JPC SYSTEMIC PATHOLOGY
REPRODUCTIVE SYSTEM
January 2022
R-N04

Signalment (JPC# 1949421): 23-month-old male Fischer 344 rat

HISTORY: Not provided

HISTOPATHOLOGIC DESCRIPTION: Testis: There are two neoplasms within this tissue section. Expanding and replacing 95% of testicular architecture and compressing adjacent seminiferous tubules is a well demarcated, unencapsulated, multilobular, densely cellular neoplasm composed of cords and indistinct nests of polygonal cells supported by a fine fibrovascular stroma. Neoplastic cells have variably distinct cell borders, abundant eosinophilic vacuolated cytoplasm, and round to oval nuclei with finely stippled chromatin and one variably distinct nucleolus. There is mild anisocytosis and anisokaryosis. Mitotic figures are less than 1 per 2.37 mm². The neoplasm is punctuated by few, up to 1 mm diameter lakes of eosinophilic, flocculent material (cystic degeneration) or hemorrhage, fibrin, and edema. The remaining seminiferous tubules are atrophied, with irregular, undulant basement membranes, lined by a single layer of Sertoli cells, devoid of germ cells, spermatocytes and spermatids, and contain variable amounts of a fibrillar to homogeneous eosinophilic material. Extending peripherally from the tunica vaginalis is a second neoplasm composed of arborizing papillary projections lined by cuboidal cells that often pile up to three cell layers thick supported by loose collagenous cores. Neoplastic cells have variably distinct cell borders, small amounts of homogenous to finely granular eosinophilic cytoplasm, and round to oval nuclei with finely stippled chromatin and a single distinct nucleolus. Mitotic figures are less than 1 per 2.37 mm². There is mild anisocytosis, few infiltrating hemosiderin-laden macrophages and mast cells, and moderately ectatic lymphatics.

MORPHOLOGIC DIAGNOSES:

1. Testis: Interstitial cell tumor, Fischer 344 rat, rodent.
2. Testis, tunica vaginalis: Mesothelioma.
3. Testis, seminiferous tubules: Atrophy diffuse, severe with aspermatogenesis.

GENERAL DISCUSSION:

• Interstitial cell tumor (ICT; Leydig cell tumor; interstitial cell adenoma)
• Most common testicular tumor of rats; also seen in the bull and dog
• Especially common in F344 rats, incidence approaches 100%
• Less common in Wistar and Sprague-Dawley rats
• Often multiple, bilateral
• Usually benign, difficult to differentiate from hyperplasia (see differential diagnosis section)
• In rats, about 10% are malignant; features of malignancy include invasion of the epididymis and (rarely) pulmonary metastasis
• Associated with concurrent hypercalcemia in rats
• Mesothelioma (see also P-N06):
  • Third most common testicular tumor of rats after interstitial cell tumor and mononuclear cell leukemia (also known as large granular lymphocytic leukemia); not uncommon to see concurrent ICT and mesothelioma
  • Tunica vaginalis is the most common site
  • Associated with mitogenic factor produced by Leydig cell tumors (Creasy et al., J Toxicol Pathol, 2012)
  • Rare in female rats; may arise in the ovarian bursa
  • All mesotheliomas considered malignant; spread by direct extension or implantation throughout the peritoneum, with invasion of subjacent tissue
  • Some metastasize through lymphatics; strangulation of the intestines may occur with abdominal mesotheliomas
  • F344 rats exposed to vinylidene chloride resulted in marked increase in malignant mesothelioma
  • Must differentiate from reactive mesothelium

TYPICAL GROSS FINDINGS:

• Interstitial cell tumors:
  • Circumscribed, lobulated, yellow-white, multiple, bilateral masses
  • Larger tumors may be hemorrhagic, necrotic, or mineralized
• Mesotheliomas:
  • Multiple firm, sessile, or pedunculated nodules, villous projections, or plaque like with fibrous adhesions
  • Gelatinous or white fibrous proliferations partially surrounding the testis
  • Shaggy yellow-tan to red-brown growths on serosal surfaces of the peritoneal cavity, tunica vaginalis propria, or both
  • Ascites and scrotal edema (can be severe)

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• Interstitial cell tumors:
  • Sheets of polyhedral to elongate cells; granular or vacuolated cytoplasm
  • Admixed with smaller cells with scant cytoplasm and hyperchromatic nuclei
  • Extend between, compress, and often replace tubules
  • Hemorrhage, necrosis, mineralization, inflammation, cystic degeneration, tubular atrophy may occur

• Mesotheliomas:
  • Diffuse or nodular foci or complex papillary structures
  • Prominent stratification and associated stromal proliferation
  • Polyhedral to cuboidal cells; abundant eosinophilic, often finely vacuolated cytoplasm
  • Round to oval nuclei; single or multiple nucleoli
  • Mitotic rate usually low

ULTRASTRUCTURAL FINDINGS:

• Interstitial cell tumor:
  • Lipid droplets, lipofuscin, abundant smooth endoplasmic reticulum, abundant mitochondria with tubulovesicular cristae and desmosomes
  • Tubulovesicular mitochondrial cristae are characteristic of interstitial cell
  • Other cells of the testis and epididymis have lamellar mitochondrial cristae
• Mesothelioma:
  • Microvillous cell membrane, junctional complexes between cells, pinocytotic vesicles, distinct basal lamina
  • Microfilaments often abundant, may be difficult to differentiate from endothelial cells

ADDITIONAL DIAGNOSTIC TESTS:

• Interstitial cell tumors:
  • Immunohistochemistry: GATA 4 positive; other such as inhibin α also reported positive in some cases
  • Elevated plasma inhibin levels
• Mesothelioma:
  • Cytoplasmic mucopolysaccharides stain weakly positive with PAS and keratin; strongly positive with Alcian blue, colloidal iron, and vimentin
  • Immunohistochemically usually positive for vimentin and cytokeratin

DIFFERENTIAL DIAGNOSIS:

Interstitial cell tumor:

• Interstitial cell hyperplasia: Hyperplasia is usually more diffuse (mice) or focal to multifocal (rats), often entraps seminiferous tubules, and does not compress peripheral testicular tissue; hyperplasia in rats and mice is defined as focal Leydig cell accumulations with a diameter equal to or greater than ½-1 times the diameter of a normal seminiferous tubule diameter (Creasy et al., J Toxicol Pathol, 2012)
• Interstitial cell carcinoma: Invasion, hemorrhage, increased pleomorphism and mitotic activity
• Sertoli cell tumor: Usually forms tubules and has a prominent fibrous stroma; IHC positive for AMH and inhibin, among others

Mesothelioma:

• Carcinomatosis
• Villous hyperplasia (mesothelial hyperplasia): No fibrovascular stalk; often associated fibrosis, low mitotic rate
• Chronic peritonitis, which may be secondary to experimental manipulation
• Polyarteritis
• Mononuclear leukemia
• Scrotal hemorrhage seen in Kilham’s rat virus (parvovirus)
• Polyarteritis
• Teratoma

COMPARATIVE PATHOLOGY:

Interstitial cell tumor:

• Most common testicular neoplasm of the dog, cat, and bull (primarily Guernsey breed), and in the Guernsey is associated with a high incidence of telangiectasia of the liver, thyroid C-cell (parafollicular) tumors, and infertility; also develops almost exclusively in cryptorchid stallions
• Rare in birds, rabbits, mice, nonhuman primates, and humans
• Malignant Leydig cell tumors with cutaneous metastasis have been reported in dogs (Kudo et al., J Vet Diagn Invest, 2019); criteria of malignancy are based upon human standards, and include size, necrosis, cytologic atypia, increased mitotic activity, increased Ki-67 expression, and vascular invasion
• Recent investigation into cellular senescence in canine testes, and using P21 and gH2AX IHC markers (nuclear), has shown an increase in cells exhibiting the senescence-associated secretory phenotype (SASP), but there is currently no evidence of the SASP leading to development of testicular neoplasms (Merz, Vet Pathol, 2019)

Mesothelioma:

• Reported in man and laboratory animals exposed to asbestos; rare in domestic animals, most commonly reported in the bull, calf, and dog
• In dogs (as in humans, mice, and rats) three types of mesotheliomas are described- epithelioid type, sarcomatoid type, biphasic or combination type
• Mesothelioma in a small sample of large felids was characterized by positive immunoreactivity for vimentin, pancytokeratin, Wilms Tumor 1 (WT1), E-cadherin; most were positive for calretinin, and half were immunopositive for MUC-1/EMA (Coe, J Vet Diagn Invest, 2021)

REFERENCES:

1. Barthold SW, Griffey SM, Percy DH. Pathology of Laboratory Rodents and Rabbits, 4th ed. Ames, IA: John Wiley & Sons, Inc.; 2016:169-170.
2. Blackshear P, Pandiri A, Nagai H, et al. Gene expression of mesothelioma in vinylidine chloride-exposed F344/N rats reveal immune dysfunction, tissue damage, and inflammation pathways. Toxicol Pathol. 2015; 43:171-185.
3. Blackshear P, Pandiri A, Ton T, et al. Spontaneous mesotheliomas in F344/N rats are characterized by dysregulation of cellular growth and immune function pathways. Toxicol Pathol. 2014; 42:863-876.
4. Canadas A, Romão P, Gärtner F. Multiple cutaneous metastasis of a malignant interstitial cell tumour in a dog. J Comp Pathol. 2016;155(2-3):181-184.
5. Caswell JL, Williams KJ. Respiratory system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol. 2. 6^th St. Louis, MO: Elsevier; 2016: 523.
6. Coe SE, Garner MM, Kiupel M. Immunohistochemical characterization of mesothelioma in 6 large felids. J Vet Diagn Invest. 2021;33(4):767-771.
7. Creasy D, Bube A, Rijk E, et al. Proliferative and non-proliferative lesions of the rat and mouse male reproductive system. J Toxicol Pathol. 2012;40:40S-131S.
8. Foster RA. Male reproductive system. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 6th ed. St. Louis, MO: Elsevier; 2017:1204, 1211.
9. Foster RA. Male genital system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 3. 6th ed. St. Louis, MO: Elsevier; 2016:474, 492-495.
10. Greaves P, Chouinard L, Ernst H, et al. Proliferative and non-proliferative lesions of the rat and mouse soft tissue, skeletal muscle, and mesothelium. J Toxicol Pathol. 2013;26 (3 Suppl): 1S-26S.
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12. Kudo T, Kamiie J, Aihara N, et al. Malignant Leydig cell tumor in dogs: two cases and a review of the literature. J Vet Diagn Invest. 2019;31(4):557-561.
13. Lopez A, Martinson SA. Respiratory system, mediastinum, and pleurae. In: Zachary JF, ed. Pathologic Basis of Veterinary Disease. 6th ed. St. Louis, MO: Elsevier; 2017:559.
14. Merz SE, Klopfleisch R, Breithaupt A, Gruber AD. Aging and senescence in canine testes. Vet Pathol. 2019;56(5):715-724.
15. Murakami T, Sassa Y. Pleomorphic malignant mesothelioma in a broiler breeder infected with avian leucosis virus subgroup J. J Comp Pathol. 2018; 160:50-55.
16. Tokarz DA, Gruebbel MM, Willson GA, Hardisty JF, Pearse G, Cesta MF. Spontaneous primary pleural mesothelioma in Fischer 344 (F344) and other rat strains: a retrospective review. J Toxicol Pathol. 2021;On-line first. DOI: 10.1177/01926233211053631.
17. Uzal FA, Plattner BL, Hostetter JM. Alimentary system In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 2. 6th ed. St. Louis, MO: Elsevier; 2016:256-257.
18. Whitney KM, Suttie AW. Testis and epididymis. In: Suttie AW, ed, Boorman’s Pathology of the Rat, 2^nd San Diego, CA:Elsevier. 2018:570-571.

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