Suppression by pregnancy of chemically induced preneoplastic cells of the rat mammary gland.

Since a previous study suggested that pregnancy either eliminated preneoplastic cells or increased their latency period in rat mammary glands, additional experiments were performed to determine the fate of these cells. Following administration of the carcinogen dimethylbenzanthracene, few mammary cancers appeared after rats completed pregnancy and lactation. Because these results are similar to those previously obtained with N-methyl-N-nitrosourea (MNU), the effect of pregnancy appears to be independent of the carcinogen used to induce cancer. For rats dosed with MNU, relatively few cancers developed either during an extended observation following pregnancy and lactation or following administration of prolactin, a growth stimulator of mammary cancers. In the latter experiment, the average number of mammary cancers per rat in the prolactin-treated virgin rats was 7.1, while prolactin-treated parous rats had only 0.8 cancers per rat; i.e., 89% fewer cancers. Thus, pregnancy appears to suppress many of the preneoplastic cells induced by carcinogens in the mammary gland of rats.