It had been reported that a higher frequency of the thymic abnormalities including thymoma and/or thymic hyperplasia with germinal centers and the effectiveness of thymectomy could be demonstrated in the patients with myasthenia gravis. The relationship between these thymic abnormalities and clinical features in myasthenia gravis, however, has not been clarified satisfactorily yet. Twenty-two thymuses obtained from myasthenic patients, who had undergone thymectomy, were investigated histologically. The hyperplastic thymuses with germinal centers were found in 52.6% of all cases including non-thymomatous regions in the thymuses with thymoma, and were observed in higher per centages in the younger myasthenic patients. The density of germinal centers was higher in the thymuses of clinically severe myasthenic patients, especially of thymomatous patients, whose thymuses had numerous germinal centers in all cases, than of the ocular myasthenia or nonthymomatous patients. These patients whose thymuses could be demonstrated numerous germinal centers deteriorated their clinical symptomes even after the thymectomy, who were followed by the improvement with the administrations of prednisolone. The older patients were, the more frequently Hassall's corpuscles in the thymic medulla were detected, although the clinical features were scarcely related with the density of Hassall's corpuscles. Of twenty-two patients, eight had thymomas; six of them were lympho-epithelial cell types, one was spindle cell type, and one was lymphocytic cell type. These results suggested that the density of germinal centers was one of the most important factors correlating with the clinical features and the responsiveness to the thymectomy or steroid therapy in the myasthenia patients.
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