Expression patterns of COX-2 and GM-CSF reported in the present study fit well with this hypothesis. COX-2 and GM-CSF are strongly expressed during early pregnancy in regions where fusion has not yet occurred, whereas the staining for the two proteins is reduced at sites of attachment (in the hybrid syncytium) as was reported for IFN-t. This supports the view that COX-2 and GM-CSF are important mediators during the elongation and apposition periods, preparing the materno-fetal interface for the adhesion stage. It is possible that COX-2 or GM-CSF levels would be reestablished after d30P when the LE is reconstituted, but this question could not be addressed in the current study.
Because the embryo attachment process and the embryonic signal (IFN-t) are very similar between the cow and the ewe, it has been hypothesized that molecular events occurring during pregnancy recognition should also be closely related. However, the expression of COXs in the endometrium differs significantly between the two ruminant species. COX-1 is expressed at steady-state levels in the sheep during the cycle and early pregnancy in both epithelial and stromal cells. In contrast, COX-1, known as a constitutive isoform in most tissues, is very weakly expressed in the bovine endometrium at all stages, confirming previous results obtained during the cycle and third-trimester pregnancy.