IntroductionThe success of normal pregnancy depends on the protection and growth of the semi-allogenic embryo within the maternal uterine microenvironment (Kirby et al., 1964; MuggletonHarris and Johnson, 1976). However, the mechanisms by which the genetically incompatible embryo escapes maternal immunological responses during early pregnancy remain unknown. Furthermore, the loss of the zona pellucidum from the blastocyst prior to implantation, and the loss of the uterine luminal epithelium at the site of the implanting blastocyst make the embryo more vulnerable to maternal insults. Thus, it is speculated that a special barrier mechanism is operative at the maternal-conceptus interface to prevent the passage of harmful stimuli to the embryo. However, the nature and regulation of this barrier system remain unknown.Pregnancy consists of several phases. After mating, the fertilized one-cell embryo grows mitotically and reaches the blastocyst stage as it enters the uterus to initiate the process of implantation and placentation. The embryo remains free and un-implanted during this preimplantation phase of pregnancy. This is followed by the attachment event of implantation phase that involves the initiation of first contact of the blastocyst trophectoderm with the luminal epithelium. This early event of implantation occurs on the midnight of day 4 or early morning of day 5 of pregnancy in laboratory mice (Das et al., 1994).