Amoeboid cells propel by generating pseudopods that are finger-like protrusions of the cell body that continually grow, bifurcate, and retract. Pseudopod-driven motility of amoeboid cells represents a complex and multiscale process that involves bio-molecular reactions, cell deformation, and cytoplasmic and extracellular fluid motion. Here we present a 3D model of pseudopod-driven swimming of an amoeba suspended in a fluid without any adhesion and in the absence of any chemoattractant. Our model is based on front-tracking/immersed-boundary methods, and it combines large deformation of the cell, a coarse-grain model for molecular reactions, and cytoplasmic and extracellular fluid flow. The predicted shapes of the swimming cell from our model show similarity with experimental observations. We predict that the swimming behavior changes from random-like to persistent unidirectional motion, and that the swimming speed increases, with increasing cell deformability and protein diffusivity. The unidirectionality in cell swimming is observed without any external cues and as a direct result of a change in pseudopod dynamics. We find that pseudopods become preferentially focused near the front of the cell and appear in greater numbers with increasing cell deformability and protein diffusivity, thereby increasing the swimming speed and making the cell shape more elongated. We find that the swimming speed is minimum when the cytoplasm viscosity is close to the extracellular fluid viscosity. We further find that the speed increases significantly as the cytoplasm becomes less viscous compared with the extracellular fluid, resembling the viscous fingering phenomenon observed in interfacial flows. While these results support the notion that softer cells migrate more aggressively, they also suggest a strong coupling between membrane elasticity, membrane protein diffusivity, and fluid viscosity.

中文翻译：

---

变形虫细胞游泳的计算模型

变形虫细胞通过产生伪足来推进，这些伪足是细胞体的手指状突起，不断生长，分叉和缩回。伪足驱动的变形虫细胞运动代表了一个复杂而多尺度的过程，涉及生物分子反应，细胞变形以及胞质和细胞外液运动。在这里，我们提出了一种由拟足动物驱动的变形虫悬浮在流体中的游泳的3D模型，该流体悬浮在流体中，没有任何粘附力，也没有任何化学引诱剂。我们的模型基于前跟踪/浸入边界方法，并结合了细胞的大变形，用于分子反应的粗粒度模型以及细胞质和细胞外液流。根据我们的模型预测的游泳细胞形状与实验观察结果相似。我们预测游泳行为会从随机样变化到持续的单向运动，并且游泳速度会随着细胞变形能力和蛋白质扩散能力的增加而增加。观察到细胞游泳的单向性，没有任何外部提示，并且是伪足动态变化的直接结果。我们发现伪足优先集中在细胞的前部，并随着细胞变形能力和蛋白质扩散性的增加而大量出现，从而增加了游泳的速度并使细胞的形状更加细长。我们发现，当细胞质粘度接近细胞外液粘度时，游泳速度最小。我们进一步发现，与胞外液相比，随着细胞质的黏性降低，速度显着提高，类似于在界面流动中观察到的粘性指法现象。尽管这些结果支持了较软的细胞迁移更为积极的观点，但它们也表明了膜弹性，膜蛋白扩散性和流体粘度之间的强耦合。