Figure 2.

Hypothetical roles of component limitation in spindle-size scaling. (A) Small-cell regime. Spindle size is limited by depletion of one or more limiting components, drawn as a red dot, from the whole cell. (B) Large-cell regime hypothesizing spindle size is not component limited. Some spindle-intrinsic mechanisms, drawn as a blue spring, limits length. Candidate mechanisms include bounded single-microtubule lengths, opposed motor activities, and a mechanochemical switch at the pole (see text for references). (C) Large-cell regime hypothesizing spindle size is component limited. The limiting component is locally activated by a reaction–diffusion system centered on chromosomes. The spindle depletes the activated component within the activation radius, leading to component limitation. A candidate reaction–diffusion system is RCC1-Ran-RanGAP1, which activates two potentially limiting spindle components, TPX2 and HURP.