Preimplantation embryonic mosaicism: origin, consequences and the reliability of comprehensive chromosome screening

Purpose of review: Embryonic mosaicism represents an ongoing challenge for contemporary comprehensive chromosome screening platforms due to the unknown reproductive potential of mosaic embryos and technical difficulties of its detection from a single embryo biopsy. Recent findings: Mosaicism in preimplantation embryos is a product of mitotic errors arising primarily from anaphase lag and chromosome nondisjunction. To date, there is high variability among estimations of prevalence of mosaicism in blastocysts, the most recent ranging from 3.3 to 83%. It has been reported that alleged mosaic embryos can develop into healthy babies, although the proper study evaluating this question remains to be completed. Technical artefacts from comprehensive chromosome screening platforms may also hinder correct classification of embryos as genuine mosaics. Summary: Although complex, embryonic mosaicism is a phenomenon that deserves further investigation. Many embryos classified as mosaic may have actual reproductive potential. The predictive value of intermediate chromosome copy number assignments for the remaining embryo and for ongoing reproductive potential needs more careful consideration. In addition, recent advancements in extended embryo culture raise the possibility of investigating whether preferential segregation, selective advantage of normal cells or surveillance of abnormal chromosome numbers occur at postimplantation stages.

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