The placenta plays a pivotal role in fetal growth, and placental dysfunction and injury are associated with embryo/fetal toxicity. Histological examination of the rat placenta for safety evaluation provides valuable clues to the mechanisms of this toxicity. However, the placenta has specific and complex biological features unlike those of other organs, and placental structure dramatically changes depending on the time during the gestation period. Thus, time-dependent histopathological examination of the rat placenta should be performed based on the understanding of normal developmental changes in morphology and function. The placentas of rats and humans are both anatomically classified as discoid and hemochorial types. However, there are differences between rats and humans in terms of placental histological structure, the fetal-maternal interface, and the function of the yolk sac. Therefore, extrapolation of placental toxicity from rats to humans should be done cautiously in the evaluation of risk factors. This review describes the development, morphology, physiology, and toxicological features of the rat placenta and the differences between the rat and human placenta to enable accurate evaluation of reproductive and developmental toxicity in studies.

Fibroadenoma (FA) is a common mammary fibroepithelial tumor. The tumor size of the FA is increased by estrogen, progesterone, prolactin, and pregnancy, whereas it decreases after menopause. These observations in humans indicate that FA is hormone dependent. In rats, the most common mammary neoplasm is also FA. Expression levels of Twist1, a transcriptional regulator of epithelial-mesenchymal transition, were examined in paraffin-embedded tissue sections of an experimental rat breast model to find physiological alternations coincident with reproductive hormonal changes. Twenty-three Fischer 344/Brown Norway F1 hybrid rats were used as 14‐ to 16-week-old adolescent rats (n=3), pregnant rats (n=4), and lactating rats (n=6) in addition to rats over 100-weeks-old that exhibited aging (n=3) and FA (n=7). Seventy-six cases of chemically induced breast carcinoma and two cases of FA in Sprague Dawley rats were also examined. Using tissue sections, we observed that Twist1-positive mesenchymal cells were predominantly located in the periductal region in adolescent and pregnant rats and in the terminal duct lobular unit in pregnant and elderly rats. Twist1 was also expressed diffusely in the mesenchymal cells of FA rats. Twist1-positive cancer-associated mesenchymal cells were found more frequently in the invasive components of breast carcinomas than in intraductal components. The expressions of Twist1 in mesenchymal cells were induced by physiological and pathological stimuli, suggesting the biological role of Twist1 in tissue structure. Further study may reveal the role of Twist1 in mesenchymal cells of mammary glands in rats.

The purpose of the present study was to evaluate effects of aflatoxin B₁ (AFB₁) on the cell cycle and proliferation of splenic cells in chickens. A total of 144 one-day-old Cobb male chickens were randomly divided into 2 equal groups of 72 each and were fed on diets as follows: a control diet and a 0.6 mg/kg AFB₁ diet for 21 days. The AFB₁ diet reduced body weight, absolute weight and relative weight of the spleen in broilers. Histopathological lesions in AFB₁ groups were characterized as slight congestion in red pulp and lymphocytic depletion in white pulp. Compared with the control group, the expression levels of ataxia–telangiectasia mutated (ATM), cyclin E₁, cyclin-dependent kinases 6 (CDK6), CDK2, p53, p21 and cyclin B₃ mRNA were significantly increased, while the mRNA expression levels of cyclin D₁, cdc2 (CDK1), p16, p15 were significantly decreased in the AFB₁ groups. Significantly decreased proliferating cell nuclear antigen (PCNA) expression and arrested G₀G₁ phases of the cell cycle were also seen in the AFB₁ groups. In conclusion, dietary AFB₁ could induce cell cycle blockage at G₀G₁ phase and impair the immune function of the spleen. Cyclin D₁/CDK6 complex, which inhibits the activin/nodal signaling pathway, might play a significant role in the cell cycle arrest induced by AFB₁.

A3, generated as a monoclonal antibody against rat malignant fibrous histiocytoma (MFH)-derived cloned cells, recognizes somatic stem cells (bone-marrow/hair follicle stem cells). We investigated the distribution of cells immunoreactive to A3 in the developing rat intestine (particularly, the colon), focusing on the ontogenic kinetics of A3-positive cells. In the rat intestine, A3 labeled spindle-shaped stromal cells localized in the submucosa and labeled endothelial cells of capillaries in the lamina propria forming villi in the early development stage. With development progression, A3-positive cells were exclusively localized around the crypts of the colon. Double immunofluorescence revealed that A3-positive cells around the crypts reacted to vimentin (for mesenchymal cells) and Thy-1 (for mesenchymal stromal cells) but not to α-SMA (for mesenchymal myofibroblastic cells) or CD34 (for hematopoietic stem cells), indicating that A3-positive cells around the crypts may have characteristics of immature mesenchymal cells. In addition, A3 labeled a few epithelial cells at the base of colon crypts. Furthermore, immunoelectron microscopy revealed that A3-positive cells lay inside myofibroblasts adjacent to the epithelium of the crypts. A3-positive cells were regarded as a new type of immature mesenchymal cells around the crypts. Collectively, A3-positive cells might take part in the stem cell niche in the colon, which is formed through epithelial-mesenchymal interaction.

Adrenal disorders are common in ferrets, but there are few studies on cystic lesions of the adrenal gland. The present study describes pathological and immunohistochemical features of adrenal cysts in eleven ferrets and discusses their histogenesis. In nine of eleven cases examined, which included seven, one, and one right, left, and bilateral cases, respectively, cysts were in the adrenal cortex and lined with epithelial cells. These epithelial cells contained an Alcian blue-negative/PAS-positive material and were positive for cytokeratin (CK) 7. The staining pattern was similar to that of biliary epithelial cells in the ferret. In five of the cases, there were small ducts adjacent to the cysts that were positive for CK7 and CK20 and negative for CK19. Based on the anatomical proximity between the right adrenal and liver, the immunohistochemical features of the small duct cells were comparable to those of hepatic oval cells. These results indicate the possibility that these adrenocortical cysts in the ferret originated from the biliary system. In the other two cases, the cysts lacked an epithelial cell lining, and there were dilated lymphoid vessels around the cysts. These cysts were assumed to have developed in the adrenal medulla, because the cyst wall was positive for glial fibrillary acidic protein and there were adrenal medullary cells positive for synaptophysin in the cyst wall. Therefore, the medullary cysts may have been associated with dilated vasculatures.

Vancomycin hydrochloride (VCM) is a glycopeptide antibiotic that is commonly used to eradicate methicillin-resistant gram-positive cocci, despite its nephrotoxic side effects. Elderly people are particularly susceptible to developing VCM-induced nephrotoxicity. However, the precise mechanism by which VCM induces nephrotoxicity in elderly people is not completely understood. Therefore, we investigated VCM-induced nephrotoxicity in mice of different ages. VCM was injected intraperitoneally into mice at 1, 3, 6, 12, and 24 months of age at a dosage of 400 mg/kg body weight for 3 and 14 days. Twenty-four hours after the last injection, we examined plasma creatinine levels and histopathological alterations in the kidneys. VCM administration increased plasma creatinine levels, and these values gradually increased to higher levels with aging. The histological examination revealed renal tubular degeneration, such as brush-border atrophy, apoptosis/necrosis of the tubular epithelium, and epithelial desquamation, that gradually became more severe with aging. Furthermore, immunohistochemical staining with anti-CD10 and anti-single-stranded DNA antibodies revealed damaged renal proximal tubules with marked dilatation, as well as numerous apoptotic cells, and these features increased in severity in 12- and 24-month-old mice receiving VCM. Based on these results, aged mice were highly susceptible to kidney damage induced by VCM administration. In addition, proximal tubular epithelial cells likely underwent apoptosis after the administration of VCM. This report is the first to document VCM-induced nephrotoxicity in mice of different ages. Thus, this mouse model could be useful for understanding the mechanisms of VCM-induced nephrotoxicity in the elderly.