Levels of estrogen and progesterone receptor in human endometrium during the menstrual cycle

Receptors for estrogens and progesterone have been assayed in 24 samples of normal human endometria at different phases of the menstrual cycle. In the proliferative endometrium the concentration of estrogen binding sites was significantly higher than that of progesterone receptor while, in the secretive endometrium, the concentration of estrogen receptor was low, compared to that of progesterone receptor. The highest concentration of both receptors was observed during the periovulatory period. From the correlation observed between estrogen and progesterone receptors and the levels of the respective steroids in the plasma, it is suggested that the levels of plasma estradiol or its own tissue receptor determine the synthesis of progesterone receptors in the periovulatory period.