Balancing between totipotency and differentiation in human embryos.

Today, it has become clear that the information on critical factors of human embryo development should result in the optimization of IVF techniques. The unique properties of human embryonic blastomeres, responsible for the ability of human embryos to recover after cell loss, became a target of our investigation. In this context, we analyzed two new markers - CAR and CCNE1 - and the WNT/ -catenin signaling in human preimplantation embryos in vitro. CAR is a gene, encoding the coxsackie and adenoviral receptor, which is induced in regenerating and developing tissues. We described a soluble CAR marking undifferentiated blastomeres and trophectoderm (TE) cells of hatching blastocysts and a transmembrane CAR relating to epithelial-like cell types via its association with tight junctions. Our further investigation was focused on CCNE1 as link between the developmental potential and the unique cell cycle characteristics. We reported ubiquitous presence of CCNE1 protein from the cleavage stages until blastocyst expansion. CCNE1 also labeled the third lineage within blastocysts’ inner cell mass (ICM), which has never been described before. We associated CCNE1 with a visceral endoderm-like phenotype and proved its critical role in human embryonic cells during normal embryo development and hESC derivation. We also performed WNT/ -catenin pathway loss- and gain-of-function studies and showed relevance of the nuclear -catenin exclusively to cleavage stages. The balanced -catenin activity was critical for the TE fate. Similar to stabilized -catenin, WNT3 protein, showing TE-specific expression in blastocysts, promoted trophoblast specification. The data from this work have led to a better understanding of early human embryogenesis, which is important for reproductive medicine.