| The technique most commonly employed for studying the deformation of passive human neutrophils, micropipet aspiration, has uncovered many interesting facts. In general, the cytoplasm of passive neutrophils behaves like a highly viscous fluid with very little elasticity. These cells enter the pipet in an isovolumetric manner (i.e. they are incompressible), without adhering to the pipet wall (i.e. they are slippery). The surface of the neutrophil is covered by a highly wrinkled plasma membrane that unfolds and smooths to accommodate the increase in apparent surface area (i.e. there is no creation of new membrane). Finally, neutrophils have an effective cortical tension which increases with the area expansion of the cell. In short, these observations suggest that from a mechanical point of view, the neutrophil may be regarded as a “slippery fluid droplet” with an effective surface tension.; In this study, several slippery droplet models were successfully solved and studied using a low Reynolds number hydrodynamics code based on finite element theory. For example, we examined several different possibilities for the rheology of the cytoplasm: Newtonian, shear thinning, and two-phase interpenetrating fluids. We also implemented improved modeling of the transport of the plasma membrane and explored the effects of both constant and area dependent surface tensions. Finally, aspiration experiments were conducted, and the results compared to the predictions of our various models.; Our systematic comparisons of mathematical models and experimental data have yielded some new insights into the mechanical nature of passive neutrophils. For example, at low rates of shear, we find that the cytoplasm can be represented quite accurately by a Newtonian fluid. However, at shear rates greater than ∼0.05 s−1, thinning of the cytoplasm occurs. Furthermore, the results confirm the dependence of the cortical tension on the area expansion of the cell. They also indicate that the membrane is attached to the cytoskeleton and has limited freedom to slip with respect to the underlying cytoplasm during aspiration. Finally, our results suggest that neutrophils has an effective surface viscosity (resistance to increasing surface area) and that an interpenetrating fluid is necessary to predict the initial rapid entry behavior. |
