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Study question How are the trophectoderm (TE) key regulators CDX2, TEAD4 and YAP expressed during early human embryogenesis? Summary answer In human embryos, the restriction of TE-specific components to the TE occurs after expansion of the embryo, when the two cell lineages (TE versus inner cells mass (ICM)) are morphologically distinguishable, suggesting a role for other molecules in the initial segregation between the ICM and TE lineages. What is known already In mice, the Hippo Signaling pathway has been described in the first lineage segregation (Nishioka et al. 2009). Differences in cell-cell contacts through differences in cell position lead to activation or inactivation of YAP, which subsequently can activate or inactivate TEAD4 in the nucleus and consequently regulate CDX2 expression in the inner versus outer cells. In the human it is not known how these Hippo signaling pathway components regulate TE specification. Study design, size, duration In this study, human preimplantation embryos wereanalysed for the expression of the transcription factors CDX2 and TEAD4, and the co-activator protein YAP. Both localization within the embryo and the time-specific expression were analysed. Participants/materials, setting, methods The study was approved by the Local and Federal Ethical Committees for research on human embryos. Good quality embryos were obtained at our IVF Laboratory after informed consent of the patients. They were fixed in 4% paraformaldehyde, permeabilized with 0,1% triton and stained for the respective proteins using immunocytochemistry. Main results and the role of chance In cleavage-stage embryos, compaction and early blastocyst stages CDX2 was found weakly cytoplasmic. From the full blastocyst stage onwards, some cells showed nuclear CDX2 expression. In expanded blastocysts CDX2 was found either in the TE nuclei or in the ICM and TE nuclei. In hatching/hatched blastocysts CDX2 was found in the ICM and TE nuclei except for one big hatched blastocyst that started to downregulate CDX2 expression in ICM nuclei. YAP and TEAD4 started to be expressed in the nuclei of some cells of late cleavage-stage and compacted embryos. In expanded, hatching and hatched blastocysts TEAD4 and YAP were expressed in ICM and TE nuclei, except for one big hatched blastocyst in which YAP started to become downregulated in the nuclei of ICM cells. Limitations, reason for caution Due to ethical concerns the amount of material is limited. This work is rather descriptive, but functional studies will be performed in the future. Wider implications of the findings Information about the key players leading to the first lineage segregation in the human embryo and the mechanisms underlying differentiation will contribute to our basic knowledge on human embryogenesis. This fundamental research is unique and crucial to the field of reproductive medicine. Even though a lot is known about this subject in mice, results obtained in the human are different. This study shows that data obtained in mice cannot always be extrapolated to humans. Study funding/competing interest(s) This research is supported by grants from the Scientific Research Foundation- Flanders (FWO-Vlaanderen) and the Research Council (OZR) of the VUB |
