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 mouse-to-mouse mammary variations. Morphology (assigned grades 0–3) ranged from a complete lack of alveoli to the presence of numerous alveoli emanating from branches. Morphological changes were driven by epithelial proliferation and apoptosis, which differed between ductal versus alveolar structures. Proliferation within alveolar epithelium increased as morphological grade increased. Extensive alveolar apoptosis was restricted to tissue exhibiting grade 3 morphology, and was ∼14-fold higher than at all other grades. Epithelial proliferation and apoptosis exhibited a positive relationship with serum levels of progesterone, but not with 17β-estradiol. Compared with other estrous stages, diestrus was unique in that the morphological grade, epithelial proliferation, apoptosis, and progesterone levels all peaked at this stage. The regulated tissue remodeling of the mammary gland was orchestrated with mRNA changes in specific matrix metalloproteinases (MMP-9 and MMP-13) and specific tissue inhibitors of metalloproteinases (TIMP-3 and TIMP-4). We propose that the cyclical turnover of epithelial cells within the adult mammary tissue is a sum of spatial and functional coordination of hormonal and matrix regulatory factors.
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