attention: Op18/stathmin; and the

gradients in Op18/stathmin, which destabilizes microtubules at least partly by tubulin sequestration in a phosphorylation-dependent manner [11–13], have been directly visualized in mitotic and interphase cells using phosphostate-specific fluorescence resonance energy transfer (FRET) probes [14]. The FRET analysis of Niethammer et al. [14] indicates that a region favoring microtubule assembly exists between the poles in mitotic cells. Phosphostate gradients in Ran, which indirectly controls microtubule assembly in mitosis via TPX2 in a GTP/GDP-state dependent manner [15,16], have been directly visualized via FRET in Xenopus egg extracts [17]. Consistent with these findings, Wollman et al. [1] show that perturbing Ran function extends the capture time 2–3 fold, suggesting that a high concentration of Ran-GTP around the chromosomes at least partially creates the favorable growth region for microtubule assembly. Previous computer modeling of RanGTP gradients suggested that the gradients would be very small in somatic cell mitosis [18]. It may be, however, that even weak gradients could translate quantitatively into relatively large catastrophe and rescue frequency gradients in the cell. In addition to controlling catastrophe and rescue, Ran-GTP may also control microtubule nucleation around the chromosomes [16]. In general, it is not yet clear how any putative molecular gradient is actually ‘read out ’ by microtubules to control their behavior spatially. Further, microtubules themselves may possess a history-dependent catastrophe that enables persistent assembly during the early part of a growth phase, with an increasing likelihood of catastrophe as elongation proceeds [6,19]. This has the effect of increasing the efficiency with which space is searched by narrowing the distribution of microtubule lengths around a length optimized for the search-and-capture process [20]. In summary, it has been appreciated for some time that chemical gradients shape the developing embryo. Evidence is accruing that they also shape the cytoplasm, which may facilitate formation of key intracellular connections, such as those between spindle poles and kinetochores during mitosis