Abstract
Osteosarcomas were found in the lung of a 62-week-old ICR female mouse having malignant systemic lymphoma. Macroscopically, the firm white nodules were located in all lobes of the lungs. Microscopically, the multiple nodules were mainly composed of irregularly shaped bone/osteoid tissues and pleomorphic or spindle-shaped osteoblast-like cells. Most of the tumors were well-differentiated and bone/osteoid tissues were abundant in the middle of the tumors, while at the tumor periphery and in a few nodules tumor cells were dominant with thin trabeculae of osteoid. These cells had hypochromatic, irregularly oval nuclei with a few small nucleoli, and, furthermore, anisokaryosis and mitosis were observed. The tumor cells were also present in the blood vessels and bronchial tubes. Immunohistochemically, the cells were positive for proliferating cell nuclear antigen (PCNA) and vimentin, but negative for cytokeratin and S-100 protein. Ultrastructurally, the cells had prominent nucleoli and dilated rough endoplasmic reticula, and collagen fibers with high electron density particles (hydroxy apatite) were observed around the cells. These findings suggest that the present tumors were malignant osteogenic neoplasms, and we diagnosed these tumors as osteosarcomas. Based on the multifocal growth pattern and presence in the blood vessels, in spite of the primary site being unknown, the tumors were considered to be metastatic.
