Morphology and progression of preneoplastic changes in methylcholanthrene-induced mouse mammary tumorigenesis

Susceptible BALB/c virgin female mice were treated with 3-methylcholanthrene in almond oil (0.5%) by skin painting and killed at monthly intervals. The mammary glands were prepared as whole mounts for stereoscopic study of the glandular tree with removal of any focal change for histology. The results showed a marked disarrangement in the architecture of the glandular tree, a prevalent ductal tree, and an overcrowding of ducts. Focal hyperplastic changes were frequently observed, including hyperplastic alveolar nodules, hyperplastic ductal nodules, noduloids and intraductal epithelial growths, and their respective rates of malignant transformation observed at histology were 22%, 15%, 7% and 50%. Data suggest a different weight of these preneoplastic changes as sources of malignant transformation and a prevalent ductal origin of the methylcholanthrene-induced neoplasms.