Micronuclei in 2-cell embryos show higher blastocyst formation rates on human embryonic development

Objective: Multinucleated blastomeres at the two-cell stage (2MNB) represent a frequently observed nuclear abnormality in early human embryos. This abnormality has been reported to significantly impact on the embryo's developmental potential to reach the blastocyst stage. However, our understanding of the embryo's developmental potential and the morphokinetics of 2MNB remains limited. This study investigates the influence of 2MNB and its subtypes on the blastocyst formation. Study design: A non-interventional retrospective study was performed in the Reproductive Medical Center of the First Affiliated Hospital of Hainan Medical University, using a time-lapse incubator. The study involved the evaluation of 4416 embryos, including 628 multinucleated embryos, from 1521 intracytoplasmic sperm injection (ICSI) cycles conducted between October 2019 and October 2021. The morphokinetic characteristics of multinucleated embryos were analyzed. Results: The results show multinucleation was the most common abnormal mitotic event during embryo development (14.22 %) in 4416 embryos. A control group of 3210 developmentally normal embryos was used in the study. The multinucleated blastomeres caused a lower blastocyst rate (52.48 % VS 64.02 %) compared to the control group. Whereas, 2MNB cause a higher blastocyst rate than them at the 4-cell stage (4MNB) (58.89 % VS 43.64 %). 2MNB can be further be further divided into 2MNB1/2cell and 2MNB2/2cell based on one multinucleated blastomere or two multinucleated blastomere appeared. Time to pronuclei fading (tPNf) is significantly longer in 2MNB2/2cell compared to 2MNB1/2cell. Furthermore, the 2MNB1/2cell embryos were divided into four subgroups (Bi-: two nuclei with almost the same size, Micro-: two nuclei with varying sizes, Poly-: more than two nuclei with almost the same size, and Cluster-: more than two nuclei with varying sizes) based on the number of nuclei and relative size. The results show that the Bi- and Micro- groups had a significantly increased blastocyst rate. The Cluster-, and Poly- groups showed significantly delayed embryonic development compared to normal controls. Bi-group has significant delays at t3, t5, and t8 and the Micro-group had a significant delay only at t8. Conclusion: 2MNB cause higher blastocyst rate than them at 4MNB. 2MNB1/2cell shows shorter tPNf compared to 2MNB2/2cell. Moreover, the Micro-, Bi- groups had a significantly increased blastocyst rate and different kinetic parameters compared to Cluster-, Poly-groups, suggesting that it is necessary to distinguish the nucleus status within 2MNB to increase the blastocyst rate. When selecting embryos for transformation from the 2MNB1/2cell, Micro- is the best choice.