We realized very recently that a missing bracket in the automatized averaging procedure for our raw data has led to the display of incorrect values for t K in figure 2(a). A corrected plot is given below with the dashed line having parameters = a 110 0 , a 1 =0.1, b = 0.8. We would like to emphasize that this correction does not affect any of the conclusions and claims of the manuscript. We apologize for any inconvenience that may have arose from the inaccurate plot.

Spindles are self-organized microtubule-based structures that segregate chromosomes during cell division. The mass of the spindle is controlled by the balance between microtubule turnover and nucleation. The mechanisms that control the spatial regulation of microtubule nucleation remain poorly understood. While previous work found that microtubule nucleators bind to pre-existing microtubules in the spindle, it is still unclear whether this binding regulates the activity of those nucleators. Here we use a combination of experiments and mathematical modeling to investigate this issue. We measured the concentration of microtubules and soluble tubulin in and around the spindle. We found a very sharp decay in the concentration of microtubules at the spindle interface. This is inconsistent with a model in which the activity of nucleators is independent of their association with microtubules but consistent with a model in which microtubule nucleators are only active when bound to pre-existing microtubules. This argues that the activity of microtubule nucleators is greatly enhanced when bound to pre-existing microtubules. Thus, microtubule nucleators are both localized and activated by the microtubules they generate.

Cellular fluids are complex media that are crowded with macromolecules and membrane-enclosed organelles on several length scales. Many studies have shown that crowding can significantly alter transport and reaction kinetics in biological but also in bio-mimetic fluids. Yet, experimental insights on how well bio-mimetic fluids can capture the complexity of cellular fluids are virtually missing. Therefore, we have combined fluorescence correlation spectroscopy (FCS) and fluorescence lifetime imaging microscopy (FLIM) to compare the spatial heterogeneities of biological and simple bio-mimetic crowded fluids. As a result, we find that these artificial fluids are capable of mimicking the average diffusion behavior but not the considerable heterogeneity of cellular fluids on the mesoscale (∼100 nm). On the nanoscale, not even the average properties are captured. Thus, cellular fluids feature a distinct, heterogeneous crowding state that differs from simple bio-mimetic fluids.