Temporal and developmental-stage variation in the occurrence of mitotic errors in tripronuclear human preimplantation embryos

Mitotic errors during early development of human preimplantation embryos are common, rendering a large proportion of embryos chromosomally mosaic. It is also known that the percentage of diploid cells in human diploid-aneuploid mosaic embryos is higher at the blastocyst than at the cleavage stage. In this study, we examined whether there is temporal and/or developmental-stage variation in the occurrence of mitotic errors in human preimplantation embryos from the first day of development onward using mitotically stable digynic tripronuclear human embryos as a model system. All the cells of the 114 digynic tripronuclear human preimplantation embryos included were analyzed by fluorescence in situ hybridization for chromosomes 1, 13, 16, 17, 18, 21, X, and Y. Embryos were grouped according to day of development (1-6) and developmental stage (2-cell to blastocyst stage). The possibility of a mitotic error was highest in the first and second mitotic divisions. The percentage of cells with mitotic errors increased during preimplantation development and was highest at the 9-16 cell stage (76%, P = 0.027). Thereafter, the percentage of cells with mitotic errors decreased to 64% at the morula and 56% at the blastocyst stage. The pattern found correlates with the activation of the embryonic genome at the 8-16 cell stage. A better insight in the timing of occurrence of mitotic errors in human preimplantation embryos could help in understanding and prevention of these errors and is relevant in the context of PGS