Pronuclear positioning during the first cell division in C. elegans is governed by a geometric optics mechanism in which line-of-sight constraints create dynamic terminator curves that gate microtubule access to surfaces, thereby sculpting the spatial distribution of motor-generated pulling forces that drive aster separation, pronuclear migration, centering, and rotation through a self-organized geometry-force feedback loop.

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.