Tumor cell types in osteosarcoma as revealed by electron microscopy. Implications for histogenesis and subclassification.

Ultrastructural studies in 26 osteosarcomas of high grade malignancy which were in diverse locations and of varied histological types revealed seven different tumor cell types. They were characterized by their features as follows: 1) anaplastic cells of malignant blast structure: 2) osteoblastic cells -- some of them with dot-like intranuclear bodies; 3) osteocyte-like cells surrounded by mineralized matrix; 4) fibroblast-like cells; 5) cells of myofibroblastic differentiation; 6) chondroblast-like cells in chondroblastic areas, and even 7) angioblastic cells that may be differentiated from the angioblastic and endothelial structures of capillaries. Histogenetically, osteosarcoma may be derived from stromal mesenchymal cells with a potential for differentiation into these seven tumor cell types, any tumor including, however, the osteoblastic and the osteocyte-like cells with tumor osteoid. This matrix serves as the specific criterion for identifying a tumor as "osteosarcoma", but almost every osteosarcoma of high grade malignancy will show these seven tumor cell types. The predominance of one or the other cell in the population may provide the basic information for achieving a cytologic subclassification of osteosarcoma in order to obtain relevant morphologic criteria in terms of prognosis.