Strain difference in neoplastic response to DMBA powder dusted onto mammary tissues between Wistar/Furth and Copenhagen strains of rats.

To investigate genetic and sex factors in the local tumorigenesis by 7,12-dimethylbenz(alpha)anthracene (DMBA) in the mammary gland, both males and females of two different inbred strains of rats, Wistar/Furth (WF) and Copenhagen (COP), were subjected to dusting with approximately 1 mg of DMBA powder directly onto the exposed inguinal mammary tissue at 30 days of age. Locally growing tumors to 2 cm in mean diameter were harvested during 28 weeks after the carcinogen application. The incidence of macroscopic tumors of mammary origin was 100% in 15 WF females, 19% in 16 WF males, 85% in 20 COP females, and 74% in 19 COP males. Histologic pictures indicated the carcinomatous pattern composing mainly of differentiated adenocarcinoma of ductular cells in 12 tumors (80%) from WF females but not in any tumors from the other groups. On the other hand, they showed the sarcomatous pattern characterized by undifferentiated sarcoma of stromal cells in 2 tumors (13%) from WF females, 3 tumors (19%) from WF males, 16 tumors (80%) from COP females, and 14 tumors (74%) from COP males. The other 2 tumors from 1 WF and 1 COP females revealed the carcinosarcomatous pattern. Therefore, mammary ductular cells of WF are highly susceptible to DMBA and may be modified by sex factors in their carcinogenesis. Mammary stromal cells of COP are extremely susceptible to DMBA independently of sex factors.