While a number of agents have been shown to induce mammary carcinogenesis i
n the rat, premalignant stages of the disease have been best characterized
in chemically-induced models, specifically those initiated by either 7,12 d
imethylbenz[alpha ]anthracene (DMBA)(4) or 1-methyl-1-nitrosourea (MNU). In
general, it appears that epithelial cells in mammary terminal end buds or
terminal ductules are the targets of carcinogenic initiation, and that a se
ries of morphologically identifiable steps are involved in the development
of mammary carcinoma. The premalignant steps include ductal hyperplasia of
the usual type and carcinoma in situ of the cribriform or comedo type; atyp
ical ductal hyperplasia has not been reported. Thus the histogenesis of les
ions occurring in chemically induced mammary carcinogenesis in the rat is s
imilar to that observed in the human; although, the spectrum of lesions obs
erved in the rat is limited. Opportunities to investigate the biological an
d molecular characteristics of premalignant breast disease in the rat are p
resented.