Parturition is Not the Mirror Image of Implantation: The Role of Progesterone
There has been an abundance of evidence from clinical, animal, and in vitro studies that progesterone (P4) is important for establishing, maintaining, and terminating a pregnancy. P4 exerts its primary action via its two receptors: progesterone receptor A (PGR-A) and B (PGR-B). Analyses of transcriptome and cistrome genome have unearthed novel members and modifiers of the P4 signaling pathway. The increase in serum P4 levels and down-regulation of PGR-B are important in the development of pinopodes, thus marking implantation. Additionally, it promotes the quiescent myometrial cell phenotype, and the inhibition of its production in myometrial cells induces labor and is the key physiologic initiator of parturition. Through genomic and non-genomic intracellular mechanisms involving these PGR isoforms, various physiologic states can be determined such as the quiescent and contractile myometrium during pregnancy.
