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bioRxiv · 10.64898/2026.04.28.721423

Geometric optics organizes organelle interactions and positioning

Farhadifar, R.·Fabig, G.·Wu, H.-Y.·Young, Y.-N.·Vogel, J.·Needleman, D. J.·Mueller-Reichert, T.·Shelley, M. J.
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Abstract

Microtubule asters are central to positioning organelles and organizing intracellular architectures. We show that the multi-stage choreography of microtubule asters which positions pronuclei during the first cell division in Caenorhabditis elegans, is governed by a cellular analog of geometric optics. Large-scale electron tomography reveals that astral microtubules reach cortical and pronuclear surfaces largely along line-of-sight trajectories, leaving complementary regions shadowed and inaccessible. Laser ablation identifies surface-anchored motors pulling on microtubules as the dominant drivers of motion. We develop a biophysical model incorporating dynamic terminator curves that separate microtubule-accessible and -inaccessible surfaces, and quantitatively recapitulate aster separation, pronuclear migration, centering, and rotation in control and genetically perturbed embryos. More broadly, robust positioning emerges from a feedback loop wherein intracellular geometry gates microtubule access, thus sculpting the distribution and magnitude of pulling forces, which in turn reshape that geometry.

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Farhadifar, R., Fabig, G., Wu, H.-Y., Young, Y.-N., Vogel, J., Needleman, D. J., Mueller-Reichert, T., Shelley, M. J.. 2026-04-29. Geometric optics organizes organelle interactions and positioning. https://doi.org/10.64898/2026.04.28.721423

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