One reason for this high failure rate is the inability of an embryo to implant. Only certain cells within the embryo can implant and form a placenta. Wiley's research laboratory uses the mouse as a model to study developmental changes that occur during preimplantation and to investigate environmental factors that cause embryo failure before implantation. Some likely factors are dioxins, which are by-products of combustion processes used in manufacturing. Mouse embryos exposed to dioxins suffer around a 70 percent implantation loss.
Dioxins also appear to be more toxic to male embryos than to female embryos.
Reproductive biologists like Wiley have found the mouse to be a useful model for the study of purported reproductive toxicants. However, there are some differences, notes Wiley. Human embryos are also much less picky about the timing of their arrival in the uterus and their stage of embryonic development. Mouse embryos follow much stricter rules in this regard. Yet, despite his work and that of others in the field, these events are still not well-understood, particularly in humans.