Published online by Cambridge University Press: 15 April 2021
The embryo is a dynamic structure that can be affected by the interaction with the surrounding environment. During its journey through the female reproductive tract from fertilization to implantation, the embryo undergoes numerous biochemical and physiological changes which are essential for a successful reproductive outcome. During successive cleavage rounds, the embryo increases in cell number, switches from maternal to embryonic genome control (embryonic genome activation; EGA) and forms cell–cell junctions. This coincides with the cells flattening and compacting at the morula stage (Coticchio et al., 2019). At the final stage of the preimplantation period, the blastomeres differentiate to form the trophectoderm and the inner cell mass cell lineages. The blastocyst undergoes remarkable events in preparation for implantation and establishment of pregnancy, including initiation of overall growth, significant rise in transcriptional activity, increased protein synthesis, and active Na+/K+ ATPase activity in the trophectoderm leading to the formation of the blastocoel cavity (reviewed by Smith & Sturmey, 2013). The blastocyst also improves homeostatic regulatory mechanisms, including defense against oxidative damage (Lane & Gardner, 2000). These changes are energy dependent, and therefore underpinned by specific metabolic pathways. Disruptions in energy production during the preimplantation period are related to embryonic developmental impairment and reduced fetal viability post-transfer (Gardner, ; Lane & Gardner, 2005b). For these reasons, metabolism is considered a key determinant of embryo competence and viability.