Die Physiologie des Implantationsprozesses

The implantation of the blastocyst is a complex process for which the embryo and the endometrium must be prepared at the correct point in time. For ethical reasons it is nearly impossible to investigate the implantation of human embryos in vivo. Due to clear species-related differences the transfer of knowledge from animal models is very limited. For these reasons 3‑dimensional models are increasingly being developed in which spheroid trophoblast or embryoid structures are confronted with a decidua-like matrix. This article summarizes the currently known factors and interactions, which are important for implantation at both the cellular and also the molecular levels. At first the preparatory processes in the menstrual cycle are described. This is followed by a description of relevant components of the immune system, whereby the properties and functions of uterine natural killer cells are dealt with in detail. Furthermore, the role of various molecular factors in the implantation is discussed (cytokines, growth factors, cell adhesion molecules, hormones). In perspective, it is to be expected that despite all the limitations studies on the abovementioned 3‑dimensional models will provide new knowledge on human embryo implantation. This could ultimately have a positive effect on the practice of reproductive medicine.