ABSTRACT: Mouse zygotes are suspected to mount adaptive responses (e.g. physiology, metabolism, gene expression) to the environment, which can be the natural environment of the female genital tract or the artificial environment of a culture medium. The jury is still out when it comes to the possible long-term effects of culture media on embryo development. In 2016 the ESHRE took a stance about this subject, saying that the environment the early embryo is exposed to can cause reprogramming of embryonic, fetal and postnatal development, with negative consequences for the conceptus (Sunde et al., Hum Reprod. 31(10):2174-82, 2016). Of course, in order to possibly measure these long-term effects, the embryo has to develop to term. On the one hand, the impact of prior adaptive responses on postimplantation development is contingent on the presence of sufficient cells in the epiblast compartment of the blastocyst-stage embryo. In mice the viable epiblast number is considered to have a minimum at 4 (Morris et al., Cell Rep. 2(4):756-65, 2012) and a maximum at 8 (Soriano and Jaenisch, Cell 46(1):19-29, 1986). On the other hand, regular embryos have reserve capacities and lie well above the minimum level, whereby detrimental effects of culture conditions are probably underestimated. In plain words, there are embryos that might have suffered from in vitro culture, but they make it because they already secured the minimum epiblast number. We reasoned that these effects may be exposed if the embryos had half the number of total cells, thereby bringing them closer to the critical minimum epiblast threshold. These half embryos live on the knife’s edge: if they make it above the threshold, then they can survive, otherwise they fail. Therefore, if any adaptation during embryo culture were to occur, it could either raise the half embryos above the critical threshold or deliver the final blow. This would give us the opportunity to work with a clear dichotomy and to measure clear responses in terms of development vs. no development. We used the model system of mouse half-embryo system to assess the adaptive responses to culture media in terms of gene expression. After in vivo zygote production and short culture in KSOM(aa) medium to complete the first cell cycle, 2-cell embryos were split, and the individual blastomeres were cultured in KSOM(aa), GM501 or SAGE 1step medium. Resultant blastocysts were compared and contrasted with each other. Transcriptome analysis revealed differences of gene expression due to culture media.
