Ultrastructural study of 28 cases of Ewing's sarcoma: typical and atypical forms.

The characteristics of 35 round cell sarcomas of bones are analyzed by optical and electron microscopical means, 28 cases of Ewing's sarcoma and 7 cases of malignant lymphomas (reticulum cell sarcoma) being distinguished. The existence of a morphological diversity within the Ewing's sarcoma group allows the authors to differentiate a conventional form of Ewing's sarcoma (21 cases) consisting of principal blastemic cells and secondary degenerative ones (dark cells) from an atypical variant (7 cases of atypical Ewin's sarcoma) whose structure adopts either an immature mesenchymal or histiocytic appearance. The ultrastructure of the atypical Ewing's cells demonstrates variability in size and in the nucleus, which is grooved and possesses prominent nucleoli. In the cytoplasm, in addition to the existence of a variable amount of glycogen, more highly differentiated structures appear when a comparison is made with the principal cells of the conventional Ewing's sarcoma. There exist filaments and mitochondria complexes, desmosomic-like junctions and profiles of endoplasmic reticulum. No transitional forms between the conventional and the atypical variants of Ewing's sarcoma have been observed. According to this analysis, when compared to other bone malignancies, the presence of glycogen as a means for identifying Ewing's sarcoma seems to be of restricted value. No clinical or anatomical differences could be established between either variant of Ewing's sarcoma.