Characterization of uterine granular cell tumors in B6C3F1 mice: a histomorphologic, immunohistochemical, and ultrastructural study.

The granular cell tumor is most often a benign neoplasm of uncertain origin. Four uterine granular cell tumors in control and treated female B6C3F1 mice were identified in chronic studies at the National Toxicology Program. Two tumors occurred in untreated control animals and 2 in treated animals receiving different compounds. Tissue sections were evaluated histologically and stained with hematoxylin and eosin, periodic acid-Schiff with diastase resistance, Masson's trichrome, toluidine blue, phosphotungstic acid-hematoxylin, and stained immunohistochemically with a panel of antibodies to muscle (desmin, alpha smooth muscle actin), neural (S-100, neuron specific enolase), epithelial (wide-spectrum cytokeratin), and macrophage (F4/80) markers. The main histomorphologic feature of tumor cells was the presence of abundant cytoplasmic eosinophilic granules that stained positive for periodic acid-Schiff with diastase resistance. Tumors varied in appearance and were comprised of sheets and nests of round to polygonal cells with distinct borders. Nuclei were hyperchromatic, pleomorphic, and centrally to eccentrically located and often contained single nucleoli. Occasional multinucleated giant cells were observed. Tumors were pale pink and homogeneous with trichrome stain and negative with toluidine blue. Three tumors had positive to weakly positive immunoreactivity for desmin, and 1 was positive for alpha smooth muscle actin. Expression of S-100, wide-spectrum cytokeratin, and neuron-specific enolase was negative for all tumors. Ultrastructurally, prominent electron-dense cytoplasmic granules were abundant and contained secondary lysosomes with heterogeneous lysosomal contents. The characteristics of these uterine granular cell tumors were suggestive of a myogenic origin.

Nitrapyrin: a scientific advisory group review of the mode of action and carcinogenicity in B6C3F1 mice.

Nitrapyrin has been registered as a nitrogen stabilizer in the United States for many years based on a robust set of regulatory data. These data demonstrated that nitrapyrin was not genotoxic and that there were no tumors elicited in rats or mice that were relevant for human risk assessment. A repeat carcinogenicity study in B6C3F1 mice, conducted at two substantially higher-dose levels (0, 125 or 250 mg/kg/day) than the original study (0, 5, 25 or 75 mg/kg/day) identified liver, stomach, epididymal and Harderian gland tumors. In order to assess the relevance of these findings for human risk assessment, a Scientific Advisory Group (SAG) examined relevant microscopic changes in these tissues and also evaluated genotoxicity and mechanistic data. The SAG determined that the maximum tolerated dose had been exceeded in mice given 125 or 250 mg/kg/day, based on 26-33% decreased body weight gains (males-250 mg/kg/day), hepatocellular necrosis and compensatory hepatocellular proliferation (males and females-125 and 250 mg/kg/day). The SAG believed that the increased incidences of hepatocellular foci of alteration and hepatocellular neoplasms represented an epigenetic response to hepatocellular necrosis and increased mitogenesis. Increased incidences of proliferative lesions in the forestomach mucosa were likely secondary to the irritant effects of nitrapyrin. Neither the liver nor forestomach effects were interpreted to be a direct carcinogenic effect. Higher incidences of Harderian gland adenomas (females) and undifferentiated sarcomas in the epididymis represented normal biological variations in incidence and were unrelated to nitrapyrin. Therefore, it was the SAG's opinion that nitrapyrin exposure that does not produce target organ toxicity in exposed individuals would not be expected to increase the risk of cancer.

A chronic inhalation toxicity/oncogenicity study of methylethylketoxime in rats and mice.

To evaluate the oncogenic potential of methylethylketoxime (MEKO), CD-1 mice (50/sex/group) and F-344 rats (50/sex/group) were coexposed 6 h/day, 5 days/wk for 18 mo (mice) or 26 mo (rats) via whole-body inhalation exposures to target vapor concentrations of 0, 15, 75, and 375 ppm (actual concentrations of 0, 15 +/- 1, 75 +/- 2, or 374 +/- 10 ppm). Satellite groups of rats and mice (10/sex/group/interval) were exposed for 12 mo (mice) and 3, 12, or 18 mo (rats) to evaluate chronic toxicity. Methyl ethyl ketone (MEK), a possible hydrolysis product of MEKO, was present at less than 1%. Treatment-related effects included increased body weight (male rats only), methemoglobin formation, hematology and clinical chemistry changes, increased liver weight, and increased spleen and testes weights (rats only). A high incidence of cataracts and corneal dystrophy occurred in both control and MEKO-exposed rats, with an earlier appearance and slightly higher incidence for these ocular lesions in MEKO-exposed animals compared to controls. Degenerative and reparative changes of the olfactory epithelium in the nasal turbinates, primarily limited to the dorsal meatus, occurred in both rats (75 and 374 ppm) and mice (15, 75, and 374 ppm). In addition, in the mice, liver changes included increased incidences of pigment in reticuloendothelial cells, centilobular hypertrophy, granulomatous inflammation, and a slightly increased incidence of necrosis (75 and 374 ppm). An increase in hepatocellular carcinomas occurred in male mice at 374 ppm. Additional MEKO-related findings in the rat included congestion of the spleen with pigment in reticuloendothelial cells and extramedullary hematopoiesis and a decreased incidence of lymphoreticular mononuclear cell leukemia. Effects observed in the liver of the rats included decreases in the incidence of both peribiliary fibrosis and hyperplasia/proliferation of the biliary duct, an increase of spongiosis hepatis in males, and an increase in the incidence of intracytoplasmic vacuoles and hepatocellular basophilic foci. The effects on the liver were generally most profound in the high-exposure groups and, with the exception of the spongiosis hepatis, occurred in both sexes. An increase in hepatocellular adenomas occurred in the male rats at 75 and 374 ppm, and hepatocellular carcinomas in the male rats at 374 ppm. In both species, the liver tumors appeared relatively late in the life of the animals, with no significant increase in tumors at 12 mo of exposure in mice and at 18 mo of exposure in rats. Lifespan shortening was not observed, as MEKO-exposed animals survived generally as well as, or slightly better than, the controls.

Methods to identify and characterize developmental neurotoxicity for human health risk assessment. II: neuropathology.

Neuropathologic assessment of chemically induced developmental alterations in the nervous system for regulatory purposes is a multifactorial, complex process. This calls for careful qualitative and quantitative morphologic study of numerous brains at several developmental stages in rats. Quantitative evaluation may include such basic methods as determination of brain weight and dimensions as well as the progressively more complex approaches of linear, areal, or stereologic measurement of brain sections. Histologic evaluation employs routine stains (such as hematoxylin and eosin), which can be complemented by a variety of special and immunohistochemical procedures. These brain studies are augmented by morphologic assessment of selected peripheral nervous system structures. Studies of this nature require a high level of technical skill as well as special training on the part of the pathologist. The pathologist should have knowledge of normal microscopic neuroanatomy/neuronal circuitry and an understanding of basic principles of developmental neurobiology, such as familiarity with the patterns of physiologic or programmed cell de

Histopathology of nasal olfactory mucosa from selected inhalation toxicity studies conducted with volatile chemicals.

In recent years, histopathologic changes have been reported in the olfactory mucosa of rodents exposed, by inhalation, to a variety of volatile chemicals. In order to better characterize these lesions, a panel of experienced pathologists reviewed microscopic lesions of the olfactory epithelium of rats reported in 10 inhalation studies conducted with 8 different chemicals. The objectives were to determine if the olfactory epithelial lesions are morphologically similar or different for the chemicals of interest, to develop and recommend appropriate diagnostic criteria and nomenclature to characterize the morphology of these olfactory lesions, and to provide specific criteria for judging the degree of severity of the olfactory changes in these studies. The results indicated that the distribution and nature of the lesions were similar in all the examined studies in which olfactory changes were observed. Recommended standardized nomenclature and diagnostic criteria and a uniform method for scoring lesion severity based on the extent of distribution and severity of tissue damage are presented.

The nomenclature of cell death: recommendations of an ad hoc Committee of the Society of Toxicologic Pathologists.

The last several years have seen considerable confusion regarding the terms "apoptosis" and "necrosis" in pathology. This situation prompted the Society of Toxicologic Pathologists to charter the Committee on the Nomenclature of Cell Death, which was charged with making recommendations about the use of the terms "apoptosis" and "necrosis" in toxicity studies. The Committee recommends use of the term "necrosis" to describe findings comprising dead cells in histological sections, regardless of the pathway by which the cells died. The modifiers "apoptotic" and "oncotic" or "mixed apoptotic and oncotic" are recommended to specify the predominant morphological cell death pathway or pathways, when appropriate. Other standard modifiers, indicating the lesion distribution and severity, may also be used in conjunction with these. "Individual cell necrosis" (also known as "single cell necrosis") may be either of the apoptotic, oncotic, or mixed types. In many cases, more traditional terms such as "coagulation necrosis" may be used to convey a meaning similar to oncotic necrosis. It is important that pathologists use terms that accurately and concisely convey the level of information appropriate to the study's needs. Furthermore, toxicologic pathologists should actively help to disseminate these recommendations to other biologists and to regulatory authorities.

Rodent Leydig cell tumorigenesis: a review of the physiology, pathology, mechanisms, and relevance to humans.

Leydig cells (LCs) are the cells of the testis that have as their primary function the production of testosterone. LCs are a common target of compounds tested in rodent carcinogenicity bioassays. The number of reviews on Leydig cell tumors (LCTs) has increased in recent years because of its common occurrence in rodent bioassays and the importance in assessing the relevance of this tumor type to humans. To date, there have been no comprehensive reviews to identify all the compounds that have been shown to induce LCTs in rodents or has any review systematically evaluated the epidemiology data to determine whether humans were at increased risk for developing LCTs from exposure to these agents. This review attempts to fill these deficiencies in the literature by comparing the cytology and ontogeny of the LC, as well as the endocrine and paracrine regulation of both normal and tumorigenic LCs. In addition, the pathology of LCTs in rodents and humans is compared, compounds that induce LC hyperplasia or tumors are enumerated, and the human relevance of chemical-induced LCTs is discussed. There are plausible mechanisms for the chemical induction of LCTs, as typified by agonists of estrogen, gonadotropin releasing hormone (GnRH), and dopamine receptors, androgen receptor antagonists, and inhibitors of 5alpha-reductase, testosterone biosynthesis, and aromatase. Most of these ultimately involve elevation in serum luteinizing hormone (LH) and/or LC responsiveness to LH as proximate mediators. It is expected that further work will uncover additional mechanisms by which LCTs may arise, especially the role of growth factors in modulating LC tumorigenesis. Regarding human relevance, the pathways for regulation of the hypothalamo-pituitary-testis (HPT) axis of rats and humans are similar, such that compounds that either decrease testosterone or estradiol levels or their recognition will increase LH levels. Hence, compounds that induce LCTs in rats by disruption of the HPT axis pose a risk to human health, except for possibly two classes of compounds (GnRH and dopamine agonists). Because GnRH and prolactin receptors are either not expressed or are expressed at very low levels in the testes in humans, the induction of LCTs in rats by GnRH and dopamine agonists would appear not to be relevant to humans; however, the potential relevance to humans of the remaining five pathways of LCT induction cannot be ruled out. Therefore, the central issue becomes what is the relative sensitivity between rat and human LCs in their response to increased LH levels; specifically, is the proliferative stimulus initiated by increased levels of LH attenuated, similar, or enhanced in human vs. rat LCs? There are several lines of evidence that suggest that human LCs are quantitatively less sensitive than rats in their proliferative response to LH, and hence in their sensitivity to chemically induced LCTs. This evidence includes the following: (1) the human incidence of LCTs is much lower than in rodents even when corrected for detection bias; (2) several comparative differences exist between rat and human LCs that may contribute, at least in part, to the greater susceptibility of the rat to both spontaneous and xenobiotic-induced LCTs; (3) endocrine disease states in man (such as androgen-insensitivity syndrome and familial male precocious puberty) underscore the marked comparative differences that exist between rats and man in the responsiveness of their LC's to proliferative stimuli; and (4) several human epidemiology studies are available on a number of compounds that induce LCTs in rats (1,3-butadiene, cadmium, ethanol, lactose, lead, nicotine) that demonstrate no association between human exposure to these compounds and induction of LC hyperplasia or adenomas. (ABSTRACT TRUNCATED)

Phenolphthalein induces thymic lymphomas accompanied by loss of the p53 wild type allele in heterozygous p53-deficient (+/-) mice.

Epidemiology studies have indicated that many human cancers are influenced by environmental factors. Genetically altered mouse model systems offer us the opportunity to study the interaction of chemicals with genetic predisposition to cancer. Using the heterozygous p53-deficient (+/-) mouse, an animal model carrying one wild type p53 gene and one p53 null allele, we studied the effects of phenolphthalein on tumor induction and p53 gene alterations. Earlier studies showed that phenolphthalein caused carcinogenic effects in Fisher 344 rats and B6C3F1 mice after a 2-yr dosing period (Dunnick and Hailey, Cancer Res. 56: 4922-4926, 1996). The p53 (+/-) mice received phenolphthalein in the feed at concentrations of 200, 375, 750, 3,000, or 12,000 ppm (approximately 43, 84, 174, 689, or 2,375 mg/kg body weight/day or 129, 252, 522, 2,867, or 7,128 mg/m2 body surface area/day) for up to 6 mo. A target organ cancer site that accumulated p53 protein in the B6C3F1 mouse (i.e., thymic lymphoma) was also a target site for cancer in the p53 (+/-) mouse. In the p53 (+/-) mouse, treatment-related atypical hyperplasia and malignant lymphoma of thymic origin were seen in the control and dosed groups at a combined incidence of 0, 5, 5, 25, 100, and 95%, respectively. Twenty-one of the thymic lymphomas were examined for p53 gene changes, and all showed loss of the p53 wild type allele. Chemical-induced ovarian tumors in the B6C3F1 mouse showed no evidence for p53 protein accumulation and did not occur in the p53 (+/-) mouse. The p53-deficient (+/-) mouse model responded to phenolphthalein treatment with a carcinogenic response in the thymus after only 4 mo of dosing. This carcinogenic response took 2 yr to develop in the conventional B6C3F1 mouse bioassay. The p53-deficient (+/-) mouse is an important model for identifying a carcinogenic response after short-term (< 6 mo) exposures. Our studies show that exposure to phenolphthalein combined with a genetic predisposition to cancer can potentiate the carcinogenic process and cause p53 gene alterations, a gene alteration found in many human cancers.

Peer review in toxicologic pathology.

Peer review of histopathology findings in safety assessment studies involving rodents and other animals is a relatively recent procedure in toxicologic pathology. It serves to ensure the integrity of the pathology evaluation in safety studies, encourages consistency of diagnostic criteria and use of common terminology, and provides a method of continuing education for participants. The use of a standardized system of pathology nomenclature and diagnostic criteria, such as the Society of Toxicologic Pathologist's Guides for Toxicologic Pathology, is of great value in the procedure. Pathology reviews may involve government-sponsored bioassay programs, in-house industrial corporations, or individual peer reviews suggested or required by government regulatory agencies. Pathology Working Groups can be an integral part of the review process. The extent of the peer review is primarily dependent on the study results; however, other variables such as confidence of the data, study size and duration, complexity, and purpose are also important considerations. Essential components of any peer review, however, include selection of tissues/lesions for review, by a reviewing pathologist, discrepancy resolution, data modification, and documentation of all aspects of the review process. Specific procedures for pathology peer review are discussed. Disagreements among pathologists discovered in peer reviews can be resolved by several methods and examples will be presented. The entire pathology peer review process should be a learning experience for all involved and can help ensure the integrity of animal toxicology studies used for important regulatory decisions involving the use of chemicals in our society.

A comparison of liver tumor diagnoses from seven PCB studies in rats.

Through a policy assumption, all polychlorinated biphenyls (PCBs) are considered probable human carcinogens by most regulatory agencies based on experimental studies in rodents where an increased incidence of liver tumors has been observed. Recognizing that new consensus criteria for the diagnoses of liver tumors in rats had been promulgated, a reevaluation of liver tumor diagnoses from seven PCB studies in rats was undertaken. These seven studies, in which rats were fed PCB mixtures containing 42, 54, or 60% chlorine, were considered to be the best studies from which to evaluate the cancer potential of PCB mixtures. The reevaluation results, where consistent diagnoses now exist across all studies, clearly indicate major differences in carcinogenic potential based on degree of chlorination. Studies of mixtures with 60% chlorination consistently resulted in a high incidence of liver tumors, whereas studies in which rats were fed mixtures with 54 or 42% chlorination showed no statistically significant increases in liver tumors. These data indicate that continuation of a science policy of assuming that all PCBs are probable human carcinogens with a potency equivalent to the mixture that contains 60% chlorine has no scientific foundation and should be reconsidered.

Proliferative lesions of the mouse lung: progression studies in strain A mice.

The progression of pulmonary neoplasia was examined in strain A/J male mice treated with a single dose of vinyl carbamate (60 mg/kg, i.p.) 6 weeks after birth. Interim sacrifices were performed at 7, 8, 10, 12, or 14 months. Proliferative lesions of the lung were divided into four categories: hyperplasias, adenomas, carcinomas arising within adenomas, and carcinomas. Grossly visible surface tumor counts, histologic diagnoses, and morphometric measurements of histologic lesions were used to evaluate progression. Vinyl carbamate-treated mice showed increased mean surface tumor counts at all time points. Diagnostic evaluation suggested that as a function of time, the relative frequency of hyperplasias decreased and the relative frequency of adenomas increased. The relative frequency of adenomas subsequently decreased, whereas the relative frequency of carcinomas increased. At all time points, carcinomas arising within adenomas were present. As time progressed, the number of carcinomas arising within adenomas decreased, whereas the number of "pure" carcinomas increased. Morphometric analysis of lesions indicated hyperplasias to be small, that adenomas were larger than hyperplasias, and carcinomas were larger than adenomas and hyperplasias, suggesting that few adenomas or carcinomas arise de novo. Collectively, these data suggest that the majority of pulmonary tumors in A/J mice treated with vinyl carbamate arise as hyperplasias, progress to adenomas, and ultimately result in carcinomas.

Tumors of the skin in the HRA/Skh mouse after treatment with 8-methoxypsoralen and UVA radiation.

8-Methoxypsoralen (8-MOP) with and without UVA radiation was administered to HRA/Skh mice (36 animals per treatment group) three times a week in the feed for a total dose of 9-80 mg/kg/week for 52 weeks. Most of the animals at the top dose of 8-MOP with UVA radiation had developed skin toxicity and/or skin tumors by 52 weeks. The skin lesions seen after treatment with 8-MOP and UVA radiation were characterized as squamous cell hyperplasia, squamous cell papilloma, and squamous cell carcinoma and are similar to what has been reported in humans after exposure to 8-MOP and UVA. Squamous cell hyperplasia and acute inflammation of the cornea were also seen in some of the treated female mice. Oral administration of 8-MOP and UVA did not result in a carcinogenic response to other organ systems. There were no increases in skin neoplasms after 8-MOP or UVA radiation alone. 8-MOP given in combination with UVA was carcinogenic to the skin of mice at dose levels similar to those used to treat psoriasis in humans.

Pulmonary effects due to subchronic exposure to oil fog.

Male and in some cases female rats were exposed to an oil fog generated by flash vaporization and subsequent condensation of light-weight lubricating oil. Exposures were for 3.5 h/d, 4d/wk for 13 wk. Males were exposed at concentrations of 1.5, 0.5, 0.2 or 0.0 mg/l (1500, 500, 200, and 0 mg/m3) and a particle size of approximately 1 micron (mass median aerodynamic diameter). A number of biologic endpoints were assessed the day after the last exposure and, in some cases, after a 4 wk recovery period. Effects of 1.5 mg/l on male and female rats were compared. Diffuse accumulation of macrophages in the alveoli was observed in all oil fog exposed groups. The degree of severity was concentration dependent. Histopathologic changes were more prominent in males than in females and represented the most notable gender-related differences. Histologic effects observed one day and 4 wk post exposure were similar. Minimal histopathologic changes and minimal increase in lavage fluid protein were the only effects observed at the 0.2 mg/l exposure level. There was a significant increase in lavage fluid protein, percent lavagable polymorphonuclear leukocytes and lung wet and dry weight following exposure to both 0.5 and 1.5 mg/l. At the highest exposure concentration effects on lung weights were still evident 4 wk post exposure. Pulmonary function endpoints including total lung capacity, vital capacity, residual volume, diffusing capacity to CO, compliance, and end expiratory volume (EEV) were unaffected by oil fog exposure with the exception of EEV in males exposed at the 1.5 mg/l level. All of the changes observed following oil fog exposure were consistent with a mild inflammatory edema.

Pulmonary effects due to short-term exposure to oil fog.

Rats were exposed to an oil fog generated by flash vaporation and subsequent condensation of lightweight lubricating oil. Exposures were for 3.5 h/d, 4 d/wk, for 4 wk, at concentrations of 1.5, 0.5, or 0.0 mg/l and a particle size of approximately 1 micron. Samples of respiratory tissues were taken for histopathologic analyses, lavage fluid samples were collected, and pulmonary function measurements were made the day after the last exposure. An accumulation of macrophages within the alveolar lumen, an increase in lavage fluid protein content, and an increase in total cell content in lavage fluid due to an influx of polymorphonuclear leukocytes was noted in rats exposed at the 1.5-mg level. Also, for this exposure group there was an increase in lung wet and dry weight and an increase in end-expiratory volume, and pneumonitis was observed histopathologically in 4 of 10 male rats exposed. Pneumonitis was not observed among six female rats examined. Oil fog had no effect on total lung capacity, residual volume, vital capacity, lung compliance, or the distribution of ventilated air within the lung. Effects following exposure to 0.5 mg/l were limited to slight accumulation of macrophages in the alveolar lumen and an increase in the total cells in lavage fluid, which could not be attributed to an increase in any particular cell type.

Factors influencing laboratory animal spontaneous tumor profiles.

In chemical carcinogenicity and drug-safety testing, a carcinogen is defined as an agent that when administered by an appropriate route causes an increased incidence of tumors in experimental animals as compared to unexposed control animals. Although a carcinogen may cause the appearance of tumors in organs where tumors do not usually occur in a given strain, the usual response is to increase the types of tumors seen spontaneously and to shorten the period of latency. The use of carcinogenesis experiments for research and safety assessment requires properly designed and well-conducted experiments and a knowledge of background data and variations in tumor incidences of control animals. Many factors can influence the reported incidences of spontaneous tumors. These include species, strain, sex, age, and source of the experimental test animal; study duration; extent of the pathology examination; dietary and environmental conditions; qualifications and experience of the study pathologist; diagnostic criteria and nomenclature conventions; and quality assurance and review procedures. This paper discusses several factors which may influence the incidence of tumors in control and test animals, and provides examples to illustrate the potential for these factors to affect the data.

Chronic toxicity and oncogenicity bioassay of inhaled toluene in Fischer-344 rats.

The chronic toxicity and oncogenicity of inhaled toluene were assessed in Fischer-344 rats. One hundred and twenty animals of each sex were exposed for 6 hours/day, 5 days/week, for up to 24 months at concentrations of toluene in air of 0, 30, 100, or 300 ppm. The calculated time-weighted average concentrations for the 24 months of exposure were 0.0, 30.1, 99.7, and 299.0 ppm, respectively. Interim sacrifices on randomly selected animals were conducted after 6, 12 and 18 months of exposure. All surviving rats were sacrificed at 24 months. A large battery of tissues and organs from all animals in the control and 300 ppm toluene group were examined for histopathology. All animals were examined for clinical changes throughout the course of the study and selected animals were used to determine ophthalmologic, hematologic, clinical blood chemistry or urinalysis effects. There were 140 unscheduled deaths over the 2-years study. Gross pathologic examination of rats dying during the course of the study, or that were sacrificed as scheduled, did not reveal any lesions attributable to toluene exposure. Histologically, a variety of proliferative, degenerative and inflammatory lesions were observed in the control and 300 ppm toluene-exposed group. These lesions were considered unrelated to toluene exposure. The results provide no evidence that toluene causes chronic toxicity or oncogenicity in Fischer-344 rats at these concentrations.

Cancer induction following single and multiple exposures to a constant amount of vinyl chloride monomer.

Vinyl chloride monomer (VCM), already identified as a human animal carcinogen, was selected as a model agent to explore an area of concern for single and intermittent low level exposure. In traditional cancer bioassay, animals are repeatedly exposed over their lifespan to a dose of suspected chemical. In the current studies rats and mice were exposed in an inhalation chamber to single one-hour doses of VCM ranging from 50 to 50,000 ppm. A second group was given 10 one-hour exposures to 500 ppm or 100 one-hour exposures to 50 ppm of the same chemical. All animals were then observed for the remainder of their lives, generally 18-24 months. Moribund animals were euthanized, and survivors were sacrificed on schedule and their tissues examined for pathological changes. Specifically, the oncogenic study demonstrated dose related effects for single one-hour exposure of VCM at high levels, i.e., 5,000 and 50,000 ppm. These concentrations increased the incidence of pulmonary adenomas and carcinomas in mice. Repeated exposure of A/J mice to the same chemical at 500 ppm X 10 one-hour exposures also increased the incidence of pulmonary adenomas and carcinomas which are considered highly one-hour exposure, no significant increase in tumors was observed. Rats exposed to identical concentrations of VCM failed to elicit a tumorigenic response.

Evaluation of procedures suggested for reducing the pathology workload in a carcinogenesis testing program.

A sampling procedure for reducing the pathology workload in evaluating a carcinogenicity bioassay was utilized for a rodent bioassay. The use of the procedure resulted in 50 percent fewer tissues being examined histologically, in the control groups and 12 percent fewer tissues in the treatment groups. The obvious saving in pathologist's time was negated by an increase in technician time, interference with quality control procedures and loss of nontumor pathology information.

Correlation between gross observations of tumors and neoplasms diagnosed microscopically in carcinogenesis bioassays in rats.

Microscopic diagnoses of spontaneous and induced neoplasms in F344 rats in two carcinogenesis bioassays were correlated with the gross observations made at the time of necropsy. The results for a negative and a positive compound with reference to carcinogenicity, indicate that of the gross descriptions indicative of tumors, 37 percent and 24 percent respectively, were not diagnosed as neoplasms microscopically. However, some of the gross lesions were diagnosed histologically as nonneoplastic lesions. Of the neoplasms diagnosed microscopically only 70 percent and 76 percent, respectively, were observed grossly. Endocrine tissues, liver and lung had the lowest correlation rates. The implications of these findings are discussed.

Renal carcinogenic and nephrotoxic effects of the flame retardant tris(2,3-dibromopropyl) phosphate in F344 rats and (C57BL/6N X C3H/HeN)F1 mice.

Tris(2,3-dibromopropyl) phosphate (TBP) was administered in the feed at one of two concentrations to groups of 55 male and 55 female inbred F344 rats and to 50 male and 50 female (C57BL/6N X C3H/HeN)F1 mice. The high and low dietary concentrations of TBP administered orally were 100 and 50 ppm for the rats, respectively, and 1,000 and 500 ppm for the mice, respectively. For each rodent type, 55 animals of each sex were used as contols. In both rodent types, renal epithelial tumors developed at incidences that were significant for male and female rats and mice that received the doses. These tumors included renal tubular cell adenomas and carcinomas that developed from the proximal convoluted tubular epithelium. Among female mice and rats, hyperplasia and/or dysplasia of the proximal convoluted tubular epithelium with or without cystic dilatation of the tubules and increase in the size of cell nuclei were dose dependent and recognized as preneoplastic and/or toxic lesions. The comparative histogenesis of renal tubular neoplasms was discussed.