Multi-nucleation in two-cell human embryos stems from spindle and metaphase plate incoherence in the first mitosis

The first embryonic division in humans is highly error-prone and a source of aneuploidies. Multi-nucleation is prevalent in two-cell human embryos, with unknown cause. Here, we live image human zygotes to elucidate the features of the first mitosis that predispose embryos to multinucleation. We show that failure to collect chromosomes into a single mass and establish a bipolar spindle during zygotic metaphase leads to severe multi-nucleation. We find that KIF10/CENP-E kinesin activity is essential to prevent the formation of multiple spindle poles and to congress chromosomes onto a metaphase plate. Furthermore, although the spindle assembly checkpoint mediates a delay in response to a highly disorganised spindle, KIF10/CENP-E-inhibited embryos ultimately undergo the first mitosis with a chaotic anaphase. Therefore, defective chromosome congression in the zygote combined with a failure to sense this error cause multi-nucleation. Remarkably, multi-nucleation can be corrected during the second mitotic division in a KIF10/CENP-E-dependent manner. We suggest that multi-nucleation may be a safeguarding mechanism to prevent chromosome loss during the highly error-prone first embryonic mitosis.