Intrauterine signaling and embryonic implantation

In mammals, the uterus is modified to be able to contain a pregnancy and nurture the developing embryo. In deciduate mammals, this is apparently due to formation of a special compartment, lined with decidual tissue, in which a semi-allogeneic (or even allogeneic, after embryo transplantation) pregnancy is accommodated. This review treats the mechanisms which have been evoked to explain the implantation of the egg and the decidualization of the implanting endometrium. At least three different neurochemicals have been considered to mediate induction of this response: histamine, prostaglandins and platelet-aggregating factor. Their importance is reviewed. The ability of the endometrium to transform into decidual tissue is contingent on the presence of the epithelium. The role of the epithelium is temporary, however, since it dies and is sloughed within a day of the induction. Studies of progesterone-dependent changes in the epithelial reaction to preimplantation pregnancy are considered.