Two concepts on the immunological aspect of blastocyst implantation.
ABSTRACT The process of blastocyst implantation is a series of interactions between the blastocyst and maternal tissues. The purpose of this process is (1) to provide nourishment to the embryo for developmental growth in appropriate physiological and endocrinological environment until a placenta is established, and (2) to protect the (semi-)allogeneic embryo from any attacks from the maternal immune system. To facilitate successful implantation, therefore, these two aspects of the embryonic demand must be satisfied in the embryo-maternal interface throughout the entire process of implantation. The first concept I present in this paper is that blastocyst implantation essential factors (BIEFs) have dual functions: one, for structural and functional modification of the endometrium to accommodate the developing embryo and provide nourishment and suitable environment for its development, and the other, for modulation, directly or indirectly, of the maternal immune system to prevent attacks by the maternal immune system. The second concept is that BIEFs convert the endometrium (or uterus) from an immunologically non-privileged site to a privileged site. This endometrial (uterine) conversion is the immunological aspect of the blastocyst implantation process. When the endometrium has become receptive for blastocyst implantation, it signifies that the immunological conversion of the endometrium by BIEFs has been sufficiently attained to let the embryo start contacting maternal tissues. During the early stages of placentation, as the trophoblast cells differentiate and make their way to the maternal blood vessels to establish the placenta, BIEFs continuously provide nourishment and immunological protection to the developing embryo. The immunological protection of the embryo/fetus from potential attacks by the maternal immune system appears to reach a peak at the time of establishment of the placenta. Thus, clarification of the roles of BIEFs in both the physiological/endocrinological aspect as well as the immunological aspect is essential for understanding the biological process of implantation.