Noble (Nb) rat strain has been used for the study of hormonal carcinogenesis of mammary and prostate glands, for its susceptibility to develop premalignant lesions as well as carcinomas in these organs by sex hormones. However, background information on the spontaneously developed mammary tumors in this rat strain is scarce. We report on the incidence rate, latency period and histopathology of mammary tumors spontaneously developed in the senile intact and untreated Nb rats compared with those induced by either combined treatments with sex steroids or 7,12-dimethylbenz[a]anthracene (DMBA) in the same rat strain. We observed that the incidence rate of spontaneous mammary tumors was 45% in female Nb rats and 3% in the males. The average age of the female Nb rats to develop palpable tumors was 14 months and rarely detected in animals less than 12 months old. It was also noted that the incidence rate of the spontaneous mammary tumors was similar to those induced by combined treatments with sex steroids for 8-10 months (46.7% for T+E2 and 55.6% for T+DES) but less than those by DMBA treatment in 8 months (over 80%). Histologically, majority of the spontaneous mammary tumors were fibroadenomas, which comprised 70% of all collected tumors and about 20% were carcinomas whereas tumors induced by steroid hormones and DMBA were all carcinomas. Distant metastases of spontaneous mammary carcinomas to lung, liver and lymph nodes were also noted, but rarely.

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