lobulo-alveolar structure in the breast is caused by
Breast development Progesterone, in conjunction with GH/IGF-1 similarly to estrogen, affects the development of the breasts during puberty and thereafter as well.[15][16][20] To a lesser extent than estrogen, progesterone contributes to ductal development at this time, as evidenced by the findings that progesterone receptor (PR) knockout mice or mice treated with the PR antagonist mifepristone show delayed (albeit eventually normal, due to estrogen acting on its own) ductal growth during puberty and by the fact that progesterone has been found to induce ductal growth on its own in the mouse mammary gland mainly via the induction of the expression of amphiregulin, the same growth factor that estrogen primarily induces to mediate its actions on ductal development.[22] In addition, progesterone produces modest lobuloalveolar development (alveolar bud formation or ductal sidebranching) starting at puberty,[15][20] specifically through activation of PRB (and notably not PRA),[23] with growth and regression of the alveoli occurring to some degree with each menstrual cycle.[15][16] However, only rudimentary alveoli develop in response to pre-pregnancy levels of progesterone and estrogen, and lobuloalveolar development will remain at this stage until pregnancy occurs, if it does.[16] In addition to GH/IGF-1, estrogen is required for progesterone to affect the breasts,[15][20] as estrogen primes the breasts by inducing the expression of the progesterone receptor (PR) in breast epithelial tissue.[23] In contrast to the case of the PR, ER expression in the breast is stable and differs relatively little in the contexts of reproductive status, stage of the menstrual cycle, or exogenous hormonal therapy.[23]