Preneoplastic lesions of the human mammary gland transplanted into the nude athymic mouse.

Morphologically normal lobules and atypical lobules postulated precancerous to ductal carcinoma were transplanted to test their biological behavior. Supravital staining disclosed the 1 to 4-mm microorgans. "Cleared" mammary fat pads of nude mice were optimal transplantation sites. Of the total of 217 transplants from 19 cancer-associated and 13 non-cancer-associated breast, 151 survived after 2 to 27 weeks. Of 61 surviving normal-appearing lobules from cancer-associated breasts transplanted without prior in vitro maintenance, 20 (30%) dedifferentiated, and of 48 surviving lobules from noncancerous breasts, 11 (20%) dedifferentiated. Fifteen of 28 histologically normal-appearing lobules (60%) obtained from cancer-associated breasts after age 50 dedifferentiated. Thirty of 36 atypical lobules isolated from the breast tissue were obtained from cancer-associated breasts and the 5 of those that dedifferentiated came from cancer-associated breasts. Twenty of 22 (90%) dedifferentiating transplants from cancer-associated breasts showed a vascularization response, whereas 3 of 7 (43%) from noncancer-associated breasts did so. If dedifferentiation in this experimental setting is indicative of a precancerous potential, the data on normal-appearing lobules obtained from cancer-associated breasts from women over age 50 suggest that these lobules carry the greatest precancerous potential. Such lobules probably belong to a type persisting after menopause and they may be hormonally autonomous. Such lobules might undergo further atypia in vivo and, eventually, cancerous transformation.