Prevention of mammary carcinogenesis in rats by pregnancy: effect of full-term and interrupted pregnancy.

In this study, the role of parity in conferring protection of the mammary gland against chemical carcinogenesis induced by 7,12-dimethylbenz(a)anthracene (DMBA) was investigated. Experiments were also carried out to determine if an 'interrupted' pregnancy was capable of reducing the incidence of mammary tumour induction. Since it has been suggested that morphological development or the proliferative pattern of the mammary gland at the time of carcinogen administration may be involved in reducing the susceptibility of the mammary gland to chemical carcinogenesis, experiments were designed to elucidate the possible influence of these two factors. Sprague-Dawley female rats were mated and were either allowed to complete pregnancy and parturition or were subjected to Caesarian section on day 5, 10 or 15 of the pregnancy. When DMBA was administered i.v. to animals which had been allowed to complete a full-term pregnancy, only 14% developed tumours, compared to 70% in age-matched nulliparous controls. Termination of the pregnancy on days 5, 10 or 15 was as effective in reducing tumour incidence as full-term gestation and parturition, but still resulted in partial and statistically significant inhibition, compared to age-matched nulliparous controls. There was no significant difference in 3H-thymidine labelling index (LI) at the time of DMBA treatment in the parous rats compared to age-matched nulliparous controls. We also observed no significant differences in the morphological development of the mammary gland in parous and nulliparous rats of the same age. These results indicate that the protective mechanism may not lie in the mammary gland per se, but may indeed be a host factor, such as hormonal or immunological changes occurring in the host as a result of the pregnancy.