ABSTRACT
After normal fertilization the human ovum has developed to the blastocyst stage during its way through the fallopian tube and implantation in the uterine cavity begins on day five to six to be complete on day 10 after fertilization (Harper, 1992). The implantation
and hemochorial placentation process is preceded by a series of maternal-embryonic interactions that are described in detail elsewhere in this book. Most of our knowledge of human implantation has been based on morphological findings, and still little is known about the regulation and molecular-cellular basis of this fundamental process. It is likely that in addition to direct cell-cell contacts between the embryo and the uterine cells, several extracellular signals pass from the embryo to the endometrium and vice versa to certify normal placentation and fetal development. Although the uterus is the most hostile place for the embryo, it is receptive for implantation only during a certain period of the menstrual cycle (implantation window), and paradoxically, implantation in the human can take place outside the uterine cavity as well. Implantation outside the uterine cavity results in ectopic pregnancy.
