Scanning Electron Micrograph (SEM) of sarcoma cells. A sarcoma is a malignant tumor originating in connective tissue. These growths are composed of densely packed cells, diffusely embedded in a homogeneous ground substance. Their degree of malignancy varies greatly. They spread by local infiltration and by blood stream invasion. The most frequent sites in which the sarcomas develop are bone, lymph nodes, and subcutaneous tissue. Sarcomas are much less common forms of malignant tumors than are carcinomas. Magnification: 4,000X at 8 x 10" print size.