An immunohistologic study of the epithelial components of 81 cases of thymoma.

Eighty-one cases of thymoma were studied immunohistologically with the use of three mouse monoclonal antibodies: one was specific for subcapsular-cortical, one for intra-cortical, and one for medullary epithelial cells. Twenty-eight (60.9%) of 46 polygonal cell thymomas were of the cortical type and 1 (2.2%) was of the medullary type. Ten (55.6%) of 18 spindle cell thymomas and 7 (41.2%) of 17 mixed cell thymomas were of the medullary type, and 1 (5.6%) of 18 spindle cell thymomas was of the cortical type. Fourteen (17.3%) of 81 thymomas were composed of epithelial cells that were triple positive immunologically; although these are unusual, they also may be present in the normal thymus. Based on these findings, triple-positive epithelium in the normal thymus consists of common stem cells that can differentiate into subcapsular-cortical, intra-cortical, and medullary epithelium; these cells may be the target cells for tumorigenesis. Epithelium in polygonal cell thymoma tends to differentiate into cortical epithelium, whereas epithelium in spindle and mixed cell thymomas differentiates into medullary epithelium.