| Research Abstract |
1. Cytokeratin (CK) patern in thymomas was compared with that in normal thymic epithelium by immunohistochemistry on the frozen sections. It was possible to classify the tumors into cortical (CK8+, 19+), medullary (CK13+, 14+, 19+), corpuscular (CK14-), and mixed types, although subcapsular type (CK7+, 14+, 19+) was not recognized.
2. Thymic carcinomas were histologically and immunohistochemically subclassified into squamous cell, adenosquamous, lymphoepithelioma-like, clear cell, mucoepidermoid, basalaid, cell carcinoma and atypical carcinoid. All the subtypes showed features compatibel with mesodermal derivation, with differntiation toward pharyngeal or medullary duct of neuroendocrine epithelia.
3. Tumor cell proliferation was studied by quantification of argrophilic nucleolar organizer regions and proliferating cell nclear antigen. The labeling indeces of these markers were not defferent between noninvasive and invasive/metastatic thymomas, but were significantky differnt between thymomas and thymic carcinomas. The results suggested that thymoma is essentially a low-grade malignant tumor regardless of the stages and that the designation of "malignant" thymoma is inadequate because of redundancy and confusion with thymic carcinoma.
4. Abnormal p53 protein and mutated p53 protein was immunohistochemically demonstrated in almost all the thymomas, but more in the abvanced stage tumors. Thymic carcinomas showed a prominent expression of the abnormal protein. Sequencing of the p53 gene revealed point mutations in low expressors of the protein as well as in high expressors, although the location of the mutation was not constant. It was concluded that abnormality in p53 gene is an early event in thymic epithelial tumors.
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