A histopathological analysis of spontaneous neoplastic and non-neoplastic lesions in aged male RccHan:WIST rats.

Histopathological information about spontaneous lesions in aged Hannover Wistar rats is limited. In this study, we describe spontaneous lesions found in 39 male RccHan:WIST rats used as a control in a carcinogenicity study. Neoplastic lesions were frequently seen in the endocrine system, such as pituitary adenomas in the pars distalis. This strain exhibited a high incidence of thymoma (10.3%), compared to other strains. We encountered an oligodendroglioma, a pituitary adenoma of the pars intermedia, and a prostate adenocarcinoma, which are comparatively rare in rats. While the variety and incidence of non-neoplastic lesions were similar to those in other strains, several interesting lesions occurred with relatively high incidence, including "harderianization" of the extraorbital lacrimal gland, common bile duct ectasia, and hyperplasia of pulmonary endocrine cells in the lung. Furthermore, comparative analyses demonstrated that the severity of chronic progressive nephropathy and murine progressive cardiomyopathy in RccHan:WIST rats was less than that in F344 rats. ©2020 The Japanese Society of Toxicologic Pathology.

Hannover Wistar rats are widely used for long-term toxicological and carcinogenic studies because they possess a number of advantageous characteristics, including a higher survival rate, a smaller size, and lower incidences of tumors, compared to those of other strains such as the Sprague-Dawley (SD) rats. RccHan:WIST, a Wistar substrain, has been recently available in Europe, the US, and Japan. Envigo (Huntingdon, UK) has provided background data for this strain, showing that there are differences in the incidence or severity of some neoplasms or toxicologically important lesions[1], [2]. In Japan, several pathologists have reported on the characteristics and biochemical parameters of RccHan:WIST rats[3], as well as spontaneous lesions that have occurred in the cornea[4] and the uterine horn[5]. However, few reports have been published so far discussing a comprehensive analysis of spontaneous lesions in aged rats of this strain, especially non-neoplastic lesions. Only Blankenship et al. reported microscopic findings of various lesions seen in routine toxicological studies[6]. The present study highlights our experience with naturally occurring neoplastic and non-neoplastic lesions in 39 male RccHan:WIST rats at 2 years of age. In addition, some non-neoplastic lesions, such as chronic progressive nephropathy (CPN), were comparatively analyzed using our specimens from aged male Fischer 344 (F344) rats.

Male RccHan:WIST rats were obtained from Japan Laboratory Animals Inc. (Saitama, Japan). The rats were provided a basal diet, CR-LPF (Oriental Yeast Co. Ltd, Tokyo, Japan), and tap water ad libitum, and individually housed in hanging-wire stainless-steel mesh cages in a room maintained at a temperature of 24°C and 55% humidity on a 12-h light–dark cycle. Of the 20 rats used as vehicle control and 20 rats used as non-treated control for a carcinogenic study, 39 were histopathologically analyzed in this study. The remaining animal developed eating disorder symptoms due to a deformed cranial incisor tooth, and thus was excluded from the analysis. Rats belonging to the vehicle group were intratracheally instilled with saline containing 0.5% Tween 80 and 0.5% sucrose 12 times every 4 weeks from the age of 10 weeks to 54 weeks. Since there was no apparent difference in the types and incidences of histological findings between the two groups (data not shown), these data were combined and described as a single data set. Necropsies were conducted whenever moribund or dead animals were found. All remaining animals were killed at 104–110 weeks of age. Thereafter, all tissues and tumor masses were excised, fixed in 10% neutrally buffered formalin, embedded in paraffin, sectioned, and stained with hematoxylin and eosin (HE). When a tissue sample was too autolytic or too small to allow histological analysis, it was excluded. For immunohistochemical staining, antigen retrieval was performed using a microwave for 10 min followed by inactivation of endogenous peroxidase. After blocking, the sections were treated with the following primary antibodies: oligodendrocyte lineage transcription factor-2 (Olig-2; IBL, Gunma, Japan: 18953, rabbit polyclonal), glial fibrillary acidic protein (GFAP; Abcam, Cambridge, UK: ab7260, rabbit polyclonal), α-melanocyte-stimulating hormone (α-MSH; Abcam: ab123811, rabbit polyclonal), adrenocorticotropic hormone (ACTH; Abcam: ab74976, rabbit polyclonal), cytokeratin 18 (PROGEN, Heidelberg, Germany: 61028, mouse monoclonal), protein G product 9.5 (PGP9.5; Abcam: ab8189, mouse monoclonal), Clara cell 16 kDa protein (CC16; Cloud-Clone, Houston, TX, USA: PAA857Ra01, rabbit polyclonal), and placental glutathione S-transferase (GST-P; MBL, Aichi, Japan: 311, rabbit polyclonal). Diaminobenzidine signals were detected with a horseradish peroxidase-secondary antibody conjugate (Agilent Technologies, Santa Clara, CA, USA: K4061).

For the comparative analysis of non-neoplastic lesions, another set of specimens was collected from 37 non-treated male F344/NJic rats at 2 years of age (obtained from Central Institute for Experimental Animals; Kanagawa, Japan). The number, area of GST-P positive foci of hepatocytes (consisting of more than five positive cells), and tissue section size were measured with an image analyzer, Image J (NIH; http://imagej.nih.gov/ij/download.html). CPN was classified into 8 grades corresponding to lesion progression[7]: Grade 0, no lesions; grade 1, minimal lesion; grade 2, mild lesion; grade 3, low-moderate; grade 4, mid-moderate; grade 5, high-moderate; grade 6, low-severe; grade 7, high-severe; and grade 8, end-stage. Murine progressive cardiomyopathy (PCM) was analyzed using HE staining and as well as Masson’s trichrome staining, confirming fibrosis, and grading was defined as the extent of lesions in the heart sections as follows[8]: Grade 0, no or only minimal changes; grade 1, <10%; grade 2, 11–40%; grade 3, 41–80%; and grade 4, >81%. The incidence of pulmonary neuroendocrine cell (PNEC) hyperplasia was evaluated by searching the sections immunohistochemically stained with the anti-PGP9.5 antibody. For each animal, 4 sections (all the from right lobe of the lung) were examined. Statistical analyses were performed using a StatLight software (Yukms. Co. Ltd; Kanagawa, Japan). Analyses of GST-P positive foci of hepatocytes and the gradings of CPN and PCM were all evaluated using Mann Whitney’s U-test. A comparison of incidence of PNEC was evaluated using Fisher’s exact test. The difference of the values was deemed statistically significant when the p value was less than 0.05.

All animal experiments were approved by the Animal Experiment Committee of the Tokyo Metropolitan Institute of Public Health.

Thirty-three of the 39 rats were killed at 104–110 weeks of age as scheduled necropsy, while four dead animals and two moribund animals were necropsied before this stage. The four dead animals were found from 85 to 110 weeks of age, two of which died of severe compression of the brain due to a large adenoma of pituitary pars distalis, while the other two died of severe anemia due to a large granular lymphocyte leukemia (LGL) in one case and a large hemangioma in the subcutis in the other case. Of the two moribund rats, one succumbed to an oligodendroglioma, whereas the other suffered from an adenoma of the pituitary pars distalis, and they were euthanized at 28 and 91 weeks, respectively. The survival rate at 104 weeks was 87.2%, and the mean survival time was 100.7 weeks. Body weight plateaued at 12–14 weeks of age, and the mean body weight increased to a maximum of 532.9 ± 63 g at the age of 94 weeks. The mean body weight at death was 520.7 ± 69 g (range: 423.6 to 730.4).

Table 1 summarizes the numbers and incidences of neoplasms observed in this study. Among the 39 rats, 30 had some form of neoplasia (76.9%), and 14 had two or more types of tumors simultaneously (35.9%).

Table 1.

Spontaneous Neoplasms Observed in Aged RccHan:WIST Male Rats

As for neoplasms of the brain, a granular cell tumor and an oligodendroglioma were observed. A large cohort analysis of RccHan:WIST rats showed that oligodendroglioma developed at an earlier age in these rats than did other brain tumors[9]. Likewise, a high-grade oligodendroglioma may have progressed aggressively and killed an adult rat at 28 weeks of age in our study; a large area of the right cerebral hemisphere appeared darkened and swollen. Histologically, a tumor involving a large hemorrhagic cyst at the center of the lesion compressed the normal brain tissue (Fig. 1A). Round tumor cells with chromatin-rich nuclei were arranged in rows, similar to the typical honeycomb structure (Fig. 1B and C). Neoplastic cells partially exhibited atypia and pleomorphism, accompanied by numerous irregularly shaped capillaries, eosinophilic extracellular substances, and necrosis (Fig. 1B). The tumor cells were immunohistochemically positive for a specific oligodendrocyte marker, Olig-2 (Fig. 1D), while they were almost all negative for a glial cell marker, GFAP (Fig. 1D inset).

Fig. 1.

Histopathological features of neoplasms found in male aged RccHan:WIST rats. A to D: An oligodendroglioma, malignant, high-grade. A: The lesion contained many cysts and capillaries and compressed the normal tissue (lower-left corner). B: The neoplastic cell was pleomorphic and accompanied by atypical endothelial cells. C: Round tumor cells were arranged like a honeycomb structure. D: The immunohistochemical outcomes were positive for Olig-2 and negative for glial fibrillary acidic protein (GFAP) (inset). E to G: Thymoma. A tumor with a cortex/medulla-like structure compressed adjacent thymic tissue. F: A high power view of E. G: The positive immunohistochemical outcome for cytokeratin 18 is seen in the serial section of F. H and I: A pituitary adenoma of the pars intermedia. H: The tumor mass (PIt) compressed the pars distalis (PD) and the pars nervosa (PN) of the pituitary gland. The intact pars intermedia (PIi) was also observed. The inset shows the positive immunohistochemistry for α-MSH. I: A high-power view of H showing highly pleomorphic cells and a mitotic figure (asterisk). J: A prostate adenocarcinoma. Neoplastic cells formed the glandular structures with marked stromal proliferation.

A greater incidence of thymoma in Hannover Wistar rats has been reported as one of the characteristics of this strain[2],[6]. Consistent with this, four thymomas occurred in this study (10.3%). One tumor showed a marked expansion with compression of unaffected tissue, and its histological structure resembled normal thymic architecture consisting of cortex-like (lymphocyte-rich) and medulla-like (epithelial cell-rich) areas (Fig. 1E and F) as previously reported[10]. Neoplastic cells were immunohistochemically positive for cytokeratin 18 (Fig. 1G). LGL was found in two dead animals, both of which showed splenomegaly and metastases in the lung and liver.

The endocrine system had the highest number of tumors, largely owing to pituitary adenomas in the pars distalis, islet cell adenomas, and pheochromocytomas, which represented 52.1% of all neoplastic lesions. In addition to these neoplasms commonly found in aged rats, one adenoma and two hyperplasias of the intermediate lobe of the pituitary gland were found in this study. Proliferative lesions of the intermediate pituitary are not frequently encountered in rats, but the incidence is higher in the Wistar strain compared to F344 or SD strains[11]. A histological analysis of the adenoma in this study showed a large nest of proliferating cells in the pituitary pars intermedia, compressing the nervous and anterior lobes (Fig. 1H). The tumor mass and intact tissue of the lobe were immunohistochemically positive for α-MSH (Fig. 1H inset) and ACTH (data not shown), whereas the pars distalis was only positive for ACTH, suggesting that tumor cells were derived from the intermediate lobe[12]. In the tumor, enlarged cells with prominent nucleoli and mitoses were frequently observed (Fig. 1I).

A prostate adenocarcinoma was encountered in one rat at the scheduled necropsy. It developed in the ventral prostate, in which variedly-sized, multi-layered and irregularly shaped ductal structures were formed by anaplastic tumor cells with chromophobic nuclei (Fig. 1J). Neoplastic cells accompanied by increased stroma and inflammatory cells had partially invaded the surrounding fat tissue, although distant metastases were not observed. While the Leydig cell tumor is one of the most common findings in aged F344 rats, it was not the case in the RccHan:WIST rats, with an incidence of only 5.1%.

The histopathological findings of non-neoplastic lesions observed in this study are summarized in Table 2. The list contains lesions that are commonly found in aged rats, such as foci of altered hepatocytes (FAH), CPN, and other inflammatory and degenerative changes.

Table 2.

Spontaneous Non-neoplastic Lesions Observed in Aged RccHan:WIST Male Rats

Concerning hyperplastic lesions, numerous lesions were observed in the endocrine organs, such as the pancreatic islet, thyroid C-cells, pituitary pars distalis, and parathyroid. In the cases of hyperplasia of the pituitary pars intermedia, cells with enlarged and increased basophilic cytoplasm were focally increased, although no marked compression of the surrounding tissue was observed (Fig. 2A). Incidence of bile duct hyperplasia (5.1%) was markedly less than that of F344 rats (96.8%)[13].

Fig. 2

Histopathological features of non-neoplastic lesions found in aged male RccHan:WIST rats. A: Hyperplasia of the pituitary pars intermedia. Basophilic cells with slightly enlarged nuclei were present near the Rathke’s cleft cyst (R). B: Alveolar macrophage aggregation. Foamy cells were densely accumulated through a number of alveoli of the peripheral region. C: Common bile duct ectasia. Epithelia were partially invaginated into the subepithelial connective tissue. High-power view of an occurrence of mitosis is shown in the upper-left corner. The inset shows the gross finding. D: “Harderianization” of the extraorbital lacrimal gland. Large Harderian gland-like acini were localized near degenerative cells with condensed nuclei and an intact lacrimal gland (seen in the lower-left corner). Pigment deposition in an acinar is shown by an asterisk. E: Pulmonary neuroendocrine cell (PNEC) hyperplasia found in the bronchiolar lumen. F: High-power view of E. PNECs had round or oval nuclei with patchy chromatin and are stained with protein G product 9.5 (PGP9.5) (F’). Clara cells with eosinophilic cytoplasm are located at the surface of the mass and stained with CC16 (F’’).

Alveolar macrophage aggregation in the lung was a non-neoplastic lesion with the highest incidence in this study (89.7%; Table 2, Fig. 2B). It was observed as clusters of foamy macrophages often seen in peripheral alveoli. There was no difference in the incidence of this type of lesion between the intact group (18 in 19) and the vehicle-treated group (17 in 20), suggesting that the aggregation may not have been induced by the vehicle treatment. Common bile duct ectasias, with a diameter of 5 mm or larger, were macroscopically observed in nine animals (23.1%). These dilated ducts were commonly composed of foamy and flattened epithelial cells, but in some animals, densely arranged columnar epithelial cells were prominent. The epithelium had folded to form glandular invaginations in the mucosa where a large number of eosinophils had infiltrated (Fig. 2C). “Harderianization” of the extraorbital lacrimal gland[14], [15] was frequently observed. Grossly visible whitish patches on the surface of the gland corresponded to an area where lobes had partially changed from normal serous acini to mucoserous and/or mucous acini (Fig. 2D) that closely resembled those of the Harderian gland. The size of the affected area depended on the individual; affected areas ranged from 0 to 50.5% of the entire section of the gland.

We examined several non-neoplastic lesions further, focusing on the difference between strains using a set of F344 samples. To obtain quantitative information about FAH, GST-P-positive foci were immunohistochemically analyzed. The number of GST-P positive foci of RccHan:WIST was largely similar to that of F344 rats (Fig. 3A). The GST-P positive area within the liver sections of each animal (Fig. 3B) and the size of each focus (Fig. 3C) were also evaluated, but again, no significant differences were observed. The size of the focus was highly variable depending upon the individual (Fig. 3C).

Fig. 3

Comparative analysis of placental glutathione S-transferase (GST-P) positive foci in the liver of aged RccHan:WIST and F344/NJic rats. A: Average number of GST-P positive foci in the liver section (± SD). B: Average GST-P positive area within the liver section (± SD) C: Size distribution of foci which were collected from all examined animals. Each colored bar shows mean value. In all of these comparisons, 33 RccHan:WIST and 36 F344/NJic rats were analyzed, and no significant changes were detected in any of them.

CPN is one of the most studied non-neoplastic lesions in aged rats. The histological grading of specimens from the two strains we examined demonstrated that the severity of the lesions was much lower in RccHan:WIST than in F344 rats (Table 3). Weber et al. reported the lower severity of CPN of RccHan:WIST rats in the comparison with SD rats[16]. It is thus suggested that resistance to spontaneous CPN may be the characteristic of RccHan:WIST.

Table 3.

Comparison of the Grade of CPN Between Male RccHan:WIST and F344/NJic

PCM was detected in 82.4% of rats in this study. The main histological findings for this lesion were focal degeneration of myocardium, mononuclear cell infiltration, and interstitial fibrosis, all of which were frequently observed in the papillary muscle and the area of ventricle close to the annulus fibrosus. All animals diagnosed with PCM exhibited some extent of fibrous deposition as evidenced by Masson’s trichrome staining, and the ratio of the positive area within the section correlated well with the histological grading (data not shown). A histological comparison of PCM between the two strains demonstrated that the severity of the lesions was slightly but significantly lower in the RccHan:WIST rats (Table 4).

Table 4.

Comparison of the Grade of PCM Between Male RccHan:WIST and F344/NJic

Finally, the occurrence of hyperplasia of PNEC in the lung was compared. PNECs are observed as single cells or clusters, called neuroepithelial bodies, in the respiratory epithelium[17]. Although little attention is generally paid to PNECs in the lungs in routine toxicity studies, it is thought that the PNEC has roles in sensing hypoxia and hypercapnia, and PNEC hyperplasia may be associated with chronic inflammation or fibrosis[18], [19], [20], [21]. Hyperplasia, defined as a lesion which consists of more than 40 cells[22], was often recognized as a polypoid or a finger-like nodule, protruding into the bronchiolar lumen (Fig. 2E). PNECs had round or oval nuclei with stippled chromatin and vacuolated cytoplasm (Fig. 2F) and were immunohistochemically positive for PGP9.5 (Fig. 2F’). On the surface area of hyperplastic nodules, a few Clara cells, immunohistochemically positive for CC16, were also involved (Fig. 2F’’). A histological analysis showed that the incidence of PNEC hyperplasia in aged rats differed between strains, in that Hannover Wistar rats displayed a slightly higher incidence compared to F344 and SD rats[22]. Our results also demonstrated that the incidence of PNEC hyperplasia in RccHan:WIST rats was statistically significantly higher than that in F344 rats (7 out of 39 [17.9%] (Table 2) vs. 0 out of 37 [0%], respectively).

Although the present study was conducted in a small number of animals and only included males, the data contributes to our overall understanding of spontaneous lesions in aged RccHan:WIST rats.

Disclosure of Potential Conflicts of Interest

The authors declare there are no conflicts of interest.