A tightly stretched membrane, squeezed into a narrow case, decides on its own where to burst. So does a cell: when it is cramped, the point of division shifts from the middle, and the two halves come out unequal. Previously, it was thought that such asymmetry is predetermined by chemical labels, but the new work shows that mechanics alone is enough.

The cell cortex, a thin contractile layer beneath the membrane, was modeled as a closed viscous surface inside an ellipsoidal shell. The equations included active stresses that depend linearly on curvature, because the cortex breaks the "up-down" symmetry. The dynamics of shape and flow were solved numerically, varying cell size and the degree of confinement. For validation, published three-dimensional data from C. elegans embryogenesis were used: eight uncompressed and seventeen compressed embryos.

In experiments, division asymmetry increased statistically significantly with decreasing mother cell volume (p < 0.001 for late germline divisions). Mechanical compression of the embryo increased asymmetry by an average of 15% (regression slope 1.151, R² = 0.917). This is the first quantitative test of how size and external confinement jointly trigger cortical instability.

The work differs from neighboring studies in that it explains division asymmetry without molecular polarization, through the pure mechanics of an active surface. The result links cell geometry to daughter cell fate and offers a unified physical mechanism for different embryonic lineages. Perhaps this is how mechanical conditions in tissues and tumors influence cell fate.

The model can be extended by including PAR protein dynamics and explicit cell-cell contacts instead of an idealized shell. A transition to a phase-field description of multicellular morphogenesis would allow self-consistent prediction of division timing, axis, and asymmetry. This would open the way to predicting cell fate in developing tissues.