A consideration of the factors which influence and control the viability and developmental potential of the preimplantation embryo.

The advent of the new reproductive technologies (including in vitro fertilization) has led to a revolution in the treatment of infertility. It has not yet led to a marked improvement in our understanding of the control of development and of viability of the early embryo. It is the poor viability of embryos, with consequent implantation failure, which is the major limiting factor to successful outcomes. While much of the research in this area has concentrated on strategies of ovulation induction, experimental models have shown that the major cause of reduced embryo viability is due to the actual process of fertilization in vitro and subsequent culture of the preembryo in synthetic culture medium. It is likely that this is due to the absence of critical nutrients or trophic factors of maternal origin and work with co-culture of embryos with somatic cells suggests improved viability can be achieved. Such co-culture is not an option for routine clinical use, however. It is essential therefore to understand by detailed study of the physiology of embryonic development their requirements for optimal development. The empirical approach of comparing different formulations of culture media is unlikely to be successful because of the vast range of parameters to be tested and the large number of pregnancies required to demonstrate a significant improvement in outcome. The strategy that is most likely to be successful in the future, therefore, is the use of appropriate experimental models, such as the developing rodent embryo, to understand the essential physiological changes in the embryo during its development, the control processes in place, and the effect of the embryo's environment on the processes. This will allow the rational design of culture media which can then be rigorously tested for improved outcome. An example of successful application of this approach is the discovery of embryo-derived platelet activating factor (PAF). The production of embryo-derived PAF was first described and validated in the rodent. In the same species it was shown to have an essential role in pregnancy and to act as an autocrine mediator of embryo viability. This fundamental observation in rodents was then confirmed in humans, and recent work has shown that supplementation of culture human embryo media with PAF results in a dramatic increase in their developmental and pregnancy potential. This example should be the first of many such improvements based on a more fundamental understanding of the embryo's developmental requirements.