This article reviews the effects of estradiol (E2), progesterone (P) and P receptor antagonists (PA) on the rhesus macaque endometrium. Ovariectomized macaques can be treated with implants of estradiol (E 2) and P to induce precisely controlled, artificial menstrual cycles. During these cycles, treatment with E2 alone induces an artificial proliferative phase marked by extensive endometrial epithelial cell proliferation and increased expression of stromal and epithelial estrogen receptor (ER) and P receptor (PR). Androgen receptor (AR) is also upregulated by E2 but is expressed only by the endometrial stroma. Progesterone acts on the E2 primed endometrium to induce secretory differentiation and causes suppression of epithelial and stromal ER, epithelial PR, and stromal AR in the functionalis zone. However, epithelial ER and PR are retained in the basalis zone during the secretory phase. When potent P antagonists (PA) are administered acutely at the end of an E2+P induced cycle, menses typically ensues similar to P withdrawal at the end of the menstrual cycle. When PAs are administered chronically there is significant blockage of all P- dependent effects including upregulation of ER, PR and AR and suppression of glandular secretory function. However, chronic PA administration also inhibits estrogen-dependent endometrial cell proliferation and growth. This endometrial antiproliferative effect is the basis of the clinical use of PA to control various diseases such as endometriosis.
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