ASPECTS OF THE MECHANICS OF LOBED LIPOSOMES

Hotani, has studied, by means of dark-field light microscopy,morpholog ical transformations which unilamellar liposomes undergo when their in terior volume decreases steadily with time as a consequence of osmosis . In a previous paper, we made a theoretical study of the initial buck ling of an originally spherical vesicle into the observed oblate spher oidal shape; and we argued that some in-plane shear elastic stiffness is required - in addition to the well-known flexural stiffness of the lipid bilayer - in order to explain the observed phenomena. In the pre sent paper, we consider a later stage in the chain of morphological tr ansitions observed by Hotani, when a series of cudgel-shaped lobes hav e sprung out of a previously axisymmetric, biconcave-shaped vesicle. S pecifically, we compare the observed shapes of such lobes with half of a series of ''peanut''-shaped vesicles that are an equilibrium confor mation of an initially spherical liposome under reduced internal volum e. We find that the shapes do not match well. On the other hand, the o bserved lobe forms do match satisfactorily portions of ''undulating tu be'' shapes which evolve from a hypothetical cylindrical vesicle, acco rding to some simple calculations. In view of this agreement, we are l ed to propose that the formation of cudgel-shaped lobes requires some sliding of one lipid monolayer over another. This conflicts, of course , with the Love-Kirchhoff hypothesis which is normally invoked at the outset of analyses of lipid vesicles by means of classical thin-shell theory; but it is in accord with previous suggestions in the context o f more obviously severe distortion of the lipid bilayer.