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The spindle is a micron-sized chromosome segregation machine built from microtubules and many other proteins. In this issue of Developmental Cell, Biswas et al. (2021) use sophisticated imaging and Xenopus egg extracts to show that the spindle's mass density is only as much as the surrounding cytoplasm, contrary to popular belief.
Figure 1. Optical diffraction tomography measures the mass density of the spindle
(A) In a medical CT scan, X-rays are transmitted across the human body in order to visualize absorptive objects, such as bones, reconstructing a three-dimensional image of the internal structures.
(B) In ODT, visible light is transmitted across the sample, and an image is reconstructed based on the object's refractive index. In ODT imaging of the spindle, only chromosomes appear in the photograph, whereas the spindle body is indistinguishable from the surrounding cytoplasm (left panel). An increase in tubulin concentration promotes microtubule polymerization in the spindle and intensifies the ODT signal accordingly (right panel). The measured refractive index value can be used to estimate the object's mass density.
