Comprehensive Chromosomal Screening from Polar Body Biopsy to Blastocyst Trophectoderm Sampling: Evidences and Considerations

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Preimplantation Genetic Screening (PGS) is a diagnostic technique increasingly used in assisted reproductive technologies (ART) to select chromosomally normal embryos for transfer. For several years, the gold-standard approach for PGS entailed blastomere biopsy at the cleavage stage analyzed by 9-chromosome fluorescent in situ hybridization (FISH). Unfortunately, this first version of PGS failed to show evidence of clinical effectiveness during randomized controlled trials (RCTs). This was mainly due to the limited coverage of FISH-based aneuploidy screening, the high technical error rate when used on singe cells, and the stage at which the biopsy was performed. In fact, blastomere biopsy has a significant impact on embryo development and its screening potential is affected from the phenomenon of chromosomal mosaicism, which reaches its highest incidence at this stage of embryo development, but especially by the technical issues associated with single-cell analysis. Experts started then a pursuit toward new approaches. Moving backwards along preimplantation development, polar bodies (PBs) approach also showed important limitations. In particular, a high false-positive and false-negative diagnosis rates were reported in predicting the chromosomal complement of resulting embryos. This is mainly due to the inability to assess meiosis I (MI) errors balancing at meiosis II (MII), to the relevant proportion of false-positive and false-negative predictions made on single polar bodies, and to the influence of male and especially mitotic-derived aneuploidies undetectable by PBs screening. Moreover, a detrimental impact of the biopsy on preimplantation embryo development was also reported occasionally for the PBs approach. Thus, this time-consuming and poorly cost-effective biopsy strategy is going to be abandoned in favor of blastocyst stage trophectoderm (TE) biopsy. This stage, besides ensuring a more robust genetic analysis and representing the most cost-effective approach, showed also no major impact of chromosomal mosaicism on diagnosis and no impact of the biopsy on embryo implantation potential. Once this new gold-standard approach is set, by exploiting the new technical advances in molecular analysis tools, comprehensive chromosomal screening (CCS) platforms replaced FISH, ensuring a more detailed and reliable diagnosis. All the RCTs published up to date, in different patient populations and through all the CCS platforms currently available, reported an average 30 % increase in sustained implantation rate per embryo transfer (ET) when PGS was performed, and a significant decrease in abortion rate and multiple pregnancy rate. These outcomes testify that CCS-based PGS on TE biopsy is reasonably the only promising approach. The future retains an increase in analysis throughput and a parallel costs reduction by next generation sequencing (NGS) implementation in PGS. A further increase in embryo developmental competence assessment will possibly derive from the novel -omics approaches and the new evidences they will bring about. These new cutting-edge technologies will hopefully help us increasing our predictive power on embryo implantation potential in in vitro fertilization (IVF).