Symmetrical Shape of the Meiotic Spindle is Dynamically Balanced

Abstract

Meiotic spindle assembly is critical for achieving accurate chromosome segregation. The spindle mainly consists of microtubules and molecular motors. Recent studies have suggested that bipolar spindle formation requires the force balance sustained by molecular motors and polymerization-depolymerization dynamics of microtubules. However, it is not well understood whether and how the two pole structures are symmetrically balanced in a spindle. In this study, we quantitatively measured the mechanical stiffness, the microtubule density, and the response to the deformation of spindle poles by micromanipulation techniques and 3D analysis. To deform the spindle self-assembled in Xenopus egg extract, we inserted two glass micro-needles into a pole region at one side and widened it perpendicularly to the pole-to-pole axis. We found that the stiffness and the microtubule density in the manipulated side of pole region reduced upon widening. Unexpectedly, the reduction was also observed after a while in the unmanipulated side, which resulted in the formation of a symmetrical defocused barrel-like shape. On the other hand, it has been reported that inhibition of the dynein function by the addition of dynein-dynactin inhibitor causes the defocusing of pole regions, such that the barrel-shaped structure is formed. We compressed one side of the barrel-shaped spindle using a pair of glass micro-needles and found that the stiffness and the microtubule density in the compressed region increased. These changes also occurred in the unmanipulated side, which was accompanied by the bipolar spindle formation. Our results suggest that symmetrical shape of the spindle is dynamically balanced for proper cell division.