Recently, we introduced an organoid-based chemical carcinogenesis model using mouse normal tissue-derived organoids. In the present review article, the histopathological and immunohistochemical characteristics of mouse normal tissue-derived organoids and tumors derived from these organoids after their in vitro treatment with genotoxic carcinogens and injection into nude mouse are reviewed. In organoids treated in vitro with genotoxic carcinogens, we confirmed macroscopic tumorigenicity and histopathological findings, including neoplastic characteristics, such as multilayered epithelia and/or invasion of epithelia into the surrounding interstitium. In contrast glandular/cystic structures with monolayered epithelia were clearly demarcated from the surrounding Matrigel/interstitium in the untreated control groups. In addition to macroscopic tumorigenicity, these microscopic epithelial changes, which are characteristic of the early stages of carcinogenesis, are included in the requirements for carcinogenicity-positive judgement of the organoid-based carcinogenesis model. Immunohistochemistry of cytokeratins (CKs), used to determine the origin of epithelia and distribution of extraductal invasive lesions, or oncogenic kinases, which reflect molecular activation in epithelia following chemical treatment, is helpful for accurate diagnosis and molecular evaluation in the early stages of carcinogenesis. This information improves our biological understanding of organoid-based chemical carcinogenesis models.

The development of in vitro toxicity assessment methods using cultured cells has gained popularity for promoting animal welfare in animal experiments. Herein, we briefly discuss the current status of hepatoxicity assessment using human- and rat-derived hepatocytes; we focus on the liver organoid method, which has been extensively studied in recent years, and discuss how toxicologic pathologists can use their knowledge and experience to contribute to the development of in vitro chemical hepatotoxicity assessment methods for drugs, pesticides, and chemicals. We also propose how toxicological pathologists should assess toxicity regarding the putative distribution of undifferentiated and differentiated cells in the organoid when liver organoids are observed in hematoxylin and eosin–stained specimens. This was done while considering the usefulness and limitations of in vitro studies for toxicologic pathology assessment.

In order to elucidate the effects of swim bladder inflation failure on swim bladder carcinogenesis, we investigated the sequential histopathological changes of swim bladders at 13, 24, 35, and 53 days post-hatch (dph) in medakas with an uninflated swim bladder, which was experimentally induced by denying access to the air–water interface between 0 and 6 dph. The reactive oxygen species (ROS) levels were measured at 24 dph. An uninflated swim bladder was induced in 47.3% of the fish denied access to the air–water interface (the denied group). The total incidence of swim bladder adenoma was 54.1% in the denied group; however, these tumors were observed in all fish with an uninflated swim bladder. In fact, these tumors were observed from 13 dph and onwards. The TBARS levels of the juveniles showed a 2.6-fold increase in fish with an uninflated swim bladder in the denied group compared to that in the control group. It is speculated that swim bladder inflation failure has some effects on the gas gland to produce ROS, leading to DNA damage in the gas glandular epithelium, which develops into swim bladder adenomas. Consequently, it is concluded that denying access to the air-water interface between 0 and 6 dph in medaka is an easy method of inducing swim bladder tumors in a short-term period, and is a useful method for producing tumor-bearing fish.

Cigarette smoking is known to increase the risk of cancer and chronic obstructive pulmonary disease (COPD). In this study, we evaluated the effects of short-term nose-only inhalation exposure to cigarette smoke in mice. Male 10-week-old C57BL mice were exposed to clean air (control) or mainstream cigarette smoke for 1 h/day, 5 days/week, for 2 or 4 weeks. Exposure to cigarette smoke increased the number of inflammatory cells, especially neutrophils, in the bronchoalveolar lavage fluid, increased inflammatory cell infiltration foci, and caused an increase in the thickness of the peripheral bronchial epithelium. Microarray gene expression analysis indicated that smoke exposure induced inflammatory responses, including leukocyte migration and activation of phagocytes and myeloid cells, as early as two weeks after the initiation of exposure. Importantly, chemokine (C-C motif) ligand 17, resistin-like alpha, and lipocalin 2 were upregulated and may serve as useful markers of the toxic effects of exposure to cigarette smoke before pulmonary histological changes become evident.

A 32-year-old woman attempted suicide by ingesting Gloriosa bulbs and died approximately 2 days later. Toxicological examination revealed a potentially fatal blood concentration of colchicine (0.096 mg/L). In addition to the increased mitotic figures in the gastrointestinal mucosa, a unique finding for acute colchicine intoxication, pathological examination showed microvesicular lipid droplets in the liver, kidney, heart, and conduction system. Furthermore, central chromatolysis of neurons was observed in the pontine nucleus, medial accessory olivary nucleus, nucleus of the solitary tract, and nucleus ambiguus. Grumose degeneration of the cerebellar dentate nucleus was also evident. These pathological findings may help identify colchicine intoxication, even in the absence of evidence suggesting ingestion during autopsy. Moreover, pathological changes in the heart and central nervous system may be associated with the development of serious complications of acute colchicine intoxication.

A 104-week-old male CD (SD) rat exhibited enlargement of the left testis. Microscopically, this mass was demarcated from the testis by fibrous connective tissue and characterized by cystic dilatation with single-layered columnar cells and papillary proliferation connected to the solid growth area without clear boundaries. In the solid growth area, cells were dissected into irregular alveolar nests by scant fibrous tissue with small blood vessels. The nuclei of proliferating cells were variable in size and round- to oval-shaped, and their cytoplasm was pale or eosinophilic and sometimes contained vacuoles or eosinophilic granules. Immunohistochemically, the tumor cells were positive for vimentin and cytokeratin (CK) 7. Since CK7 was exclusively positive in the rete testis epithelium of the naïve rat, it was valuable to diagnose this tumor as rete testis-originated. Based on these results and the lack of apparent pleomorphism, mitotic figures, and metastasis, the present case was diagnosed as rete testis adenoma.

The optic tectum of Japanese quail embryos with in ovo exposure to methotrexate 100 ng/g egg on embryonic day 4 was examined from 3 to 24 hour after treatment. At 9 hour after methotrexate exposure, several apoptotic neuroepithelial cells appeared in the ventricular zone of the optic tectum; these increased in number and were diffusely distributed throughout all layers of the ventricular zone of the optic tectum at 12 hour. At 24 hour, neuroepithelial cells in the ventricular zone of the optic tectum were eliminated and showed sparse cell density. Throughout the experimental period, proliferation of neuroepithelial cells in the ventricular zone of the optic tectum of methotrexate-treated embryos was inhibited. These results suggest that neuroepithelial cells in the ventricular zone of the optic tectum in Japanese quail embryos can be affected by folic acid antimetabolites, methotrexate, at an early embryonic stage.

Vestibular organs consist of the maculae staticae, which are located in both the utricle and saccule, as well as the semicircular ducts and their ampullas. There have been no reports on specimen preparation methods for vestibular organs, including maculae staticae or semicircular ducts. In this study, we investigated highly reproducible methods of preparing vestibular organ specimens for histopathological examinations. We established a method that allows researchers to observe the utricle and saccule, including otoliths, the ampulla of a semicircular duct, and parts of semicircular ducts. This highly reproducible method is useful for histopathological analysis of mice with symptoms of abnormal equilibrium caused by medical toxicity and genetic modification.