Neoplasms Occurring in Aged Fischer Rats, With Special Reference to Testicular, Uterine, and Thyroid Tumors2

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Abstract
Inbred fischer rats were set aside as nonbreeders specifically to obtain tumors of endocrine organs and endocrine-responsive tissues that might be used in studies of hormone responsivity. The average age at death or autopsy was 675 days for 102 females, and 725 days for 92 males. The tumor data are recorded for those animals that were autopsied (91%). Sixty-eight percent of the males developed testicular interstitial (Leydig) cell tumors and in three fourths these were bilateral. One half of the 20 tumors transplanted into isologous hosts grew to a palpable size. In all instances where growth occurred it was more rapid in castrate than in intact male recipients. At least four of these transplanted tumors produced androgen. Twenty-one percent of the females had macroscopically evident uterine polypoid tumors that were endometrial in origin. Nine of these were transplanted into isologous hosts; 4 grew as sarcomas and 5 maintained their original benign, polyp-like microscopic appearance. Three of these latter tumors grew more rapidly in intact females than in intact males or castrate hosts of either sex. Nine of the aged rats developed grossly evident tumors of the thyroid that morphologically were typical type-γ nodules. A papillary adenocarcinoma was found in the contralateral lobe of the thyroid of 1 of these rats. Only 1 type-γ nodule was transplanted, and after 1 year it has grown to approximately 1 cm in diameter in 1 of the 2 untreated male recipients. The incidence of breast, adrenal medullary, and pituitary tumors, as well as of other miscellaneous tumors, was not notably different from that reported by several investigators observing aged rats of different genetic background.

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