Central spindle instability and ectopic furrow organization mark fragmentation-prone first mitosis in human embryos

Abstract Severe cell fragmentation compromises blastocyst formation and correlates with aneuploidy, but the mitotic events associated with fragmentation remain unclear. Using non-invasive live-cell imaging of DNA, tubulin, and/or actin in discarded human two-pronuclear embryos, here we show how central spindle dynamics, cleavage furrow positioning, and actin-ring organization are associated with fragmentation. During the first mitosis, fragmentation-prone embryos frequently show unstable central spindle positioning, ectopic cleavage furrow ingression, and multiple contractile actin rings. These features are accompanied by larger furrow–central spindle offsets, delayed central spindle compaction toward midbody assembly, and severe fragmentation. In embryos with limited fragmentation, central spindle compaction progresses earlier and furrow positioning shows smaller offsets. Notably, these abnormalities are largely confined to the first mitosis; by the second mitosis, fragmentation is reduced, central spindle compaction occurs earlier, and embryos form a single contractile ring. Together, these findings indicate that fragmentation during human zygotic mitosis is associated with spatial miscoordination among the central spindle, cleavage furrow, and actin cortex.