Involution of the thymus: a possible diagnostic pitfall

There were investigated 22 cases from which the thymic tissue was removed either during surgery for cardiovascular malformations (n = 14), or for myasthenia gravis (n = 8). Histological sections were stained with routine morphologic methods, and immunohistochemistry was performed for cytokeratin, CD20, CD3, and S100 protein. Aspects characteristic for thymus involution were found in 11 cases without myasthenia gravis and in all cases with myasthenia gravis. Morphological changes of the thymus of involution are age-dependent. There were characterized stages of involution, with special reference to cortical - medulla inversion, lymphocyte depletion and sequestration. In advanced-stage of involution, epithelial cells are arranged in cords or compact islands, and could mimic a thymoma or a metastatic carcinoma. The immunohistochemical profile is similar but not identical to the active thymus. We noticed a decreased expression of cytokeratin, and a reduced number of CD3, CD20, and S100 positive cells. Morphologic and immunohistochemical peculiarities of the thymus of involution are discussed in relation with the specific pathology of the organ.