Cellular Turnover in the Mammary Gland Is Correlated with Systemic Levels of Progesterone and Not 17β-Estradiol During the Estrous Cycle1

Adult mammary tissue has been considered "resting" with minimal morphological change. Here, we reveal the dynamic nature of the nulliparous murine mammary gland. We demonstrate specific changes at the morphological and cellular levels, and uncover their relationship with the murine estrous cycle and physiological levels of steroid hormones. Differences in the numbers of higher-order epithelial branches and alveolar development led to extensive …