Expression of LIF,VEGF, CD57 and CD68 after the transfer of rat embryos to mouse uteri

The high failure rate of interspecific pregnancy is a major obstacle to the successful interspecific cloning of mammals. To investigate the reasons for the failure of inter-specfic pregnancy between rats and mice, we transferred rat blastocysts into mouse uteri on the third day of pseudopreg-nancy (D3). Our previous study showed that intact rat em-bryos could still be observed in mouse uteri on D9. In the present study, we found that expression of CD57 and CD68 increased significantly at the maternal-fetal interface fol-lowing the transfer of rat embryos. Similarly, Leukaemia inhibitory factor (LIF) expression increased, but vascular endothelial growth factor (VEGF) expession decreased. In a co-culture system, the percentage of rat ectoplacental cones (EPCs) with adhesion and outgrowth and outgrowth area on mouse uterine decidual cells were less than that of mouse EPCs. These results indicate that an increase in the immu-nological rejection response and a decrease in the invasive-ness of rat embryos may be important reasons for the failure of interspecific pregnancy between rat and mouse.