Stability of cylindrical vesicles under axial tension

In recent experiments, a thin (presumably cylindrical) tubular ''tether'' is mechanically pulled from a roughly spherical fluid-phase lipid-bilayer vesicle. As a first step to understanding these experiments in terms of the commonly used models for vesicle shapes, we examine the mechanical stability of cylindrical vesicle surfaces under axial tension. This should be an adequate description of the extruded tether under the influence of the pulling force, and it provides a starting point for the calculation of the full vesicle shape. It turns out that there is not much difference between an isolated cylindrical vesicle and one where the spherical part of the whole shape is taken into account in an approximate way, giving us reason to believe that our calculation is also relevant to the tether-pulling experiments. Comparison of our results with the experiments shows that the observed tethers fall comfortably within the predicted range of cylindrical stability.