Hydrodynamic narrowing of tubes extruded from cells

We discuss the pulling force f required to extrude a lipid tube from a living cell as a function of the extrusion velocity L. The main feature is membrane friction on the cytoskeleton. As recently observed for neutrophils, the tether force exhibits a "shear thinning" response over a large range of pulling velocities, which was previously interpreted by assuming viscoelastic flows of the sliding membrane. Here, we propose an alternative explanation based on purely Newtonian flow: The diameter of the tether decreases concomitantly with the increase of the membrane tension in the lipid tube. The pulling force is found to vary as L(1/3), which is consistent with reported experimental data for various types of cells.