Migration speed of nucleolus precursor bodies in human male pronuclei: a novel parameter for predicting live birth

Purpose To study the relationship between the migration speed of nucleolus precursor bodies (NPBs) in male and female pronuclei (mPN; fPN) and human embryo development during assisted reproduction. Methods The migration speed of 263 NPBs from 47 zygotes was quantitated, and embryonic development was observed until the blastocyst stage. The central coordinates of mPN, fPN, and NPBs were noted at multiple timepoints. Then, the distance traveled by the NPBs between two sequential images was measured, and migration speed was calculated. Additionally, we investigated the relationship between NPB migration speed and ploidy status ( N = 33) or live birth/ongoing pregnancy (LB/OP) ( N = 60) after assisted reproduction. Results The NPB migration speed in both mPN and fPN was significantly faster in the zygotes that developed into blastocysts ( N = 25) than that in the zygotes that arrested ( N = 22). The timing of blastulation was negatively correlated with NPB migration speed in the mPN. Faster NPB migration was significantly correlated with LB/OP. In multivariate logistic analysis, NPB migration speed in the mPN was the only morphokinetic parameter associated with LB/OP. In a receiver-operating characteristic curve analysis of LB/OP by the NPB migration speed in the mPN, the cut-off value was 4.56 μm/h. When this cut-off value was applied to blastocysts with preimplantation genetic testing for aneuploidy, 100% of the blastocysts faster than or equal to the cut-off value were euploid. Conclusion The NPBs migrated faster in zygotes having the potential to develop into a blastocyst, and eventually into a baby. This predictor could be an attractive marker for non-invasive embryo selection.