Membrane rupture and independent extension of sister membranes drive cytokinesis in C. elegans embryos

Cell division, the physical separation of a cell into two daughter cells, is classically thought to require continuous plasma membrane invagination driven by equatorial contractile forces. Here, we challenge this paradigm by uncovering an unexpected mechanism in C. elegans embryos: the plasma membrane ruptures at metaphase or earlier, generating physically independent sister membranes. This separation of sister membranes persists into later cytokinesis, suggesting that the extension of free-ended membrane could direct cytoplasmic partitioning. The rupture of the mother cell membrane and the independent extension of individual sister membranes create widely distributed membrane discontinuities in early C. elegans embryos, yet massive cytoplasmic leakage does not occur. EM observations suggest that exposed cytoplasmic regions are shielded by plasma membranes from neighboring cells or by extracellular matrix components. These findings overturn the long-standing assumption that cytokinesis requires persistent membrane integrity and offer fresh insights into cell division.