The fine structure of thymoma, with emphasis on its differential diagnosis. A study of ten cases.

In this study, thymoma is defined as a neoplasm of the epithelial-reticular framework cells of the thymus. As in the normal thymus, these cells regularly displayed branching tonofilaments, macuale adherens, elongated processes, and basal lamina. These characteristics proved useful in the differential diagnosis of thymoma from a variety of anterior mediastinal tumors including thymic carcinoid, lymphoma, germinoma (seminoma type), and fibrous mesothelioma. Lymphocytes in the thymomas often showed mitotic activity and a moderate degree of transformation. The significance of this and the gland-like spaces, vacuolated epithelial cells, starry-sky appearance, emperipolesis, and perivascular spaces is discussed.