This study sought to investigate the abnormal swelling in the right femur of an NOD mouse. A one-year-old male NOD mouse was obtained from the Animal Facilities of the Center for Animal Resources Development, Wonkwang University, Korea. In the previous 3 weeks, an abnormal swelling and growth was identified in the distal portion of the right femur in the animal in question. Urine analysis was conducted with an Urostick and the result was the following: as leucocytes-, nitrogen-, urobilinogen-, protein 30 mg/dL, pH 5.0, blood >200 erythrocytes/µL, specific gravity 1.030, ketones-, bilirubin-, glucose-. Grossly, a large mass on the distal femur was observed in the right femur. Lesions were poorly marginated, associated with destruction of the cancellous and cortical elements of the bone, and showed ossification within the soft tissue component. Necropsy was conducted and the organs were submitted to gross examination and trimmed. The trimmed tissues were fixed in 10% neutral buffered formalin, and embedded in paraffin. Four µm sections were made and stained with hematoxylin and eosin for histopathological examination.

Gross examination of the necropsied animal revealed abnormal swelling of the right femur which had a hard consistency. In the subcutaneous layer of the right femur lesion, jelly-like degenerative materials were found. The cut surface of the 4×5 cm femoral mass was poorly marginated, and white (Figure 1). Also, swelling of inguinal lymph nodes and atrophy of testes were detected. Histologically, the femoral mass was identified as a poorly differentiated sarcoma. Histopathologic examination of the femoral bone mass revealed an invasive proliferation of poorly differentiated neoplastic mesenchymal cells forming streams, bundles, and nests, which resulted in destruction of normal bone (Figure 2). Neoplastic cells exhibited random variation in cellular appearance and arrangement, as well as matrix composition and abundance. Haphazard and often intermingling patterns of osteogenic, chondroblastic, lipoblastic, and angiogenic tissues were present. Larger areas of neoplastic bone and hyaline cartilage contained multiple large areas of hemorrhage and necrosis bordered by neoplastic cells. In cases of inguinal lymph nodes, a major histopathological finding was infiltration of lymphocytes. In the liver, infiltration of lymphocytes was a major lesion. In the testes, major histopathological findings were degeneration of seminiferous tubules and infiltration of lymphocytes. There were no metastatic lesions in the internal organs. The neoplastic cells were found only in the right femoral mass.

The mass in this study was diagnosed as an osteosarcoma. Primary bone sarcomas that include osteosarcomas are divided into two categories, central and peripheral, with central tumors being higher in incidence and involvement [20]. Tumor types are typically named for the matrix they produce; however, when tumors contain multiple cell types and matrices, determination of histologic classification may be problematic. According to the World Health Organization classification of bone and joint tumors, an osteosarcoma is a primary malignant neoplasm of mesenchymal tissue that gives rise to a variety of patterns, but always includes the production of bone by malignant osteoblasts [20]. In animals, dogs have a relatively high occurrence of osteosarcoma [2,10]. However, spontaneous osteosarcoma of mouse is rarely reported [14-17]. Osteosarcomas arise commonly in the proximal humerus, distal radius, distal femur, and proximal and distal tibia in the dog [9-11,21,22]. Primary osteosarcomas rarely cross joints or invade adjacent bones during early tumorigenesis, but as the tumor increases in size, both events occur more commonly [8,20,23]. In domestic animals, osteosarcomas can be classified as poorly differentiated, osteoblastic, chondroblastic, fibroblastic, telangiectatic, and giant cell types [10,20]. Mesenchymal tissue is known to possess high differentiation potential, and this explains the multipotential characteristics of the neoplasm in this case.

The mass in this study was diagnosed as an osteosarcoma. To our knowledge, this is the first spontaneous osteosarcoma in an NOD mouse.