The life and death of an organism rely on correct cell division, which occurs through the process of mitosis. Although the biochemical signalling and morphogenetic processes during mitosis are well understood, the importance of mechanical forces and material properties is only just starting to be discovered. Recent studies have revealed that the layer of proteins beneath the cell membrane—the so-called cell cortex—stiffens during mitosis, but it is as yet unclear whether mechanical changes occur in the rest of the material in the cell, contained in the cytoplasm. Here we show that, in contrast to the cortical stiffening, the interior of the cell undergoes a softening and an increase in dissipative timescale, similar to viscoelastic relaxation. These mechanical changes are accompanied by a decrease in the active forces that drive particle mobility. Using optical tweezers to perform microrheology measurements, we capture the complex active and passive material states of the cytoplasm using six relevant parameters, of which only two vary considerably during mitosis. We demonstrate a role switch between microtubules and actin that could contribute to the observed softening.

有机体的生死依赖于正确的细胞分裂，细胞分裂是通过有丝分裂过程发生的。尽管有丝分裂过程中的生化信号和形态发生过程已广为人知，但机械力和材料特性的重要性才刚刚开始被发现。最近的研究表明，细胞膜下方的蛋白质层——即所谓的细胞皮层——在有丝分裂过程中变硬，但尚不清楚细胞质中包含的其他物质是否发生机械变化。在这里，我们表明，与皮质硬化相比，细胞内部经历了软化和耗散时间尺度的增加，类似于粘弹性松弛。这些机械变化伴随着驱动粒子移动的主动力的减少。使用光学镊子进行微流变学测量，我们使用六个相关参数捕获细胞质的复杂主动和被动材料状态，其中只有两个在有丝分裂期间变化很大。我们展示了微管和肌动蛋白之间的角色转换，这可能有助于观察到的软化。