Highlights d Microtubule nucleation scales spindles in zebrafish embryos and Xenopus egg extracts d Microtubule nucleation is regulated by the surface of the cell d Microtubule polymerization only contributes to scaling below a critical cell size d Microtubule number and length are regulated using distinct geometric cues

Cellular organelles such as the mitotic spindle adjust their size to the dimensions of the cell. It is widely understood that spindle scaling is governed by regulation of microtubule polymerization. Here we use quantitative microscopy in living zebrafish embryos and Xenopus egg extracts in combination with theory to show that microtubule polymerization dynamics are insufficient to scale spindles and only contribute below a critical cell size. In contrast, microtubule nucleation governs spindle scaling for all cell sizes. We show that this hierarchical regulation arises from the partitioning of a nucleation inhibitor to the cell membrane. Our results reveal that cells differentially regulate microtubule number and length using distinct geometric cues to maintain a functional spindle architecture over a large range of cell sizes.

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