Fig. 6: Model of how spindle instability causes multinucleation during the first mitosis.
From: Shape of the first mitotic spindles impacts multinucleation in human embryos
After the PNBD, the spindle formation begins at the poles. There were two pole-number patterns: two and three poles. In bipolar spindles, high-aspect-ratio (AR) spindles are stable (a), have focused poles, and lead to mononuclei. However, in bipolar spindles, low-AR spindles are unstable (b), and centrosomes inherited from the sperm cannot maintain microtubule nucleation centers. Consequently, these spindle poles often become defocused, leading to multiple nuclei. Although multipolar spindles (c) have three poles, they are stable and do not change until the anaphase begins. As a result, all multipolar spindles lead to multinuclei.
