Herein we review the morphological and physiological effects of estradiol and progesterone (P) on the nonhuman primate uterus. Progesterone action acts to prepare the endometrium for embryo implantation, which normally occurs only during a brief period in the mid-luteal phase of the menstrual cycle. During this window of implantation, P stimulates secretory morphological differentiation and suppresses estrogen receptor alpha (ERalpha), in the endometrial functionalis zone. Reduced endometrial ERalpha is a definitive physiological marker for the onset of endometrial receptivity in primates. These actions of P are specific for the functionalis zones, and P does not fully inhibit ERalpha in the glands of basalis zone of nonhuman primates. Paradoxically, during the secretory phase of the cycle, progesterone receptor (PR) is also reduced in the glandular epithelium of the progestin-responsive functionalis zone. Therefore, P action on the epithelium in the functionalis zone may be mediated by paracrine factors arising from the PR-positive cells in the stroma. Genomic analysis of the endometrium of women and nonhuman primates has revealed numerous secretory phase genes that may contribute to differentiation of the endometrium. However, the exact nature and function of these putative factors have been elusive. We propose that nonhuman primates, especially macaques, can provide a valuable animal model for experimentally testing the functional role of P-regulated genes on endometrial receptivity.

The hormonally driven expression and cell-specific localization patterns of the progesterone receptor membrane components (PGRMC1 and PGRMC2) in the macaque endometrium during the menstrual cycle are unknown. Additionally, the expression and localization patterns of PGRMC1 and PGRMC2 in the secretory eutopic endometrium of primates afflicted with endometriosis are also unknown. Therefore, we used real-time PCR to quantify transcript expression levels of the PGRMCs in well-defined samples of endometrium collected from artificially cycled macaques during the menstrual cycle, and in the secretory phase endometrium of naturally cycling macaques afflicted with endometriosis. In situ hybridization and immunocytochemistry were used to localize PGRMC1 and PGRMC2 mRNA and protein, respectively. We compared the patterns of expression and localization of the PGRMCs with the expression and localization patterns of nuclear progesterone receptor (PGR). PGRMC1 and PGR were elevated during the proliferative phases of the cycle, and then declined to nearly undetectable levels during the late secretory phase of the cycle. Levels of PGRMC2 were lowest during the proliferative phases of the cycle and then increased markedly during the secretory phases. Strong staining for PGRMC2 was localized to the luminal and glandular epithelia during the secretory phases. When compared with artificially cycled disease-free animals, macaques with endometriosis exhibited no changes in the expression or localization patterns for PGR and PGRMC1 but exhibited strikingly reduced levels of PGRMC2 transcript and altered intracellular staining patterns for the PGRMC2 protein. Collectively, these results suggest that membrane-bound PGRMC2 may provide a pathway of action that could potentially mediate the non-genomic effects of progesterone on the glandular epithelia during the secretory phase of the cycle. Further, reduced levels of membrane-bound PGRMC2 may be associated with the progesterone insensitivity often observed in the endometrium of primates afflicted with endometriosis.

Objective To use contrast enhanced ultrasound (CEU) to quantify blood flow in the macaque uterus during early pregnancy. Design Prospective nonhuman primate study. Setting National Primate Research Center. Animals Naturally cycling female rhesus macaques (Macaca mulatta). Interventions Female macaques were mated on days 11–14 of the cycle. Females were then imaged by CEU and Doppler ultrasound (DUS) once every 3 days from day 21 through day 39 of the fertile cycle. Main Outcome Measures Visualization and quantification of uterine vascular perfusion. Results CEU identified the primary placental disc and underlying vessels ~2 days earlier than DUS was able to observe endometrial thickening. CEU revealed spatial differences in vascular perfusion between the endometrium, myometrium, and the endometrial-myometrial (junctional) zone. Myometrium displayed the highest rate of blood flow (>10 mL/min/g tissue). There was less blood flow in the endometrium and junctional zone (<3 mL/min/g). A brief fall in progesterone was observed during early implantation, which was correlated with reduced blood flow to all three uterine compartments, but did not reduce flow to the placenta. Conclusions CEU provides a sensitive, non-invasive method to assess vascular perfusion of the uterus during embryo implantation in macaques. We propose CEU as a new diagnostic tool to monitor vascular changes associated with early pregnancy in women.