Role of progesterone in normal breast physiology

The extant literature indicates that progesterone significantly influences normal mammary growth and differentiation. However, most breast tissue rese arch is conducted using malignant cells, in which the progesterone activity differs greatly from that in normal cells. Although progesterone has been demonstrated to support cyclic proliferation in the breast during the menst rual cycle and pregnancy, in vitro studies have been inconsistent in their assessments of progesterone's role in proliferation. Similarly, mitotic act ivity in the breast reaches its peak during the progesterone-dominant lutea l phase of the menstrual cycle, and therefore some researchers claim that p rogesterone plays a major role in breast cancer; however, several clinical observations have found evidence dismissing progesterone as a key factor. T hus, researchers seek to expand our current understanding of the role of pr ogesterone in breast physiology.