A tightly stretched membrane, confined in a narrow case, decides on its own where to burst. So does a cell: when it's crowded, the rupture point shifts from the center, and the two halves end up unequal. Previously, such asymmetry was thought to be set in advance by chemical labels, but new work shows that mechanics alone suffices.
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.
In C. elegans, the germline divides asymmetrically from the start, while somatic cells initially divide almost equally but become asymmetric as they shrink. This was reproduced with a single model without fitting to each lineage.