Progesterone concentration is highly correlated with that of PRL and increases at the time of renewed blastocyst development. Treatment of ovariectomized skunks with progesterone maintains embryo viability but fails to induce implantation. In the present study, uterine LIF mRNA expression increased at early embryo activation, during which progesterone is the predominant ovarian steroid, suggesting that it might be responsible for the up-regulation of LIF expression. Constant administration of progesterone to ovariectomized skunks significantly increased uterine weight but failed to alter uterine LIF expression. LIF mRNA or protein is most abundant in the human, mink, and rabbit endometrium when progesterone concentration is elevated. Administration of progesterone results in increased staining for LIF protein in the uterus of unmated rabbits, whereas in the ewe, progesterone results in decreased staining for LIF in the luminal epithelium and increased staining in the glandular epithelium and stroma of the uterus. ampicillin antibiotic
During delayed implantation in the mouse, an injection of estrogen induces implantation and is associated with a significant rise in LIF mRNA. LIF protein increases in response to an injection of estrogen, whereas injection of progesterone does not stimulate LIF protein expression in ovariectomized mice. Treatment of ovariectomized sheep with estrogen maintains or reduces immunoreactive LIF in the endometrium. In our study, constant administration of physiologic concentrations of estradiol increased uterine weight in ovariectomized skunks to levels similar to those observed during blastocyst activation, but failed to alter uterine LIF mRNA concentrations compared to those in ovariectomized controls.