Thymomas and Thymic Hyperplasia in Praomys (Mastomys) natalensis. Concomitant Myositis, Myocarditis, and Sialodacryoadenitis

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Abstract
Twenty-seven thymomas and 11 hyperplastic thymuses were observed in a series of 113 untreated Mastomys from which histologic sections of the thymus were available for examination. Most of the Mastomys with thymic lesions were over 2 years old, and two-thirds of them were females. The histologic pattern of the thymomas corresponded closely with that of the thymomas in man in which the normal relationship of cortex to medulla is effaced and the different thymic cellular and structural components appear in abnormal relationships to each other. In the hyperplastic thymuses, the pattern of cortical and medullary markings was reproduced, although lacking somewhat the regularity seen in the noninvoluted thymus of young animals. Myositis and atrophy of skeletal muscle were present in one Mastomys with thymic hyperplasia and in 7 with thymomas. Nearly one-third of the animals with thymomas had severe myocarditis, and over two-thirds had chronic inflammation often associated with degeneration and atrophy of the extraorbital and intraorbital lacrimal, lingual, harderian, or one or more of the salivary glands. These lesions were noted, but less frequently, in the Mastomys with hyperplasia of the thymus. The possibility is considered that the thymomas, thymic hyperplasia, myositis, myocarditis, and other inflammatory lesions might have some relationship to myasthenia gravis or other diseases of man believed by some to be associated with autoimmunity. Many of the 38 Mastomys with hyperplasia of the thymus or thymoma also had neoplasms of other organs, but these were estimated to be no more frequent than in our entire colony.