Thymoma is a neoplasm of thymic epithelial cells. This definition excludes other tumors that may affect the thymus, such as lymphoma and germ cell tumors. Although rare, thymoma is the most common tumor of the anterior superior mediastinum. The term lymphoepithelioma has been used in cases in which the thymoma contains a large number of lymphoid cells.
Normal thymic epithelium tissue arises from the third branchial cleft and the third and fourth branchial pouches. Dendritic cells and macrophages found in large quantities at the corticomedullary junction arise from mesodermal tissues (bone marrow). The epithelial cells and these other stromal tissues of the thymus influence the selection and maturation of the T lymphocytes. Dysregulation of this system in thymoma is believed to be a cause of accompanying paraneoplastic syndromes.
In the normal thymus, bone marrow–derived precursor cells destined to become thymocytes (or T lymphocytes) enter the thymus at the corticomedullary junction and differentiate as they pass through the thymus. These cells can be characterized in their developmental progression by changes in expression of 3 cell surface markers: CD4, CD8, and the T-cell receptor (TCR)–CD3 complex.
Initially, the cells undergo positive selection; thus, those cells that fail to receive a signal (ie, do not recognize self) die by apoptosis or become inactive. The cells that pass through the corticomedullary junction undergo negative selection; the thymocytes expressing TCRs that have an excessively high affinity for self-proteins are eliminated. These cells are believed to recognize self too strongly and to have autoimmune potential. From the corticomedullary junction, the cells enter the medulla or circulate in the periphery to other lymphoid structures (ie, lymph nodes). The lymphocytes’ selection process and developmental progression are influenced by direct contact between the TCR-CD3 complex on the thymocyte and the major histocompatibility complex (MHC)–antigen complex on thymic epithelial cells, dendritic cells, and B lymphocytes. The cytokines involved in thymocyte development and selection include interleukin (IL)–1, IL-2, IL-3, IL-4, IL-6, and IL-7.