Progesterone Signaling and Mammary Gland Morphogenesis

Progesterone was identified as a mammogenichormone several years ago but until now its precise rolein mammary development has remained obscure. Recentlywith the generation of several transgenic mouse models and development of reagents for analysisof progesterone receptor expression, the role ofprogesterone signaling in mammary development isbecoming more clear. The most significant observationsto emerge from these studies are (1) progesteronereceptors (PR)4 are present in a heterogeneous manner inthe epithelial cells and undetectable in the surroundingfat pad; (2) they are essential for lobuloalveolar and not for ductal morphogenesis; (3)progesterone signaling through progesterone receptors,leading to lobuloalveolar development, is initiated inthe epithelium and may occur through paracrinemechanisms; and (4) a regulated expression of the twoisoforms of progesterone receptor is critical formaintaining appropriate responsiveness to progesteroneand hence, epithelial cell replicative homeostasis.These studies also reveal that the consequences ofprogesterone signaling through progesterone receptor maydepend on the cell context, cell-cell andcell-extracellular matrix interactions, the dynamics ofPR turnover and the fate of PR positivecells.