Institute for Theoretical Physics and BioQuant, Heidelberg University, Heidelberg, Germany.
Cell Biology and Biophysics Unit, European Molecular Biology Laboratory (EMBL), Heidelberg, Germany.
PLoS Comput Biol. 2018 Nov 15;14(11):e1006588. doi: 10.1371/journal.pcbi.1006588. eCollection 2018 Nov.
Cytoplasmic flows are an ubiquitous feature of biological systems, in particular in large cells, such as oocytes and eggs in early animal development. Here we show that cytoplasmic flows in starfish oocytes, which can be imaged well with transmission light microscopy, are fully determined by the cortical dynamics during surface contraction waves. We first show that the dynamics of the oocyte surface is highly symmetric around the animal-vegetal axis. We then mathematically solve the Stokes equation for flows inside a deforming sphere using the measured surface displacements as boundary conditions. Our theoretical predictions agree very well with the intracellular flows quantified by particle image velocimetry, proving that during this stage the starfish cytoplasm behaves as a simple Newtonian fluid on the micrometer scale. We calculate the pressure field inside the oocyte and find that its gradient is too small as to explain polar body extrusion, in contrast to earlier suggestions. Myosin II inhibition by blebbistatin confirms this conclusion, because it diminishes cell shape changes and hydrodynamic flow, but does not abolish polar body formation.
细胞质流动是生物系统的普遍特征，特别是在大型细胞中，如早期动物发育中的卵母细胞和卵子。在这里，我们展示了海胆卵母细胞中的细胞质流动完全由表面收缩波期间的皮质动力学决定，这些流动可以通过透射光显微镜很好地成像。我们首先表明卵母细胞表面的动力学在动物-植物轴周围具有高度的对称性。然后，我们使用测量的表面位移作为边界条件，通过数学方法求解变形球体内部的 Stokes 方程。我们的理论预测与通过粒子图像测速法定量的细胞内流动非常吻合，证明在此阶段，海胆细胞质在微米尺度上表现为简单的牛顿流体。我们计算了卵母细胞内部的压力场，发现其梯度太小，无法解释极体的挤出，与早期的建议相反。blebbistatin 抑制肌球蛋白 II 证实了这一结论，因为它减小了细胞形状的变化和流体动力学的流动，但没有消除极体的形成。
