Enhancement of spontaneous mammary tumorigenesis at advanced age in mice by temporary stimulation of mammary growth during youth.

As a possible step to clarify the relationship between mammary gland growth during youth and mammary tumorigenesis at advanced age, the long-term effects of the temporary stimulation of mammary gland growth at youth on spontaneous mammary tumorigenesis was studied in a high mammary tumor strain of SHN virgin mice. They received a single pituitary graft under the kidney capsule or a subcutaneous implantation of progesterone for 60 days between 30 and 90 days of age. Both treatments enhanced mammary gland growth and the effect of pituitary grafting was higher. Spontaneous mammary tumorigenesis paralleled mammary gland conditions at youth. The results indicate that the stimulation of mammary gland growth during youth enhances spontaneous mammary tumorigenesis at advanced age in mice, which is quite contrary in rats, and they explain the cause of the higher mammary tumor potential in breeders than in virgin mice.