Implantation (embryology)
Implantation, also known as nidation,[1] 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.[2] Implantation is the first stage of gestation, and, when successful, the female is considered to be pregnant.[3] An implanted embryo is detected by the presence of increased levels of human chorionic gonadotropin (hCG) in a pregnancy test.[3] 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.[4] The endocannabinoid system plays a vital role in this synchrony in the uterus, influencing uterine receptivity, and embryo implantation.[5] 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.[5][6][7]
There is an extensive variation in the type of trophoblast cells, and structures of the placenta across the different species of mammals.[8] Of the five recognised stages of implantation including two pre-implantation stages that precede placentation, the first four are similar across the species. The five stages are migration and hatching, pre-contact, attachment, adhesion, and invasion.[8] The two pre-implantation stages are associated with the pre-implantation embryo.[9][10]
In humans, following the stage of hatching that takes place around four to five days after fertilization, the process of implantation begins. By the end of the first week, the blastocyst is superficially attached to the uterine endometrium. By the end of the second week, implantation has completed.[11]