Malignant tumor of the breast with bone formation.Studies by light and electron microscopy

A malignant tumor of the breast showing bone formation was studied by light and electron microscopy. The basic pattern consisted of undifferentiated cells separated by wide but poorly demarcated stromal bands. Giant cells were also present. Bony trabeculae, smaller osteoid masses and (rarely) cartilage were scattered through the tumor. The undifferentiated cells were pleomorphic by electron microscopy but showed desmosomes and evidence of an ability to produce collagen and ground substance. Many cells adjacent to osteoid and bony trabeculae resembled normal osteoblasts but others were similar to the undifferentiated cells. The stromal bands contained “normal-looking” and “primitive-looking” fibroblasts which shared features of the undifferentiated cells. The question of a mesenchymal versus epithelial origin of the tumor could not be settled but the authors believe that production of stroma and bone is not inconsistent with epithelial origin. The many shared features amongst the various cell types suggest that all, including bone and stroma-producing cells, had a common malignant origin.