nidation

Implantation, also known as nidation, is the stage in the mammalian embryonic development in which the blastocyst hatches, attaches, adheres, and invades into the endometrium of the female's uterus. Implantation is the first stage of gestation, and, when successful, the female is considered to be pregnant. An implanted embryo is detected by the presence of increased levels of human chorionic gonadotropin (hCG) in a pregnancy test. The implanted embryo will receive oxygen and nutrients in order to grow.

For implantation to take place the uterus must become receptive. Uterine receptivity involves much cross-talk between the embryo and the uterus, initiating changes to the endometrium. This stage gives a synchrony that opens a window of implantation that enables successful implantation of a viable embryo. The endocannabinoid system plays a vital role in this synchrony in the uterus, influencing uterine receptivity, and embryo implantation. The embryo expresses cannabinoid receptors early in its development that are responsive to anandamide (AEA) secreted in the uterus. AEA is produced at higher levels before implantation and is then down-regulated at the time of implantation. This signaling is of importance in the embryo-uterus crosstalk in regulating the timing of embryonic implantation and uterine receptivity. Adequate concentrations of AEA that are neither too high or too low, are needed for successful implantation.