During childbearing years, the breast parenchyma undergoes cyclic variations under hormonal influence. In the follicular phase of the menstrual cycle, epithelial cells proliferate. In the luteal phase, the connective tissue around acini becomes edematous, and there is angiogenesis and lymphocyte infiltration.
During pregnancy/lactation, the lobules enlarge, the acini increase in number with dilated lumina containing milk. Secretory vacuoles appear in the apical portion of cytoplasm in epithelial cells.
In peri- and post-menopause, the glandular tissue is progressively atrophied. Progressive involution of the acini leads to decreased size and number of lobules. Fibrous tissue is replaced by adipocytes. The use of hormone replacement therapy can significantly slow these degenerative changes.
(Guinebretiere 2005)
Breast physiology
diagram of breast stromal changes with age
Image: www.imaginis.com/breast-health-non-cancerous/breast-anatomy-and-physiology-1