Osteopontin expression in uterine stroma indicates a decidualization-like differentiation during ovine pregnancy

Osteopontin (OPN) is a component of the extracellular matrix that interacts with cell surface receptors, including integrins, to mediate cell adhesion, migration, differentiation, survival, and immune function. In pregnant mice and primates, OPN has been detected in decidualized stroma and is considered to be a gene marker for decidualization. Decidualization involves transformation of spindle-like fibroblasts into polygonal epithelial-like cells that are hypothesized to limit conceptus trophoblast invasion through the uterine wall during invasive implantation. Decidualization is not considered characteristic of species with noninvasive implantation, such as domestic animals. However, the extent of trophoblast invasion between sheep and pigs differs, with sheep exhibiting erosion of the uterine luminal epithelium (LE) and fusion of trophectoderm with LE to form syncytia, and pigs maintaining an intact LE throughout pregnancy. Therefore, the present study measured changes in the decidualization marker genes OPN, desmin, and alpha smooth muscle actin (alphaSMA) in ovine and porcine uterine stroma throughout pregnancy. The morphology of endometrial stromal cells in pregnant ewes changes following conceptus attachment, with cells increasing in size and becoming polyhedral in shape by Day 35 of pregnancy. Expression of OPN mRNA and protein, as well as desmin and alphaSMA proteins, was observed in this same uterine stromal compartment. In contrast, no morphological changes in uterine stroma nor induction of OPN mRNA and protein, or desmin protein, were detected during porcine pregnancy. Interestingly, alphaSMA protein was absent on Day 20, but prominent in uterine stroma of pregnant pigs on Day 45. Collectively, these results indicate that the uterine stroma of sheep undergoes a program of differentiation similar to decidualization in invasive implanting species, whereas porcine stroma exhibits differentiation that is more limited than that in sheep, rodents, or primates. Results suggest that uterine stromal decidualization is common to species with different types of placentation, but the extent is variable and correlates with the depth of trophoblast invasion during implantation.