Physiological and molecular aspects of human implantation.

A greatly increased amount of knowledge on the preimplantation embryo and the molecular biology of the uterus have led to the development of new concepts on the adhesion of the embryo to the uterus and on the implantation process. This brief review covers cleavage and blastocyst formation, and the preparations of the embryo for implantation. The molecular factors involved in human implantation are then described, including the role of the steroid hormones, cytokines, adhesion molecules and substrates. The changes occurring in the embryo and uterus during implantation are described. Brief descriptions of extravillous trophoblast and the role of immune-type responses in the uterus are given. Finally, the molecular aspects of human implantation are considered.