The thymus according to Schambacher: medullary ducts and reticular epithelium of thymus and thymomas

Thymomas exhibit histologically characteristic features of differentiation with great similarity to structures in fetal, normal and involuted thymus. They include pleomorphic ducts, cysts and glandular structures, which cannot be explained with the conventional views on thymus development as introduced by Hammar, but find a simple histogenetic clarification if Schambacher's concepts are accepted. With suitable fetal material Schambacher's findings can be easily confirmed. They indicate that the thymus is composed of two different types of epithelium, called here the "reticular epithelium" and the "medullary duct epithelium." The origins and development are described. The features of fetal thymus, of normal and involuted thymus, of cystic thymic lesions and of dysplastic thymus are re-examined and compared with structures found in thymomas. The observed findings suggest that thymomas are complex "organoid" epitheliomas with diversified reticular epithelial and medullary duct differentiation. Like other epitheliomas they grow slowly and may show local invasive propensities, but distant metastases remain exceptionally rare.