The M5076 tumor, a reticulum cell sarcoma of histiocytic origin that arose spontaneously in the ovary of a C57BL/6 mouse, is highly invasive and metastatic. Regardless of the site of the primary tumor, this neoplasm rapidly and preferentially metastasizes to the liver and spleen, killing the host. Numerous other organs also are involved, including the lungs and bone marrow. This tendency to metastasize to osseous tissues appears to be a characteristic of the M5076 tumor that rarely is found with other rodent neoplasms. However, these extrahepatic foci are evident only microscopically and are seen relatively late in the progression of tumor growth. We describe the sequential gross and microscopic lesions that develop in syngeneic mice during the metastatic spread of the M5076 tumor. Since viable tumor cells form small tumor colonies in most organs, we suggest that the gross pattern of metastasis, with the apparent predilection for hepatic tissue, is caused by variations in tumor cell proliferation rather than by distinctive patterns of neoplastic cell spread and entrapment.
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