The cytogenetic constitution of human blastocysts: insights from comprehensive chromosome screening strategies

Embryos that are able to form blastocysts have succeeded in activating their genome and differentiating into two cell types-an external layer of trophectoderm cells, which will go on to form extra-embryonic tissues such as the placenta, and the inner cell mass, which will give rise to the embryo proper. Culturing embryos to the blastocyst stage has become an increasingly popular IVF practice over the past decade. Additionally, it has been proposed that the identification and transfer of euploid blastocysts could significantly improve IVF outcomes. Toward this end, comprehensive molecular cytogenetic methods have been developed. The application of such methods in both clinical and research contexts has yielded cytogenetic data from large numbers of blastocysts. Questions have been raised concerning the implantation potential of blastocysts diagnosed 'euploid' or 'aneuploid', and there has been particular debate over the detection and viability of embryos categorized as 'mosaic'-composed of a mixture of normal and aneuploid cells.

This review aims to summarize data from studies using comprehensive molecular cytogenetic methods to examine blastocyst-stage embryos, describing current knowledge related to rates of euploidy, uniform aneuploidy and mosaicism. Issues associated with the developmental capacity of blastocysts of different cytogenetic constitutions will also be addressed. Guidelines on the clinical management of blastocysts with varying chromosome complements will be considered.

Rates of euploidy, uniform aneuploidy (in which all cells have the same abnormal karyotype) and mosaicism were determined via a thorough literature search (PubMed). The keywords used in the search were as follows: preimplantation embryo development, blastocyst stage, embryonic aneuploidy, meiotic chromosome malsegregation, post-zygotic chromosome malsegregation, comprehensive chromosome screening, array comparative genomic hybridization, single-nucleotide polymorphism array, next-generation sequencing, embryo mosaicism and implantation of mosaic embryos. Relevant articles written in English and published up to March 2018 were reviewed.

Different types of aneuploidy, including some complex forms, are capable of persisting to the blastocyst stage. As expected, euploidy rates decreased with advancing female age, whereas uniform aneuploidy increased. Analysis of multiple cells from individual blastocysts revealed that most of those classified 'abnormal' contained no euploid cells (due to meiotic errors arising in the gametes and therefore present in every cell), some having additional mosaic (post-fertilization, mitotic) errors. Blastocysts with a mix of normal and aneuploid cells were observed less frequently than other classes of embryo. The transfer of embryos with diploid-aneuploid mosaic biopsy specimens is reportedly associated with higher miscarriage and lower implantation rates, compared to embryos in which only euploid cells are detected.

Detailed investigations into the chromosome constitution of human blastocysts suggest that a significant proportion is euploid in every cell, although the exact fraction is strongly influenced by female age. These findings do not support the notion that mosaic chromosome abnormalities are a natural part of embryo development. Mosaic aneuploidies, arising post-zygotically, were detected by various different comprehensive molecular cytogenetic methods, suggesting that the majority of these represent genuine findings. However, it remains possible that certain comprehensive molecular cytogenetic methods may carry a risk of mosaicism being incorrectly assigned, in a minority of samples, as a result of technical artifact. This may be a consequence of degraded DNA in the trophectoderm biopsy or other technical issues. According to published studies, blastocysts considered to have uniform aneuploidy and, to a lesser extent, those with mosaic abnormalities were associated with poorer clinical outcomes in comparison with euploid embryos.