What does an embryo need?

This review argues that the question "What does an embryo need?" cannot be adequately answered in quantitative terms to allow the formulation of media for culturing early mammalian embryos. It can be shown experimentally that "needs" in terms of the nutrients an embryo chooses to consume, and their rates of consumption, vary widely, as they are determined by the concentration of the nutrients under consideration and other constituents in the culture medium. Similarly, it is impossible to define "needs" from knowledge of the kinetic properties of nutrient transport systems. Measurements of nutrient consumption, are, however, valuable in determining overall metabolic activity and the balance between oxidative and glycolytic metabolism, in demonstrating qualitative requirements for specific nutrients and in providing markers of normality or abnormality against which to devise methods for diagnosing embryo health. On the basis of these and other considerations, a strategy is proposed for the formulation of embryo culture media that promotes metabolism that is "quiet" rather than "active", reduces the concentrations of nutrients to match those in the Fallopian tube, selects the "quietest" embryos for transfer, and trusts the autonomy of the embryo.