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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.
Before considering the basis of embryo culture medium, it is worthwhile reflecting on its function. A detailed overview of embryo metabolism is given in Chapter 4; however, in brief, the embryo must satisfy changing demands for energy by consumption of nutrients from the external milieu (Lewis & Sturmey, 2015). In an in vivo setting, these needs are catered for in a dynamic manner by the secretions of the oviduct; in an in vitro situation, these requirements must be satisfied by the embryo culture medium. In addition to the provision of energy substrates, the medium must also satisfy basic physicochemical requirements. Primarily, the medium must facilitate buffering of pH in response both to changing environments to which the embryo is exposed and to excretion of metabolic waste products, notably lactic acid, which is released by cells with accompanying protons, causing pH to fall. Moreover, the culture medium must avoid inducing osmotic stress. One of the major consumers of cellular energy is the maintenance of intracellular ion composition, maintained through the action of ion pumps. Providing suitable osmolarity and pH are among the most basic requirements of any embryo culture medium.