Induction of ductal carcinomas by intraductal administration of 7,12-dimethylbenz(a)anthracene in Wistar rats.

Postpartum Wistar inbred rats (weaned on the 9th puerperal day) were injected intraductally in one mammary gland with 7,12-dimethylbenze (a) anthracene (DMBA) to selectively induce ductal carcinoma. The incidence of ductal hyperplasia increased with time until it peaked at 7 weeks (12/13 animals) and then decreased. Ductal carcinoma first developed at 9 weeks in 3/12 (2 non-invasive and 1 invasive lesion) and the incidence increased with time until invasive ductal tumors were observed in 9/11 at 20 weeks. Tumors developed only in the DMBA-treated mammary glands and no systemic effects of the carcinogen were observed. Degeneration and detachment of epithelioglandular cells were seen here and there in the ducts and terminal ducts, and epithelioglandular cells proliferated in terminal duct until 2 weeks. Residual trace DMBA powder was detected in terminal ducts and the epithelioglandular layer until 7 weeks. This trace DMBA was considered to be the cause of the development of atypical epithelial cells, inducing ductal carcinomas.