Implantation (embryology)
Implantation, also known as nidation is the stage in the embryonic development of mammals in which the blastocyst hatches, attaches, adheres, and invades into the wall 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. The endocannabinoid system is important in successful uterine receptivity and implantation, as high level of cannabinoid receptor CB1 placed on the blastocyst, are bound for attachment with the endocannabinoid anandamide, an N-Acylethanolamine, produced at higher levels by the uterus before implantation and then down-regulated at the time of implantation by fatty acid amide hydrolase (FAAH), at the uterine lining (endometrium) to accomplish implantation. There is an extensive variation in the type of trophoblast cells, and structures of the placenta across the different species of mammals. 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. The two pre-implantation stages are associated with the pre-implantation embryo. 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. There are five recognized stages of implantation in mammals, including two pre-implantation stages that precede the formation of the placenta. They are: migration and hatching, pre-contact, attachment, adhesion, and invasion. The first four stages are similar across the species with the process of invasion being variable.
