ABSTRACT
The human preimplantation embryo is exposed to specific nutrients within the reproductive tract.1 Analysis of the fluids within the fallopian tube and the uterus have revealed that glucose levels are at their lowest (0.5 mM) and pyruvate (0.32 mM) and lactate (10.5 mM) levels at their highest in the fallopian tube at the time when the fertilized oocyte and early-cleavage-stage embryo is present, while in the uterus, glucose is present at a much higher level (3.15 mM), whereas pyruvate (0.1 mM) and lactate (5.87 mM) are significantly lower than in the fallopian tube.1 This nutrient availability reflects the energy requirements of the developing preimplantation embryo and is mirrored in the culture media used for in vitro fertilization (IVF).2–5 The early-cleavage-stage embryo is characterized by pyruvate uptake, low levels of oxidative metabolism and low oxygen consumption, whereas at the blastocyst stage, it exhibits both high levels of glycolysis and high oxygen consumption.6–8 Sequential media (cleavage and blastocyst stage media) were therefore developed to maintain the embryo's metabolic needs in culture up to day three and from day three through day five postfertilization, respectively. However, single-step continuous media have lately been introduced, aiming to provide an uninterrupted culture from day one through day five and enabling the embryo to choose and consume the ingredients it requires at each specific stage of preimplantation development.9–14 To date, no statistically significant differences have been observed between sequential and one-step media with regard to implantation and pregnancy rates as well as among different media that belong to the one-step or the sequential category.9–14
