Ultrastructure of spontaneous and urethan-induced thymomas in buffalo rats

Normal thymuses from Buffalo and Long-Evans rats of various ages, and spontaneous and urethan-induced thymomas in Buffalo rats, were examined by electron microscopy. Histological variabilities among thymomas of the lymphoid, mixed, and epithelial cell types were a reflection of the number of lymphoid cells within the network composed of neoplastic epithelial reticular cells. In the cytoplasm of these cells, development of tonofilaments and membrane-bound bodies and inverse development of the rough-surfaced endoplasmic reticulum were recognized in the sequential process from the lymphoid cell type to the epithelial cell type. An important role of the development of the rough-surfaced endoplasmic reticulum for thymic function was suggested. Phagocytic activity of the neoplastic epithelial reticular cells was revealed, and some of the membrane-bound bodies in these cells, especially those with moniliform structures, were regarded as remnants of damaged lymphocytes. Evidence for neoplastic epithelial reticular cell-lymphoid cell transformation could not be established from study of the thymoma tissue. No virus-like structures were observed in these thymomas.