Role of progesterone in normal breast physiology.

The extant literature indicates that progesterone significantly influences normal mammary growth and differentiation. However, most breast tissue research 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 menstrual cycle and pregnancy, in vitro studies have been inconsistent in their assessments of progesterone's role in proliferation. Similarly, mitotic activity in the breast reaches its peak during the progesterone-dominant luteal phase of the menstrual cycle, and therefore some researchers claim that progesterone plays a major role in breast cancer; however, several clinical observations have found evidence dismissing progesterone as a key factor. Thus, researchers seek to expand our current understanding of the role of progesterone in breast physiology.