Mammary Gland Morphogenesis and Function during Pregnancy

In response to progesterone and prolactin, the murine mammary epithelium executes an accelerated proliferative program that is required to generate numerous ductal side branches and alveoli during early pregnancy. With prolactin exposure later in pregnancy, alveolar epithelial cells differentiate to enable the synthesis and secretion of enough milk components to meet the nutritional demands of an average litter size. Termination of the suckling stimulus at weaning triggers regression of the alveolar-ductal network through proteinase- and apoptotic-mediated remodeling steps, collectively termed involution. Following postlactational involution, the remodeling of the mammary gland to a ductal structure similar to that of the prepregnant mouse completes this mammary cycle of development. The fact that the mammary gland can enter this developmental cycle with each successive pregnancy strongly supports the existence of a resident mammary stem cell.