Shapes of phosholipid vesicles with beadlike protrusions

Abstract.

Giant phospholipid vesicles obtained by the method of electroformation were observed by the phase contrast microscope. Most of these vesicles contain a protrusion which shortens in a slow shape transformation process until it is absorbed into the main vesicle body. We are concerned with the last stages of this shape transformation process, where the protrusions attain a beadlike shape. The number of "beads" decreases one by one in consecutive steps, and it is demonstrated that each such step consists of two distinguishable phases. During the first phase the beadlike shape does not change and the necks connecting the "beads" are narrow. During the second phase the width of the protrusion necks increases. On the basis of the assumption that these shape transformations are driven by the decrease of the equilibrium difference between the outer and the inner membrane monolayers areas, the system behavior is analyzed in terms of the generalized bilayer couple model. The theoretical results confirm the observed time sequence: at a given number of "beads" the protrusion has in the first phase the shape that consists of spheres connected by infinitesimal necks, and during the second phase the protrusion is a single prolate unit with open necks. The discrepancies between the observed and the predicted widths of the necks are interpreted by the repulsive forces between the neighboring "beads" induced by the membrane thermal fluctuations. The analysis presented extends the existing catalog of vesicle shapes to the region of larger differences between the areas of membrane monolayers, and confirms the applicability of the generalized bilayer couple model to the description of the shape behavior of phospholipid vesicles containing beadlike protrusions.