Self-centering activity of dynamic microtubule arrays. (a) Self-centering by nucleation and dynamic instability, for a microtubule-nucleating bead. Images are separated by 3 minutes, scale bar is 10 μm (From Proc Natl Acad Sci U S A). (b) Self-centering by motors. Left: diagram of crosslinked motors reorienting microtubules. Right: fluorescence image of an aster formed in a microwell by this mechanism (reprinted by permission from Macmillan Publishers Ltd: Nature 389:305–308, copyright 1997 ).