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Fibro-osseous lesions are observed in the sternebrae and other bones of a number of strains of mice as spontaneous and induced lesions, especially in females. The lesions occur earlier and are more severe in mice given estrogens (Highman et al., 1981; Sass and Montali, 1980). These lesions consist of partial or complete replacement of the marrow cavity with an eosinophilic matrix that contains spindle-shaped fibroblasts, osteoclasts and a few normal appearing cells (Figs. 320 and 321). The matrix is rich in collagen and may contain a few spicules of partly mineralized osteoid and a few trabeculae of woven or lamellar bone. Some lesions are accompanied by a dense network of bony trabeculae, giving a sclerotic appearance to the affected sternebra. Similar changes have also been reported in long bones. These changes do have some morphologic resemblance to fibrous osteodystrophy. However, the kidneys and parathyroids in these mice are usually histologically normal.
Osseous metaplasia is occasionally seen in areas of old injury. It has been found in the kidney (Fig. 322), urinary bladder and lung.
Osteoma and osteosarcoma are rare in most strains of mice but have been seen in more than 20% of OF-1 mice (Wilson et al., 1985), usually occur in aged animals and are more common in female than in male mice (Frith et al., 1982). The incidence can be increased with estrogens (Highman et al., 1981). Three morphological types have been described. The osteoblastic type is characterized by the presence of prominent osteoid formation being produced by the neoplastic osteoblasts (Figs. 323 and 324). The fibroblastic type contains primarily a fibrous stroma with minimal production of osteoid. The mixed type contains prominent areas of both of the preceding types.
Osteosarcomas develop in the calvarium, the long bones of the limbs, the vertebrae and occasionally the pelvic bones. Osteosarcomas probably metastasize more frequently than any other malignant naturally occurring neoplasm in the mouse (Frith et al., 1981b). Pulmonary metastases are the most common, but occasional hepatic (Fig. 325) and renal metastases have been seen. FBJ virus includes periosteal osteosarcomas which do not metastasize in mice (Ward et al., 1978).
Chondrosarcomas are extremely rare in mice. Fig. 326 shows a chondrosarcoma.
This malignant mesenchymal neoplasm is derived from skeletal muscle. The neoplasm is composed of large prominent spindle-shaped cells with occasional giant cells (Fig. 327). Mitotic figures are usually prominent. Cross striation may be visible with the PTAH stain, but it may be necessary to positively confirm the presence of myocyte organelles with electron microscopy.
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