Concentrations of steroid receptors in normal human endometrium in relation to the day of the menstrual cycle.

Concentrations of receptors for estrogen (ER), progestin (PR) and androgen (AR) were measured in the cytoplasm and nucleus of human endometrium, throughout the normal menstrual cycle. Endometrial cellular ER and PR levels gradually increased during the preovulatory and immediate postovulatory phases, remained constant at the maximal level during the mid-secretory phase and gradually decreased to the lowest level at the end of the cycle. There was a difference between ER and PR nuclear levels in the mid-secretory endometrium, where maximum levels of PR and low levels of ER were noted. This PR abundance indicates the necessity of progesterone action for implantation. During the menstrual cycle, AR sites did not change noticeably and far fewer were found than those of the other receptors. This finding indicates that androgen plays a less important role in female reproduction.