A study based on cryo-electron tomography (cryoET) shows that mitotic spindles, the machinery that segregates chromosomes during cell division, are locally organized by microtubule density and the activity of the kinesin Eg5. The work provides a high-resolution three-dimensional picture of the spindle's internal architecture in its native state, free of the distortions introduced by conventional chemical fixation.

The mitotic spindle has been a central object of study in cell biology for decades, but its internal organization at the nanometre scale has proven difficult to resolve. Classical models describe the spindle as a structure globally organized by centrosomes and kinetochores, while more recent work has suggested that local assembly of microtubules plays a relevant role. Cryo-electron tomography addresses this question by vitrifying the sample and imaging it in its hydrated state, without staining or sectioning.

The technique involves tilting the vitrified sample in the electron beam and acquiring a series of projections that are reconstructed into a three-dimensional volume. From these reconstructions, individual microtubules are segmented and the distribution of their orientations and densities is analysed in different spindle regions. The analysis reveals that local microtubule density and the activity of Eg5, a kinesin that slides antiparallel microtubules, determine spindle organization on local scales.

The implication is that the spindle cannot be explained solely by a global organization imposed from its ends, but emerges from local rules combining microtubule abundance and the molecular activity of kinesins. This connects with the open debate on how spindles self-assemble in the absence of centrosomes in many cell types and in mammalian oocytes. The approach also opens the door to studying how mutations or drugs that alter Eg5 modify spindle architecture at the nanometre scale.