithelium that formed adenomatous patterns; and 3) teratoid medulloepitheliomas. Teratoid medulloepitheliomas, which we considered the most advanced and malignant lesions, consisted of heterogeneous, highly mitotic, invasive cells and contained heteroplastic elements including striated muscle, undifferentiated mesenchymal tissues, and hyaline cartilage. We suggest that MAM-Ac induces hyperplasia of retinal cells followed by establishment of aberrant growth zones containing miscoded cells that give rise to medulloepitheliomas.

Development of aquarium fish models for environmental carcinogenesis: tumor induction in seven species

For small fish species to be utilized as models for carcinogenicity testing they should be capable of developing neoplasms, preferably in multiple tissues, when exposed to known carcinogens. Seven species of small fish were exposed to methylazoxymethanol acetate (MAM-Ac) and tumor development was monitored. Specimens 6-10 days old were exposed to nominal concentrations of MAM-Ac up to 100 mg 1(-1) for 2 h, then transferred to carcinogen-free water. Hepatic neoplasms developed in the Japanese medaka, guppy, sheepshead minnow, Gulf killifish, inland silverside, rivulus, and fathead minnow. Additionally, neoplasms occurred in other organs and tissues of the medaka (retina, various mesenchymal tissues, exocrine pancreas, kidney, and nervous tissue), guppy (mesenchymal tissue, exocrine pancreas, and kidney), and sheepshead minnow (choroid gland, mesenchymal tissues, and nervous tissue). All tumors were diagnosed in specimens within 1 year post-exposure. Early signs of liver tumors appeared in medaka and guppy at about 1 month post-exposure. These studies show that both medaka and guppy would be good models because they appear sensitive to carcinogens, develop tumors in multiple tissues and are easy to breed and maintain. Certain other small fish species also may prove to be good models because of habitat preferences, breeding strategies, or genetic attributes.