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Fig. 6 Figure 6. Hierarchical Regulation of Spindle Scaling Schematics of the different regimes of spindle scaling. For large cells, spindles reach an upper limit because activation and availability of microtubule nucleators is restricted to a reaction-diffusion volume. Below this volume, spindles scale with cell size by regulating microtubule number, which depends on the surface area of the cell through the sequestration of a microtubule nucleation inhibitor. When the surface to volume ratio exceeds a critical value, components that regulate microtubule dynamics become limiting, leading to a change in microtubule length for small cells.