Djungarian hamsters (Phodopus sungorus) and Roborovski hamsters (Phodopus roborovskii) are small cricetid rodents (6), which have become popular companion pet animals (3). Domestic dwarf hamsters frequently could occur spontaneous neoplastic disease, and most of them are integumental origin, particularly cutaneous atypical fibromas and fibrosarcomas (4,5). Atypical fibromas and fibrosarcomas are cutaneous tumors with large ganglion cell-like (GL) cells supported by variable amounts of collagen fibers (5). GL cells have a small round nucleus with a prominent nucleolus and abundant basophilic foamy cytoplasm (7). GL cells are normally observed in the dermis or subcutaneous adipose tissue of the abdominal and thoracic skin in Djungarian hamsters (1,3). These cells are rarely observed in young animals; however, they increase in number and extent with age, especially in males. Atypical fibromas and fibrosarcomas in Dungarian hamsters have been reported (1,3-5).

Although several case reports have been published, GL cells’ origin, behaviors, and roles remain unclear (7,8). According to Immunophenotypic results, GL cells arise from peculiar undifferentiated cutaneous mesenchymal cells (3); however, a more recent study found that neoplastic GL cells showed a neuroectodermal phenotype (8). This communication provides detailed histologic and immunohistochemical characteristics of atypical fibroma, fibrosarcoma, and the tumorigenesis of GL cells in Djungarian hamsters.

Biopsy samples (n = 62) of hamsters were submitted to the Department of Veterinary Pathology at the Jeju National University between 2018 and 2021 (Table 1). These samples were requested for pathologic diagnosis from local animal hospitals in Korea. Among them, 10 cutaneous masses were diagnosed as atypical fibroma (n = 9) and fibrosarcoma (n = 1) derived from cutaneous GL cells. The signalment for the hamsters is summarized in Table 2. All hamsters were Djungarian hamsters breed. Among these patients, nine were males, and one was female. The median age was 19.2 months, with an age range of 12-24 months. Nine male hamsters had tumors in the ventral area of the body including five in the abdomen (Fig. 1A) and four in the thoracic region (Fig. 1B), respectively. But the female hamster had a mass on the foreleg.

Table 1 . Diagnosis of 62 biopsy samples from hamsters with sex predilection.

Organ system	Diagnosis	No. of cases	Sex
			Female	Male
Skin	Atypical fibroma	9		9
	Atypical fibrosarcoma	1	1
	Apocrine gland adenoma	7	7
	Apocrine complex carcinoma	2	2
	Apocrine cystadenoma	3	2	1
	Cutaneous lymphoma	3	2	1
	Pyogranulomatous dermatitis	3		3
	Trichoepithelioma	2	2
	Cutaneous histiocytoma	2		2
	Sebaceous gland adenoma	2		2
	Cutaneous plasmacytoma	2	1	1
	Apocrine adenocarcinoma	1		1
	Apocrine ductal carcinoma	1	1
	Calcinosis cutis	1		1
	Cutaneous fibrosarcoma	2		2
	Cutaneous hemangiopericytoma	1	1
	Cutaneous lipoma	1		1
	Cutaneous liposarcoma	1		1
	Cutaneous mast cell tumor	1	1
	Foreign body granuloma	1	1
	Sebaceous gland epithelioma	1	1
	Squamous cell carcinoma	1		1
Genital	Uterine adenocarcinoma	2	2
	Pyometra	2	2
	Uterine leiomyosarcoma	2	2
	Epididymal abscess	1		1
	Ovarian adenoma	1	1
	Uterine adenoma	1	1
	Testicular abscess	1		1
	Testicular myxosarcoma	1		1
	Uterine fibroma	1	1
	Uterine leiomyoma	1	1
Mammary gland	Mammary gland complex adenoma	1	1
Total		62	33	29

Table 2 . Signalment of ten Djungarian hamsters with atypical fibroma and fibrosarcoma.

No.	Sex	Age (months)	Location of mass	Histologic diagnosis
1	Male	24	Thoracic	Atypical fibroma
2	Male	12	Abdomen	Atypical fibroma
3	Male	12	Thoracic-lateral	Atypical fibroma
4	Male	21	Abdomen-lateral	Atypical fibroma
5	Male	24	Thoracic-lateral	Atypical fibroma
6	Male	24	Abdomen	Atypical fibroma
7	Male	20	Abdomen	Atypical fibroma
8	Male	24	Abdomen	Atypical fibroma
9	Male	12	Thoracic-lateral	Atypical fibroma
10	Female	19	Foreleg	Atypical fibrosarcoma

Figure 1. Gross findings. Skin mass. Hamsters. Irregular round protruding masses (arrow) in the abdominal (A, case 1) and thoracic (B, case 6) skin.

Cutaneous masses were processed and embedded in paraffin wax then sectioned into 4 μm slices and stained with hematoxylin and eosin (H&E) for microscopic examination. Replicate sections were also stained with special stains, such as Masson’s trichrome, and immunohistochemistry (IHC) using the EnVision System (Dako, Tokyo, Japan) was performed to determine the origin of tumor cells. The primary antibodies used were vimentin (1:100; DAKO, Glostrup, Denmark), desmin (1:100; DAKO), smooth muscle actin (SMA) (1:100; DAKO), S100 (1:400; DAKO), neuron-specific enolase (NSE) (1:200; DAKO), glial fibrillary acid protein (GFAP) (1:500, DAKO), synaptophysin (1:100, DAKO), SOX10 (1:200; Abcam, Cambridge, United Kingdom), claudin-1 (1:500; Abcam), and androgen receptor (1:100; Abcam).

Histologically, 9/10 hamsters had a well-circumscribed mass with thin fibrous connective tissues. Neoplastic cells were accumulated from the superficial dermis to the deep dermis. They were composed of variable numbers of spindle-shaped and polygonal cells with a GL cell appearance, interspersed with an abundant variably vascularized collagenous extracellular matrix (Fig. 2A). The neoplastic cells had basophilic to amphophilic granular cytoplasm and round nuclei with one or two prominent nucleoli (Fig. 2B). There were scattered multifocal necrotic foci with/without mineralization throughout the cutaneous mass. The overall histologic features in the female case differed from those in nine male cases. The cutaneous mass was poorly demarcated, and unencapsulated with connective tissues and composed of densely proliferated spindle-shaped cells. Neoplastic cells showed moderate-to-severe anisocytosis and anisokaryosis with strong invasive tendencies, and formed bi- or multi-nucleated giant cells or bizarre mononuclear giant cells (Fig. 2C). There were 12 mitosis per 10 high-power fields under the light microscope.

Figure 2. Histologic findings. (A) Neoplastic foci were composed of variable numbers of spindle-shaped or polygonal cells with moderate amounts of collagenous extracellular matrix in case 3. (B) Neoplastic cells had basophilic to amphophilic granular cytoplasm and round nuclei in case 3. (C) Neoplastic cells showed severe anisocytosis and anisokaryosis with binucleated giant cells (arrow) in case 10. H&E. Bar, A = 50 µm; B-C = 20 µm.

The cytoplasm of the neoplastic cells was stained blue with Masson’s trichrome special stain. Based on the IHC, neoplastic cells were stained for vimentin (Fig. 3A), S100 (Fig. 3B), and NSE (Fig. 3C) in all 10 cases. However, these neoplastic cells were negative for other markers including desmin, SMA, GFAP, synaptophysin, SOX10, and claudin-1. Of these 9 male cases, 3 showed positive reactions to the androgen receptor (Fig. 3D).

Figure 3. Immunohistochemical findings. Neoplastic cells demonstrated positive reactions to vimentin (A) in case 1, S100 (B) in case 5, NSE (C) in case 2, and androgen receptor (D) in case 4. IHC. Bar, A-D = 20 µm.

According to previous studies, atypical fibromas or fibrosarcomas arise predominantly in the ventral abdominal or medial limb region in male hamsters (1,3). In this study, nine tumor cases occurred in males and were observed in the ventral area of the body. Only one case was found in the forelimb of a female hamster. The anatomical location and sex predilection of these tumors in this study were similar to those in previous studies (1,4). Interestingly, atypical fibrosarcoma was observed in a 19-month-old intact female hamster.

According to the previous literature on the incidence of spontaneous tumors in hamsters, Djungarian hamsters are almost exclusively affected by integumental neoplasms such as mammary tumors, atypical fibroma, and papilloma; in contrast, Syrian hamsters commonly have hematopoietic tumors including plasmacytoma and lymphoma (4). Integumental tumors, particularly atypical fibromas or fibrosarcoma (10/62 cases = 16.1%), were the most common lesions in this study. All affected hamsters were of the Djungarian breed. Although there have been several case reports of atypical fibromas originating from GL cells, the characteristics of GL cells are unclear until today. To clarify the identity of GL cells, we performed an immunohistochemical assay using various antibodies. Based on histologic and immunohistochemical characteristics, non-neoplastic and neoplastic GL cells are assumed to originate from undifferentiated mesenchymal cells of the dermis or subcutaneous adipose tissue (3), or fibroblasts with an uncommon phenotype (1). A recently published report revealed that GL cells express nestin, NSE, and melan-A, suggesting a neuroectodermal origin of GL cells (8). In this study, all neoplastic GL cells were positive for vimentin, S100, and NSE; however, they were negative for other markers including desmin, SMA, GFAP, synaptophysin, SOX10, and claudin-1.

A previous study examined the number and distribution of GL cells across four groups of hamsters (intact male and female, castrated male, and neutralized female) under nonneoplastic conditions. GL cells were the most abundant and showed the widest distribution in intact male hamsters, particularly in the ventral skin. In comparison, intact and neutralized female hamsters and castrated male hamsters had considerably fewer GL cells (7). A variety of skin cells, including fibroblasts and hair follicles, have been documented to respond to androgen directly or indirectly via insulin-like growth factor or other growth factors (2,10). Male Djungarian hamsters reach sexual maturity within 4 weeks after birth. The number of GL cells increases in male hamsters during the period of sexual maturation. Kashida et al. (3) reported that staining for androgen and estrogen receptors was positive in the nuclei of under 30% and 0% of GL cells from 11-month-old hamsters, respectively. Kondo et al. (5) reported two cases of atypical fibrosarcomas in male (22-month-old) and female (12-month-old) hamsters, and these neoplastic cells showed positive reactions for androgen receptors in both cases, while estrogen receptor α in female case. These results suggest a correlation between serum androgen concentration and the number of GL cells. In this study, nine patients were male, and only one was female. Immunohistochemistry showed that 33.3% (3/9 male cases) were positive for the androgen receptor. Moreover, in these 3 cases, the nuclei showing positivity for androgen receptor were present in fewer than 10% of GL cells. In contrast to previous literatures (3,5), the overall prevalence of androgen receptor positivity was relatively low in this study. The underlying reason for these observed outcomes remains unknown. A notable limitation of this study is the absence of information regarding the neuter status of the male hamsters. This limitation could potentially influence the data. Further investigations incorporating comprehensive data on the neuter status of all animals are warranted for more thorough understanding and accurated interpretation the observed findings.

Vimentin is a cytoskeletal protein of type III intermediate filaments found in various non-epithelial cells, especially mesenchymal cells (11). S100 has a broad tissue and cell distribution, including in glial cells, neurons, chondrocytes, Schwann cells, melanocytes, phagocytic and antigen-presenting histiocytic cells, myoepithelial cells, some glandular epithelium, and muscles. In veterinary oncology, S100 distinguishes between schwannomas, neurofibromas, and fibrosarcomas. Numerous cells are S100-positive in schwannomas, few in neurofibromas, and none in fibrosarcomas (9). In a previous study, < 30% of neoplastic and non-neoplastic GL cells were labeled (3); however, no immunolabeling was observed in a more recent study (5,8). In this study, all 10 patients showed diffuse positive reactions for S100 in neoplastic and non-neoplastic GL cells. NSE is an isoenzyme of 2-phospho-D-glycerate-hydrolase, which is not only present in neurons of both the central and peripheral nervous systems, but also in various neuroendocrine tumors and in normal tactile cells of the skin (13). Furthermore, the expression of this enzyme aligns with the specific metabolic circumstances, such as an increase in energy requirement. Additionally, it serves as a pro-survival factor for tumor cells (8,12). The immunolabelling pattern for NSE observed in this study aligns with findings from a previous report (8). In that study, GL cells in atypical fibromas were moderately labeled for NSE in two dwarf hamsters (8). Although tumor cells have shown positive reactions for NSE, further studies are needed to demonstrate the origin of tumor cells.

Hamsters are very popular as pets in worldwide. Neoplasms are an important cause of death in domestic hamsters and a major target for surgical excision in pet animal veterinarians. This study provides clinical information and histopathologic and immunohistochemical features of 9 atypical fibromas and one fibrosarcoma in domestic Djungarian hamsters. This valuable information and diagnostic approach are helpful for pet owners, veterinarians and pathologists to obtain rapid, practical, and precise diagnoses for these tumors.

This research was supported by the 2023 Scientific Promotion Program funded by Jeju National University.

The authors have no conflicting interests.