Kavli Affiliate: Daniel J. Needleman
| First 5 Authors: Colm P Kelleher, Daniel J Needleman, , ,
| Summary:
To prepare gametes with the appropriate number of chromosomes, mammalian
oocytes undergo two sequential cell divisions. During each division, a large,
long-lived, microtubule-based organelle called the meiotic spindle assembles
around condensed chromosomes. Although meiotic spindles have been intensively
studied for several decades, as force-generating mechanical objects, they
remain very poorly understood. In materials physics, coarse-grained theories
have been essential in understanding the large-scale behavior of systems
composed of many interacting particles. It is unclear, however, if this
approach can succeed in capturing the properties of active, biochemically
complex, living materials like the spindle. Here, we show that a class of
models based on nematic liquid crystal theory can describe important aspects of
the organelle-scale structure and dynamics of spindles in living mouse oocytes.
Using our models to interpret quantitative polarization microscopy data, we
measure for the first time material properties relating to stress propagation
in living oocytes, including the nematic diffusivities corresponding to splay
and bend deformations. Unlike the reconstituted amphibian spindles that were
previously studied in vitro, nematic elastic stress is exponentially screened
in the microtubule network of living mammalian oocytes, with a screening length
of order one micron. This observation can be explained by the relatively high
volume fraction of embedded chromosomes in mammalian meiotic spindles, which
cause long voids in the microtubule network and so disrupt orientational stress
propagation.
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