Culture of preimplantation embryos: facts and artifacts

Mammalian preimplantation embryos normally develop within the protected environment of the female reproductive tract, which virtually precludes studies on embryogenesis in situ. Information must therefore be derived from experiments on cultured embryos. Consequently, studies on the epigenetic regulation of embryogenesis have long been interwoven with efforts to formulate culture media capable of sustaining normal development. In this review, comparative information on epigenetic regulation of embryo development is discussed, including information on energy substrate and amino acid preferences of embryos. Advantages of simple versus complex culture media, and of substituting serum albumin or synthetic macromolecules for serum, are discussed. Some potential pitfalls of co-culture are described. Culture appears to induce anomalies in embryo metabolism, which may derive from disturbed intracellular pH. Rationales for selecting endpoints to evaluate the outcome of experiments are considered, including incorporation of timing of embryo development into the analysis. Poor experimental design and/or data analysis can detract from or even negate the value of data obtained from embryo culture; examples are examined to help correct this problem. All of these points are discussed with a view to using data on the needs of embryos for making improvements in the design of culture media, so that higher yields and increased viability of embryos are achieved.