Nine mast cell tumors were found in 4, 000 mice used in chronic toxicological tests or carcinogenicity tests. Seven cases were diagnosed as malignant mastocytosis and the other two cases were diagnosed as mastocytoma. These neoplastic mast cells had a slightly larger cytoplasm and clear nuclei as compared with the normal mast cells, but their cytoplasmic granules showing a marked metachromasia by toluidine-blue stain were very fine similar to those of normal mast cells. Consequently, these cells were considered to be well-differentiated. The incidence of mast cell tumor was higher in the male than in female mice. Other features of mast cell tumors in mice were: a paucity of infiltration of eosinophils, proliferation of collagen fibers, edema, and other stromal reaction, were considered to be characteristic as compared with the mast cell tumors in other animals.

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The spontaneous tumors in Sprague-Dawley rats were studied in 510 females and 450 males which have been used as control groups of various chronic toxicity tests. The incidence of spontaneous tumor was 86.9% in males and 96.0% in females, respectively. In female rats, the most frequent tumor was pituitary adenoma, and followed by breast tumor, insuloma, and adreno-cortical adenoma. In contrast, pituitary adenoma, insuloma, adrenal pheochromocytoma, and hepatic tumor were common tumors in males. The incidence of pituitary adenoma, mammary tumor, thymoma, and adrenocortical adenoma was higher in females than males. On the other hand, the incidence of lung tumor, hepatocellular carcinoma, follicular cell adenoma of the thyroid, pheochromocytoma, and renal tumor was higher in males. Leukemia was also noted about 2.3% in males and 0.8% in females, and all of them were myelogenic leukemia. Moreover, various other tumors were found in other organs or tissues, but their incidences were low.

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Spontaneous neoplastic and non-neoplastic lesions in ACI/N, F344/DuCrj, Slc: Wistar and NRC: Donryu rats used as controls in carcinogenicity studies, were examined. Tumors of the endocrine organs, genital orgams, and mammary gland were the most common spontaneous tumors in all strains of rats examined. Besides these tumors, various tumors were also detected in many organs at low incidences. From the results on the organ distribution and histological types of spontaneous tumors, and the characteristic non-neoplastic lesions, it was indicated that Donryu rats were different from ACI, F344, and Wistar rats. ACI rats were also different from F344 and Wistar rats, except for the fact that interstitial cell tumors of the testis were observed with high incidence. On the other hand, there was no difference between F344 and Wistar rats with respect to the organ distribution and histological types of spontaneous tumors observed.

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Pancreatic acinar cell hyperplastic lesions and neoplasms were examined for nuclear DNA content by microspectrophotometry. Hyperplastic acinar cell lesions induced in rats by azaserine displayed a wide range of DNA contents, 1.5C-7C for acidophilic cell type and 1.5C-7.5C for basophilic cell type compared with the euploid pattern of 1.5C-5C in normal pancreas. On the other hand, the modal DNA values of rat acinar cell adenomas were distributed over a wider range of 2C-8.5C. In acinar cell carcinoma, the DNA histogram showed an aneuploid pattern of 2C-9C. In human pancreas of autopsy cases, hyperplastic acinar cell lesions composed of eosinophilic cells displayed a slightly wider range of DNA content (1.5C-6.5C) than that in normal acinar cells (1.5C-5C) or that in surrounding acinar cells without pathological change (1.5C-6C). The DNA histogram of basophilic cell type hyperplastic lesions were of an aneuploid pattern (2.5C-8.5C) similar to that of an acinar cell carcinoma (2C-10.5C). Acinar cell adenoma showed a polyploid pattern in its histogram (1.5C-8.5C). These findings suggest that acinar cell hyperplastic lesions in rat and human probably are precursors for acinar cell neoplasms.

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Dose-response studies on early lesions and carcinogenesis of urinary bladder were carried out in male rats treated with N-butyl-N-(4-hydroxybutyl) nitrosamine (BBN). In experiment 1, BBN was given in 6 different concentrations as solutions in drinking water to examine early lesions of the urinary bladder to F344, 6-week-old rats for up to 12 weeks. Doses of more than 0.01% BBN induced histologically simple hyperplasia and papillary or nodular hyperplasia, and scanning electron microscopically pleomorphic microvilli, short, uniform microvilli, and ropy or leafy microridges. 0.005% BBN treatment also induced ropy or leafy microridges. In experiment 2, BBN was given in 3 different doses to Wistar, 8-week-old rats for up to 82 weeks. The highest dose, 0.005% induced urinary bladder carcinoma and the lowest dose, 0.0005% did not develop any changes of the urinary bladder.

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Promotion potential of uracil-induced urolithiasis on rat bladder carcinogenesis of male F344 rats pretreated with 0.05, 0.01, 0.005 or 0.001% of N-butyl-N-(4-hydroxybutyl) nitrosamine (BBN) was investigated. Enhancement of induction of papillomas and carcinomas by 3% uracil was observed only in rats given 0.05% BBN. Initiation activity of uracil on urinary bladder carcinogenesis was also studied using promotion by NaHCO₃. No tumor was seen in a uracil-NaHCO₃ group whereas increased incidences of tumors were observed in a BBN-NaHCO₃ group.

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Nine mutagenic heterocyclic amines, 5 foodstuffs, and 4 anti-tumor drugs were tested for tumor initiating activity in a two stage skin carcinogenesis model using 12-O-tetradecanoyl phorbol-13-acetate (TPA) as the promoter. All compounds were topically applied twice weekly for 5 weeks on the dorsal skin, and then followed by similar TPA administration for 47 weeks. Test chemical groups with TPA promotion induced skin tumors in 5 to 35% of the mice, wheter no tumors appeared in the groups treated with test chemicals alone or TPA alone. Statistical analysis according to either Fisher exact test or Peto trend test revealed significant differences for tumor appearance in the Trp-P-1, Trp-P-2, MeAαC, Phe-P-1, AF-2, and BHA followed by TPA groups as compared to the TPA alone. Zinc Chloride, chromic chloride, cadmium chloride, nickel chloride, mercuric chloride, and arsenic acid were tested for possible tumor promoting activity in the skin of emale SENCAR mice pretreated with 7, 12-dimethylbenz (a) anthracene (DMBA). At a dose of 200 μg applied twice weekly for 26 weeks, none of the metal compounds tested enhanced skin tumor development. They also showed no tumor-developing potency when continuously applied in the smae manner without DMBA initiation. The results indicated that Trp-P-2 and AF-2 have an initiating activity, and Trp-P-1, MeAαC, Phe-P-1 and BHA show possible initiating potency in two-stage mouse skin model, but the heavy metal compounds tested do not have any tumor-promoting or tumorigenic activity in the mouse skin.

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The growth behavior of two transplantable osteosarcomas, spontaneously occurring and a carcinogen, 4-hydroxyamino-quinoline 1-oxide (4-HAQO), induced in Fischer 344 rats are described. The transplantability of those neoplasms was more than 80% and their doubling time ranged from 2.8 to 5 days. At the present time, the tumor occurring spontaneously is in the 17th generation and the 4-HAQO induced one is in the 14th generation. In spontaneously occurring osteosarcoma, numerous osteoid formation was observed but by serial transplantation osteoid formation decreased and disappeared by the 10th generation. On the other hand, in 4-HAQO induced osteosarcoma, osteoid formation was maintained by serial transfer and pulmonary metastasis occurred in animals receiving subcutaneous transplantation. These results provide a useful experimental model for studying tumor cell differentiation in forming osteoid and pulmonary metastasis.

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Recently a new mutant causing hereditary hepatitis was established from non-inbred Long-Evans rats. Hepatitis with severe jaundice appears spontaneously in rats 4 months after birth. Some affected rats die of submassive hepatic necrosis, while others survive for a long life. Liver cancers appear in these long survived rats. The hereditary hepatitis rats thus provide an animal model useful for the basic and clinical studies of hepatitis and liver cancer.

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Properties of hyperplastic hepatic nodules, presumptive preneoplastic lesions of hepatocellular carcinomas, were investigated in mosaic rodents of which livers comprise two genetically-different lineages of cells. In the first experiment, we explored the monoclonality of hyperplastic nodules induced by DEN and phenobarbital in female C3H×spf-ash F1 mice of which livers are composed of hepatocytes with and without ornithine carbamyl transferase (OCT) activity. Simultaneous staining for OCT and γ-glutamyltranspeptidase activities demonstrated that the nodules were composed entirely of one or the other of 2 lineages, representing their monoclonality. The second experiment was carried out to investigate the phenotypic reversion of early hyperplastic nodules by using transplantation system between analbuminemic rats (NAR) and SD×NAR F1 rats. Hyperplastic nodule cells isolated from NAR were transplanted into the liver of SD×NAR F1 rats of which livers are positive for albumin. In those recipient mosaic rats, hyperplastic nodules were virtually negative for albumin and the surrounding nonhyperplastic nodule cells were positive. It became clear that, although hyperplastic nodules express biochemical markers during administration of promoting stimuli, they revert to become phenotypically normal after removal of the stimuli.

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N-methyl-N-nitrosourea (MNU) 50mg/kg b.w. was administered to 50-day-old LE, WFN and F₁A (WFN×LE) rats maintained in our laboratory and after 24 weeks of MNU treatment they were sacrificed and served for pathological study. The developed tumors were mainly kidney, mammary tumors, and leukemia. Kidney tumor was histologically considered to be of mesenchymal origin similating a fibrosarcoma, and mammary tumors were tubular, papillary or compact type adenocarcinomas and fibromas. The mesenchymal tumors of kidney was observed only in WFN rats and its incidence was as high as 46.7%. DNA synthesis of the kidney at 50 days was examined. Bromodeoxyuridine (BrDU) 20mg/200g b.w. was injected into peritoneal cavity in LE, WFN, F₁A rats and after 60min they were sacrificed and incorporation of BrDU in the kidney cells was examined by anti-BrDU monoclonal antibody and ABC method. The mean incorporated cell numbers in one kidney by 10 fields were LE 4.3±3.4, WFN 16.3±7.3, F₁A 2.5±2.6. BrDU uptake in WFN rat kidney was significantly (P<0.01) elevated. This finding well correlated with the occurrence of kidney tumor.

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Occurrence of spontaneous tumor in experimental animals is of great concern in long-term carcinogenic and chronic toxicity studies. Among spontaneous tumors, mammary tumor is one of the most predominant tumor in mice. However, it is well known that mouse mammary tumor virus (MMTV) is widely involved in all cells in the mice in a proviral form. Based on endogenous viral gene patterns, all strains of mice including inbred or non-inbred carried 2-6 such genes, though the incidence of mammary tumor is different depending on the strains. Administration of urethane in the drinking water of breeding mice lead to a higher incidence of mammary tumor at an early age in these strains. Urethane, however, did not induce or enhance endogenous retroviral antigen expression or increase in MMTV-DNA sequences in mammary tumor by radioimmunoassay or molecular hybridization. These results showed that MMTV was not involved in chemical carcinogenesis of the mouse mammary glands. In regard to mammary tumors in carcinogenic or chronic toxicity studies, we must examine further whether endogenous MMTV is activated or not.

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Five-week-old male Crj: CD (SD) rats were treated with 10% sodium chloride after a total dose of 20 Gy of X-ray administered to the gastric region in two equal fractions separated by 3 days. After treatments, the animals were sacrificed seqentially and analyzed quantitatively on the medial and small arteries in the stomach. In the early phase after X-irradiation, fibrous thickening of the intima was observed in the medium-sized arteries. Slight fibrosis of the adventitia appeared in the late phase after X-irradiation. The small artery with the fibrinoid necrosis of the serosa in the glandular stomach was recognized at 9 weeks after treatment with X-ray plus NaCl. These lesions were expanded into the small artery of the muscular layer, the submucosal and the mucosal layers, and the medium artery of the serosa. In quantitative analysis of the arteries using an image analyser, thickness of the arterial wall and the diameter of arteries became enlarged progressively in rats treated with X-ray plus NaCl.

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