38 endometrial sarcomas were studied light microscopically, 5 of them additionally with the electron microscope. According to their histological appearance these tumours were classified as homologous stromal sarcomas (13 cases), pure heterologous (4) and mixed mesodermal sarcomas (21). The ultrastructure is described with reference to the cytoplasmic differentiation of the tumour cells. A basic immature cell type was present in all endometrial sarcomas irrespective whether they were of pure homologous or of mixed type. In regard to the striking similarity between this cell and cells of the early proliferating endometrium an origin of all endometrial sarcomas from immature stromal cell is suggested. The mixed tumours contain a number of highly differentiated cells like smooth muscle cells, rhabdomyoblasts, fibroblasts etc. As there was a number of cells intermediate in cytoplasmic appearances between immature and differentiated end stages it seems likely that the latter develop from the former. Thus the undifferentiated cell represents the stem cell from which the differentiation processes of all cell lines take their origin. A transition from neoplastic epithelium to neoplastic mesenchyme could not be demonstrated in thed that the mixed epithelial-mesenchymal endometrial tumours originate by a simultaneous malignant transformation of cells with a fixed differentiation potency.

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