Embryo implantation is the process by which a blastocyst-stage embryo attaches to the endometrium, which is the inner wall of the uterus. It consists of 4 main stages:
- First, the embryo is detached from the zona pellucida, leaving only the trofoectoderm (purple cells in the image) and the internal cell mass (blue color in the image).
- It is then positioned in the endometrial epithelium, with the embryonic pole (where the internal cell mass is) towards the endometrium.
- Subsequently, the trofoectoderm and endometrial epithelium are joined by adhesion molecules and pinopods, which are extensions of the endometrium that attract the blastocyst.
- Finally, invasion occurs, after which the blastocyst is embedded in the endometrial stroma.