Expression pattern of different gap junction connexins is related to embryo implantation.

Successful implantation in mammals requires a close interaction between the embryo and the uterus. Direct cell-cell communication via gap junctions seems to play an important role in the preparation of the uterus for embryo implantation and in the regulation of trophoblast invasion. During preimplantation in the rat the gap junctional proteins connexin (cx) 26 and cx43 are suppressed. This loss of cell-cell communication seems to be important for transformation of the endometrium into the receptive phase. The suppressive effect is mediated by progesterone as demonstrated by the application of antigestagens. At implantation, however, a spatial and temporal pattern of connexin expression is induced in response to embryo recognition. cx26 is locally expressed in the uterine epithelium of the implantation chamber, cx43 in the surrounding decidua prior to invasion. With progressing invasion, the decidual cells surrounding the invading trophoblast in addition to cx43 reveal cx26. In this phase, the invasive partner, the blastocyst, is characterized by coexpression of cx43 and cx31. During trophoblast invasion however, cx31 becomes restricted to the cells of the invasive ectoplacental cone, cx43 to the embryo proper. It seems that compartmentalization of the trophoblast and the inner cell mass is established by two different connexins. During placental differentiation connexin expression switches from cx31 to cx26 and cx43, indicating the end of the invasive phase. The highly regulated pattern of connexin expression in the endometrium as well as in the trophoblast suggests a key role of this different intercellular pathways in regulating the invasion process of the trophoblast into its host tissue, the endometrium.