Thymoma.An ultrastructural study

Ultrastructural study of two thymic tumors has revealed, within both, a lymphocytic component morphologically indistinguishable from the general population of lymphocytes. The reticulum cells were dissimilar in their fine structure with respect both to nuclear morphology and to the content of cytoplasmic organelles. These divergences parallel the variations in light microscopic appearance exhibited by this class of neoplasm. Cytoplasmic fibres and desmosomes were observed in both instances but their quantity and distribution differed. The presence of cytoplasmic fibres and desmosomes—particularly the latter—would justify the designation of the reticular component of these neoplasms as epithelial reticular cells. This categorization is in harmony with the epithelial aspect manifest by thymic tumors under the light microscope. The extent, if any, to which mesenchymal reticular cells participate in processes of thymic neoplasia must await further ultrastructural inquiry.