ABSTRACT

Implantation is a process which requires interaction of the blastocyst and subsequently the developing embryo and placenta with the endometrium. Initially during this process, the blastocyst establishes contact with the surface epithelium of endometrium. Subsequently, during a series of exquisitely controlled steps, the blastocyst is gradually implanted in the underlying stroma. Formation of the placenta, the so-called “placentation”, completes the implantation process and establishes a means of supporting the embryo to the end of the pregnancy period. Most of the information regarding the phases of human implantation is derived from the specimens available in the Carnegie collections. Based on this material, implantation has been divided into various stages (Table 16.1). At stage 4a, trophoblasts in different species use one of the following modes of invasion of the endometrium (Schlafke and Enders, 1975):

1. Displacement penetration (mouse, rat): In this mode of endometrial invasion, surface epithelial cells detach from their basement membrane and from each other, they degenerate and then are phagocytozed by trophoblasts. As a consequence, the process of implantation is initiated by the exposure of the trophoblasts to the bare underlying basement membrane.