Cell kinetics of the human mammary lobule during the proliferative and secretory phase of the menstrual cycle.

We evaluated the mitotic index and nuclear volume of the epithelium of the human mammary lobule during the menstrual cycle. Thirty patients were selected and divided at random into two groups, i.e., 15 women in the proliferative phase and 15 in the secretory phase. Normal mammary tissue was obtained on the occasion of fibroadenoma enucleation. Mitotic indices (/1000) and nuclear volume (micron 3) were 0.0 and 53.8, respectively, during the proliferative phase, and 11.6 and 130.0, respectively, during the luteal phase, the difference being statistically significant (p < 0.05). These data indicate that the proliferative stimulus of the mammary lobule is more intense during the luteal phase of the normal cycle and may probably be due to a synergistic action between progesterone and estradiol.