Controversy exists as to whether individual blastomeres from two-cell-stage mouse embryos have identical developmental properties and fate. We show that the transcription factor Cdx2 is expressed in the nuclei of cells derived from the late-dividing but not the first-dividing blastomere of two-cell embryos and, by lineage tracing and RNA interference knock-down experiments, that this lagging cell is the precursor of trophectoderm. Cdx2 mRNA is localized toward the vegetal pole of oocytes, reorients after fertilization, and becomes concentrated in the late-dividing, two-cell-stage blastomere. The asymmetrical distribution of Cdx2 gene products in the oocyte and embryo defines the lineage to trophectoderm.

Genital tract infections caused by gram-negative bacteria induce abortion and are one of the most common complications of human pregnancy. This study was carried out to decipher the mechanism of gram-negative bacterial lipopolysaccharide (LPS)-induced pregnancy loss, using a mouse (Park strain) model. Since many of the biological effects of LPS are mediated by interleukin (IL)-1a, the role of IL-1a in LPS-induced pregnancy loss was studied. Pregnant female animals were injected intraperitoneally (i.p.) with different doses (1 to 50 mg) of LPS from Salmonella minnesota Re-595, on day 0.5 of pregnancy. We found that 250 mg/kg body weight (i.e. 5 mg/female mouse) of LPS when given on day 0.5 of pregnancy was the 'minimum dose' (MD) required to completely inhibit the implantation of the blastocyst in the mouse. The effect of this dose on the pathophysiology of the various reproductive organs (i.e. uterus, ectoplacental cones, developing fetus, ovaries etc.) was assessed on day 14 of pregnancy. The effects of this dose on the level and pattern of expression of the proinflammatory cytokine IL-1a in the maternal uterine horns and preimplantation stage embryos were studied by RT-PCR. A single dose (100 ng/mouse) of recombinant mouse IL-1a was given i.p. to pregnant females on day 1 of pregnancy to study its effect on implantation. Our results show that treatment of the pregnant animals with LPS may alter cell proliferation and induce leukocyte infiltration, degeneration of luminal glandular epithelium, and hyperplasia in the various reproductive organs, and may also alter both embryonic and uterine IL-1a expression. IL-1a administration also caused implantation failure similar to that of LPS. The observations suggest that the determined MD of LPS may alter the expression of developmentally important proinflammatory cytokines such as IL-1a, which could, in turn, inhibit the normal processes of blastocyst implantation. Therefore, it is proposed that the LPS-induced histopathological alterations in the various reproductive organs of pregnant animals could be mediated by IL-1a and this may be one of the causes of failure of blastocyst implantation in the mouse.

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Certain uterine prostaglandins (PGs) are elevated at implantation sites and are needed to trigger the events of blastocyst implantation that include blastocyst-uterine attachment and stromal decidualization with vascular permeability changes. Several decades of investigations showed that treatment with PG synthesis inhibitors, prior to or during the time of implantation, resulted in either complete inhibition or a delay in implantation or reduction in the number of implantation sites with diminished decidual tissue. Consistent with these findings, we observed that whereas a selective PG endoperoxide synthase (Ptgs) 1 inhibitor SC-560 failed to inhibit implantation, a selective Ptgs2 inhibitor SC-236 showed significantly reduced number and size of implantation sites in progesterone-treated ovariectomized pregnant hamsters. It is known that Ptgs2 expression and Ptgs2-derived prostacyclin (PGI 2 ) synthesis at implantation sites are needed for implantation in the mouse (a rodent that needs ovarian estrogen for implantation). However, it is unknown which Ptgs and PG synthases produce which PGs at implantation sites of the hamster (a rodent that does not need ovarian estrogen for implantation). Here we demonstrate that as blastocyst implantation proceeds, a reduction in Ptgs1 expression from uterine luminal epithelial cells and a gradual induction in Ptgs2 expression exclusively in luminal epithelial and adjacent decidual cells occurred at implantation sites of hamsters. Results also reveal that PGE 2 , but not PGI 2 , is the major PG at implantation sites where Ptgs2 and microsomal type PGE synthases but not PGI synthases are co-expressed. This elevated uterine PGE 2 at implantation sites may serve to initiate or amplify physiological signals required for specific aspects of the implantation process in hamsters.

Once these inflammatory mediators have increased in the feto-maternal tissues, it may be too late or harmful to try and prevent the adverse outcomes of pregnancy.