Embryonic signals and the initiation of blastocyst implantation.

The earliest sign of blastocyst implantation is an increase in endometrial vascular permeability which is localized in areas adjacent to blastocysts. The localized nature of this response suggests that it occurs in response to a signal from the blastocyst. It has been suggested that this signal may be physical in nature, or may be due to blastocyst-produced histamine, oestrogen or prostaglandins. The evidence for each of these is reviewed. At present, it is not possible to exclude any of these signals, which are not mutually exclusive, with certainty. The bulk of the evidence suggests that prostaglandins have an obligatory role in the initiation of implantation, but they may not be necessarily be of blastocyst origin.