Origin of spindle-scale symmetry-breaking at the spindle mid-plane

Determine the physical origin of the spindle-scale symmetry-breaking observed in electron tomography analysis regions centered at the spindle mid-plane of HeLa metaphase spindles, despite the expectation of symmetry in microtubule polarity along the spindle axis at that location under the active liquid crystal framework.

Background

Within the active liquid crystal model, coupling between microtubule density and orientation through active polar transport (the v1 term) explicitly breaks nematic symmetry. Such symmetry breaking is anticipated in LC-PolScope data analyzed over a half-spindle where coherent polar ordering is present.

However, the electron tomography analysis focuses on the spindle mid-plane, where microtubule polarity along the spindle axis is expected to be symmetric. Despite this, the measurements indicate spindle-scale symmetry-breaking in that region, and its origin is not established. The authors suggest that detailed measurements of the microtubule polarity field and its fluctuations may clarify the mechanism.