Spontaneous Eosinophilic Granulated Round Cell Tumors in Rats

Spontaneous globule leukocyte tumor accompanied by inflammatory cells in a Wistar Hannover rat

We encountered hematolymphoid neoplastic lesions in the form of many nodules in the spleen and liver in a 110-week-old male Wistar Hannover rat (Crl:WI (Han)). The lesions contained atypical proliferative cells, eosinophils, lymphocytes, and macrophages. The proliferative cells comprised various atypical cell types with or without cytoplasmic eosinophilic granules. The granules were positively stained using periodic acid-Schiff and elastase stains, were bluish purple using phosphotungstic acid and hematoxylin, and showed no metachromasia using toluidine blue. In immunohistochemical staining, the proliferative cells with or without granules were positive for granzyme B, rat mast cell protease II, and Ki67. Electron microscopic examination revealed that single to multiple high-density granules of variable size were covered by a membrane. These findings led to a diagnosis of globule leukocyte tumor. The accompaniment of this tumor by inflammatory cells is likely evoked by mast cell-like active mediators contained in the granules of the globule leukocytes.

Malignant mast cell tumor of the thymus in an Royal College of Surgeons (RCS) rat

A 152-week-old male Royal College of Surgeons (RCS) rat kept as a non-treated animal in a long-term animal study presented with a soft mass in the anterior mediastinum, which adhered to the pleura of the lung. Histopathologically, the mass mainly consisted of round to short spindle-shaped tumor cells that had infiltrated through the hyperplastic thymic tissue. The tumor cells were arranged in loose to dense sheets. Nuclei were moderate in size and round to spindle-shaped, with small nucleoli. Almost all tumor cells exhibited abundant eosinophilic cytoplasm, including eosinophilic granules of a range of sizes. The granules of tumor cells exhibited metachromasia with toluidine blue stain and were positive for c-kit and mast cell protease II. These findings indicate that the tumor described here represents a rare case of spontaneous malignant mast cell tumor with thymic epithelial hyperplasia.

The metrial gland in the rat and its similarities to granular cell tumors

Metrial glands are normal structures located in the mesometrial triangle of the pregnant rat uterus from gestational day (GD) 8 through termination of pregnancy. Metrial glands are composed of a dynamic mixed cell population of granulated metrial gland (GMG) cells, endometrial stromal cells, trophoblasts, blood vessels, and fibroblasts. Collections of similar cells may be seen in association with pseudopregnancy and other hormonal disturbances. Granulated metrial gland cells are the hallmark cell of the metrial gland. They are bone-marrow-derived, perforin-positive, natural killer cells that proliferate in the pregnant uterus. Understanding the normal histogenesis of the metrial gland and recognizing the possible existence of GMG cells and a reactive metrial gland in the nonpregnant state are important when examining any uterine lesion that contains granulated cells. This report demonstrates that the cellular composition, morphology, and immunohistochemical staining profile of normal metrial glands are similar to reported granular cell neoplasms in rats and mice. The possibility of a non-neoplastic lesion involving the metrial gland should be considered when proliferative lesions involving granulated cells are observed in the uterus of mice and rats from nonclinical toxicity studies. Positive immunohistochemical staining for perforin and S100 would assist in the classification of such lesions as a reactive metrial gland or decidual reaction.

Spontaneous tumors of small mammals

The most common tumor of guinea pigs is bronchogenic papillary adenoma; of hedgehogs is mammary gland adenocarcinoma; of hamsters is adrenal cortical adenoma; of gerbils is ovarian granulosa cell and theca cell tumors; of mice is pulmonary carcinoma; and of rats is mammary fibroadenoma. A relatively low incidence of tumors is described for chinchillas and hamsters, whereas the incidence of tumors is high for gerbils, hedgehogs, mice, and rats. Limited literature regarding neoplasia exists for prairie dogs, sugar gliders, and chinchillas.