Relationship between onset age of mammary tumour and reproduction in four strains of mice with different mammary tumour potentials.

The relationship between the onset age of mammary tumours and some reproductive parameters was examined in 4 strains of mice with mammary tumour potential in the order of SHN > SLN > GR/A > C3H/He. The onset age of mammary tumours of SHN, in which no difference was observed between the parous and virgin animals in the mammary tumour potential, was effected little by reproduction. However, in this strain, the age at first parturition was earlier in the individuals with early onset age of mammary tumours than in those with late onset age. In GR/A, the onset age of mammary tumours had a significantly negative correlation with rearing rate and the rearing rate was higher in the individuals with early onset age than those with late onset age. While the onset ages of mammary tumours had significant correlations with some reproductive parameters in both SLN and C3H/He, the degree was much higher in C3H/He than in SLN, well paralleling mammary tumour potential. These findings have demonstrated that the higher mammary tumour potential in parous mice than in virgins is ascribed principally to the stimulation of mammary gland growth and mammotrophic hormone secretion, which, in turn, results in an increase in the risk of malignant transformation of the cells and their progression, and that the influence of reproduction is greater in the lower mammary tumour strains.