Malignant mixed osteogenic tumours of the breast. An ultrastructural study of two cases.

Two cases of malignant mesodermic tumours of the human mammary gland with osteogenic and chondrogenic structures were analysed by optical and electronic microscopical means. One of them was associated with an adenocarcinoma of the gland. The histological pattern was similar to that of those cases previously examined under the optical microscope in the mammary gland and in extraskeletal osteogenic sarcomas of soft tissues. When investigated under the electron microscope, the chondroblasts possessed a highly developed RER in active synthesis with an amorphous material which contributed to the building up of the ground substance matrix of the tumorous cartilage. Osteoid fields with scattered osteoblasts appear throughout the tumorous stroma and were associated with calcium deposits. They were continuous with fibroblasts and mesenchymal undifferentiated cells of a very immature character. Giant cells of osteoclastic type were included within the mononucleated mesenchymal cells mimicking bone osteoclastoma. The presence of all these cell types suggests the existence of a common malignant origin, the stem cell being differentiated into epithelial carcinomatous and mesenchymal sarcomatous chondral and osteogenic tissues.