Starfish vesicle

Shapes of vesicular aggregates range from tubular to spherical, from more exotic large compound (LCV) and starfish vesicles to simpler extended lamellae. Both unilamellar [75] and multilamellar ( onions ) [47,76] vesicles have been observed. One of the possible morphologies formed in solution are tubular vesicles, also known as tubes (rods) [77,78], Soft, water-filled polymer tubes of nanometer-range diameters and several tens of millimeters in length have been prepared via self-assembly of amphiphilic ABA triblock copolymer in aqueous media (Fig. 4). The tubes were mechanically and chemically stable and could be loaded with water-soluble substances [23],... 

For a fixed value of a, the two parameters v and Ado determine the shape of vesicles which can be placed in a two-dimensional phase diagram as shown in Figure 14a (143). There are numerous shapes including spherical, tubular, oblate, and starfish vesicles. Vesicles with outwardly curved shapes and prolate vesicles are in the upper part of the diagram for Ado > 1, and inwardly curved shapes and oblate vesicles are in the lower part for Ado > 1- In this phase diagram, all the shapes are drawn to scale, have the same area, and differ only in their values of V and Ado. Lipid and block copolymers exhibit a similar multitude of shapes as shown for PB-PEO block copolymer vesicles in Figure 14b (144). 

Budding Transition. In cells, the budding of vesicles from internal organel-lae or the plasma membrane is an ubiquitous process and a key process for cellular traffic. In case of giant lipid vesicles, asymmetric insertion of fluorescently labeled PEO with cholesteryl anchor groups into the outer monolayer lead to an increase of the spontaneous curvature and induced budding of starfish vesicles (136). The addition of the cholesteryl anchor increases both components of the effective spontaneous curvature Ado, the area difference, and the local spontaneous curvature as locally the membrane curves away from the polymer (147). 

Vesicle-Tubule Transition. Transitions between spherical and tubular vesicles have been reported for a glycopolymer-containing amphiphilic block copolymer by varying the ratio of the co-solvents THF and DMF in aqueous solutions, and by changing the temperature at which the aggregates were prepared (62). Shape transitions involving tubular and starfish vesicles as well as connected tubules have been observed for PS-PEO as a fimction of water content, polymer concentration, and added ions (25). 

Minimizing the total energy, F = F,. + now leads to pears, budding and a multitude of starfish vesicles, some of which are shown in Figure 7.3 [7], These shapes depend not just on the reduced volume v but on one more parameter which basically is the number difference of lipid molecules in the two layers. These shapes of lowest energy can be arranged in a two-dimensional phase diagram [15,18]. As... 

Figure 7.3 Two calculated starfish vesicles obtained by minimizing F + Fade- Reproduced from Europhys. Lett. 33, 403 (1996) by permission of the European Physical Society.

Figure 10.11 Comparison of theoretical and experimental nonaxisymmetric starfish vesicles [71], A seven-armed star and an H-shape are shown. These vesicles are swollen fiom the single phospholipid SOPC. Thus, highly branched shapes can be formed by homogeneous membranes.

Figure 10.12 Complex starfish vesicle with minimal symmetry [79]. The mean shape is mirror symmetric to the focal plane of the microscope and a plane orthogonal to it through the center of gravity of the vesicle.

To the left of the point CEP (see Figure 10.4), the upper spinodal of the oblate phase turns into a real second-order transition from oblates to nonaxisymmetric shapes [72]. These shapes resemble starfish at small reduced volume and connect the prolate and oblate vesicle phases. They will be described in the next section. 

Induction of the acrosome reaction is a key event that occurs in sea urchins, starfish, ascidians and mammals. However, it is not clear that in the amphibian Xenopus sperm undergo the same type of acrosome reaction, because thus far it has not been possible to detect a morphologically identifiable acrosome vesicle in these sperm. The acrosome reaction in echinoderms and in mammals has been well studied. During the acrosome reaction the acrosomal vesicle fuses with the plasma membrane and the acrosomal components are externalized. Several proteolytic activities thought to be involved in penetration through the extracellular matrix of the egg are deteeted in the acrosomal contents. In addition, exposure of egg coat binding molecules on the surface of the sperm occurs (see below). 

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