Tsai M A, Frank R S, Waugh R E
Department of Biophysics, University of Rochester School of Medicine and Dentistry, New York.
Biophys J. 1993 Nov;65(5):2078-88. doi: 10.1016/S0006-3495(93)81238-4.
The mechanical behavior of the neutrophil plays an important role in both the microcirculation and the immune system. Several laboratories in the past have developed mechanical models to describe different aspects of neutrophil deformability. In this study, the passive mechanical properties of normal human neutrophils have been further characterized. The cellular mechanical properties were assessed by single cell micropipette aspiration at fixed aspiration pressures. A numerical simulation was developed to interpret the experiments in terms of cell mechanical properties based on the Newtonian liquid drop model (Yeung and Evans, Biophys. J., 56: 139-149, 1989). The cytoplasmic viscosity was determined as a function of the ratio of the initial cell size to the pipette radius, the cortical tension, aspiration pressure, and the whole cell aspiration time. The cortical tension of passive neutrophils was measured to be about 2.7 x 10(-5) N/m. The apparent viscosity of neutrophil cytoplasm was found to depend on aspiration pressure, and ranged from approximately 500 Pa.s at an aspiration pressure of 98 Pa (1.0 cm H2O) to approximately 50 Pa.s at 882 Pa (9.0 cm H2O) when tested with a 4.0-micron pipette. These data provide the first documentation that the neutrophil cytoplasm exhibits non-Newtonian behavior. To further characterize the non-Newtonian behavior of human neutrophils, a mean shear rate gamma m was estimated based on the numerical simulation. The apparent cytoplasmic viscosity appears to decrease as the mean shear rate increases. The dependence of cytoplasmic viscosity on the mean shear rate can be approximated as a power-law relationship described by mu = mu c(gamma m/gamma c)-b, where mu is the cytoplasmic viscosity, gamma m is the mean shear rate, mu c is the characteristic viscosity at characteristic shear rate gamma c, and b is a material coefficient. When gamma c was set to 1 s-1, the material coefficients for passive neutrophils were determined to be mu c = 130 +/- 23 Pa.s and b = 0.52 +/- 0.09 for normal neutrophils. The power-law approximation has a remarkable ability to reconcile discrepancies among published values of the cytoplasmic viscosity measured using different techniques, even though these values differ by nearly two orders of magnitude. Thus, the power-law fluid model is a promising candidate for describing the passive mechanical behavior of human neutrophils in large deformation. It can also account for some discrepancies between cellular behavior in single-cell micromechanical experiments and predictions based on the assumption that the cytoplasm is a simple Newtonian fluid.
中性粒细胞的力学行为在微循环和免疫系统中都起着重要作用。过去有几个实验室开发了力学模型来描述中性粒细胞变形能力的不同方面。在本研究中，对正常人中性粒细胞的被动力学特性进行了进一步表征。通过在固定抽吸压力下的单细胞微量移液器抽吸来评估细胞力学特性。基于牛顿液滴模型（Yeung和Evans，《生物物理杂志》，56：139 - 149，1989）开发了一个数值模拟，以根据细胞力学特性来解释实验结果。确定了细胞质粘度是初始细胞大小与移液器半径之比、皮质张力、抽吸压力和全细胞抽吸时间的函数。测得被动中性粒细胞的皮质张力约为2.7×10⁻⁵ N/m。发现中性粒细胞细胞质的表观粘度取决于抽吸压力，当用4.0微米移液器测试时，在98 Pa（1.0 cm H₂O）的抽吸压力下约为500 Pa·s，在882 Pa（9.0 cm H₂O）时约为50 Pa·s。这些数据首次证明中性粒细胞细胞质表现出非牛顿行为。为了进一步表征人类中性粒细胞的非牛顿行为，基于数值模拟估计了平均剪切速率γm。表观细胞质粘度似乎随着平均剪切速率的增加而降低。细胞质粘度对平均剪切速率的依赖性可以近似为幂律关系，用μ = μc(γm/γc)⁻ᵇ描述，其中μ是细胞质粘度，γm是平均剪切速率，μc是特征剪切速率γc下的特征粘度，b是材料系数。当γc设定为1 s⁻¹时，被动中性粒细胞的材料系数确定为正常中性粒细胞的μc = 130 ± 23 Pa·s和b = 0.52 ± 0.09。幂律近似具有显著能力来协调使用不同技术测量的细胞质粘度已发表值之间的差异，尽管这些值相差近两个数量级。因此，幂律流体模型是描述人类中性粒细胞在大变形时被动力学行为的一个有前途的候选模型。它还可以解释单细胞微机械实验中的细胞行为与基于细胞质是简单牛顿流体假设的预测之间的一些差异。
