Of mice and (wo)men: factors influencing successful implantation including endocannabinoids.

BACKGROUND One in six couples suffer from infertility problems requiring in vitro fertilization therapy; however, the average birth rates in the past decade per IVF-embryo transfer cycle have remained static at around 25%. Although implantation failure is considered a major cause of infertility in otherwise healthy women, inadequate uterine receptivity is considered to be responsible for only two-thirds of implantation failures and problems with the embryo itself are responsible for the other third, such that only 30% of oocytes that are contacted by sperm result in successful human term pregnancies. Due to technical and ethical considerations, most research into the factors affecting the success of embryo implantation has been performed in mice, but this may be less than ideal. METHODS Selected relevant literature detailing the similarities and differences between rodent and human reproductive physiology surrounding implantation were nominated for inclusion. Primary papers and review articles (and primary sources within these), published between 1975 and 2012, with a clear indication for a particular ligand or cell being involved in the implantation process or placentation in the mouse or woman, were thoroughly examined and used to construct the review. RESULTS Mice have been identified as suitable models for investigating the processes of early pregnancy in women, for many reasons including their predictable, relatively short gestation and the ability to deliberately breed mice with the absence of a desired gene. There is, however, increasing evidence to suggest that the reproductive systems of humans and mice differ considerably when considering early pregnancy events. CONCLUSIONS In this review, we examine what is already known about the normal implantation process and the intrinsic factors that affect implantation, and then compare the differences between mice models and women in the context of early pregnancy. We highlight numerous differences between the mice and women and conclude it is becoming clear that all of the data from mouse studies cannot be confidently extrapolated to human reproduction.