New challenges for conservation

Lamins A and C lend stiffness to the cell nucleus but are not expressed in stem cells until gastrulation. Therefore, stem cell nuclei are physically much less stiff than differentiated cells. This property may make it easier for stem cells to migrate, adopt different shapes, and remodel nuclear DNA. David Pajerowski et al. characterized nuclear deformation in stem cells and an epithelial analog. The authors applied suction to cells by using micropipettes and studied nuclear shear by using fluorescence microscopy. Embryonic stem cells were the most deformable, followed by hematopoietic stem cells (HSCs) that also lack lamins A and C. Deformation followed a power law as a function of time: for the first 10 s, the exponent indicated a viscoelastic f low, which was followed by a phase of plastic, irreversible deformation. To verify that lamins were responsible for stiffness, the authors developed a line of epithelial cells in which lamin expression had been knocked down by 85%. Initially, the epithelial cells had been as stiff as any differentiated cell; without the lamins, they became as pliable as HSCs. The authors also investigated the flow properties of highly condensed chromatin, using GFP-histones to observe the irreversible distortion that occurs at long timescales. — K.M.