Implantation

Implantation is the moment when the embryo stops floating and settles into the wall of the uterus. By then it is no longer a single cell: it is a blastocyst, a hollow ball of cells, and what it does is embed itself in the endometrium, the inner lining of the uterus. It usually happens during the second week of development.

First it has to shed its covering. The blastocyst comes wrapped in the zona pellucida, a coat that protected it up to that point, and it has to shed that coat completely once it enters the uterine cavity. Then it works its way toward the wall with adhesion molecules, which are like cellular Velcro, and with polarity signals.

Nothing is improvised on the other side either. The uterus is in its secretory phase, with high estrogen and progesterone, and those hormones make the glands and arteries of the endometrium coil up and the tissue grow lush. It is a seed that takes root only if the soil was watered and loosened before it arrived.

The site matters. Normally it implants high up, near the fundus of the uterus. If it implants too low, the placenta can end up covering the exit, which is called placenta previa.