FIGURE 2.

Diagrammatic representation of inter-individual “Goldilocks zone.” (A) shows a hypothetical metabolic range for a theoretical embryo. This embryo has a green “optimal” range of metabolism where energetic needs for development are sufficiently high to satisfy demand, but sufficiently low to prevent unintended damage, for example from reactive oxygen species, that are a by-product of elevated metabolism. Within the “green” zone, there is space to increase or decrease metabolism as required (sometimes referred to as metabolic scope), without falling into the damaging ‘red’ zones. (B,C) show 5 different theoretical embryos, with inherent metabolic variation. In scenario (B), a minimal exposure to a stressor, such as a low stress environment (for example, culture in optimal conditions with low oxygen tension, optimal nutrient supply and minimal disturbance), keeps more of the embryos within their individual “goldilocks zone” such that they are able to develop. From a selection perspective, embryo 5 is the least viable. In scenario (C), the range of stressors is wide (for example, culture in high oxygen conditions, in poor quality medium and frequent disturbance) meaning that embryos need to respond. Those with the widest inherent “goldilocks zone” (illustrated by Embryo 1) are best able to tolerate the stress and from a selection perspective, Embryo 1 is therefore the most viable embryo.