On the Requirement for Estrogen for Establishing Pregnancy in the Non-Human Primate
Pregnancy establishment and progression in primates is considered solely dependent on progesterone (P) and the need for estrogen (E), if any, in this process is debated. Estrogen participates in the initial preparation of the endometrium and is required in particular for the induction of progesterone receptors1. It is, however, not clear if continual maintenance/turnover of progesterone receptors is influenced by estrogen levels. Progesterone is known to suppress its own as well as estrogen receptor concentrations2. During the critical period of luteal rescue, signalling the start of the preparedness of the uterus for implantation or pregnancy, the luteal tissue of most primates produces both estrogen and progesterone in increasing amounts and in the bonnet monkey at least3 there exists a distinct difference in the serum E:P ratio of the cycling vs pregnant4 monkey (Fig. 1). The endometrial nuclear receptor ratio for progesterone:estradiol of rhesus monkeys slated to undergo pregnancy has also been shown to register an upward swing during this critical phase. Besides induction of progesterone receptors it appears estrogen, along with progesterone, could be having a regulatory role in the synthesis of a few key proteins presumed to be involved in pregnancy maintenance (Table. 1). More recently, estradiol has been observed to stimulate in vitro protein synthesis by first trimester human placental minces10 and one of the induced glycoproteins, a 45 kd protein, has also been identified11.
KeywordsLate Luteal Phase Female Monkey Bonnet Monkey Luteal Tissue Blastocyst Implantation
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