Spherical vesicle

Artificial membrane systems can be prepared by appropriate techniques. These systems generally consist of mixtures of one or more phospholipids of natural or synthetic origin that can be treated (eg, by using mild sonication) to form spherical vesicles in which the lipids form a bilayer. Such vesicles, surrounded by a lipid bilayer, are termed liposomes. 

Phospholipids or similar water-insoluble amphiphilic natural substances aggregate in water to form bilayer liquid crystals which rearrange when exposed to ultrasonic waves to give spherical vesicles. Natural product vesicles are also called liposomes. Liposomes, as well as synthetic bilayer vesicles, can entrap substances in the inner aqueous phase, retain them for extended periods, and release them by physical process. 

The structure of the blood capillary wall is complex and varies in different organs and tissues. It consists of a single layer of endothelial cells joined together by intercellular junctions. Each endothelial cell, on an average, is 20-40 pm long, 10-15 pm wide, and 0.1-0.5 pm thick, and contains 10,000-15,000 uniform, spherical vesicles called plasmalemmal vesicles. These vesicles range in size between 60 and 80 nm in diameter. About 70% of these vesicles open on the luminal side of the endothelial surface, and the remaining open within the cytoplasm. Plasmalemmal vesicles are believed to be involved in the pinocytic transport of substances across the endothelium. The transition time of pinocytic vesicles across the cell is... 

The relation between head-group area and extended chain length and volume of the apolar groups favors formation of vesicles or bilayers from these twin-tailed surfactants. However, it seems that DDDAOH forms vesicles only in dilute solution, because the solution becomes viscous with increasing [surfactant] which is incompatible with the presence of approximately spherical vesicles. It appears that the solution then contains long, non-vesicular aggregates (Ninham et al., 1983). 

This model for the modulated state of tubules leads to an interesting speculation on the kinetic evolution of flat membranes or large spherical vesicles into tubules.68,132 In this scenario, when a flat membrane or large spherical... 

For lipid bilayers, equation (4) can be simplified. Above we have seen that the flat unsupported bilayer is without tension, i.e. y(0, 0) = 0, and therefore the first two terms must cancel y0 = — kcj. As argued above, JQ = 0, and thus also the third term drops out. The remaining two terms are proportional to the curvature to the power two. For a cylindrical geometry only, the term proportional to J2 is present. For spherical vesicles, the two combine into one ( kc + k)J2. The curvature energy of a homogeneously curved bilayer is found by integrating the surface tension over the available area ... 

The total curvature energy of a spherical vesicle is given by 4tt(2/cc + k). As all experimental data on phospholipids indicate that kc is not small, one is inclined to conclude that the vesicles are thermodynamically unstable the reduction of the number of vesicles, e.g. by vesicle fusion or by Ostwald ripening, will reduce the overall curvature energy. However, such lines of thought overlook the possibility that k is sufficiently negative to allow the overall curvature free energy of vesicles to remain small. 

Figure 23. Radial segment density profile through a cross-section of a highly curved spherical vesicle. The origin is at r = 0, and the vesicle radius is very small, i.e. approximately r = 25 (in units of segment sizes). The head-group units, the hydrocarbons of the tails and the ends of the hydrocarbon tails are indicated. Calculations were done on a slightly more simplified system of DPPC molecules in the RIS scheme method (third-order Markov approximation), i.e. without the anisotropic field contributions...

One approach could be the attempt to include the lipids into the stabilization process. Lipid molecules bearing polymerizable groups can actually be arranged as planar monolayers or as spherical vesicles and polymerized by high energy irradiation within these membrane like structures under retention of the orientation of the molecules (8,9,36). 

In the previous chapters it has been shown that stable cell membrane models can be realized via polymerization of appropriate lipids in planar monolayers at the gas-water interface as well as in spherical vesicles. Moreover, initial experiments demonstrate that polymeric liposomes carrying sugar moieties on their surface can be recognized by lectins, which is a first approach for a successful targeting of stabilized vesicles being one of the preconditions of their use as specific drug carriers in vivo. 

Peroxisomes are spherical vesicles bounded by a single membrane. They contain enzymes that catalyse oxidations that produce hydrogen peroxide which is degraded by the enzyme catalase. For example, very long or unusual fatty acids that are present in the diet but have no function are completely degraded in the peroxisomes. 

A liposome is a spherical vesicle with a membrane composed of a phospholipid and cholesterol (less than 50%) bilayer. Liposomes can be composed of naturally derived phospholipids with mixed lipid chains (such as egg phosphatidylethanolamine) or... 

For example, PPI dendrimers functionalized with azobenzene chromophores appended with aliphatic side chains assemble into large spherical vesicles in water below pH 8 (Fig. 11.14 Tsuda et al. 2000). 

The long-chain a-amino acid esters (40), (41), and (42) form bilayers on sonication in water under acidic conditions. Liposomes prepared from (40) and (42) precipitate if the aqueous medium is neutralized by titration with NaOH. Only liposomes made from (41) are stable even in basic solutions, as shown by electron microscopy52). Polypeptide formation in oriented spherical vesicles was confirmed by FT-IR spectroscopy. The liposomal solution of (41) was freeze-dried and the spectrum obtained from the residue was comparable with one of the polycondensed monolayers. The formation of polypeptide vesicles is illustrated in Scheme 4. 

Phospholipids are the most important of these liposomal constituents. The major component of cell membranes, phospholipids are composed of a hydrophobic, fatty acid tail and a hydrophilic head group. The amphipathic nature of these molecules is the primary force that drives the spontaneous formation of bilayers in aqueous solution and holds the spherical vesicles together. 

Fig. 17 (a-d) Cryo-TEM images of diblock (sphere-rod) liposomes comprised of liquid-phase lipid nanorods (white arrows) connected to spherical vesicles. The lipid nanorods are stiff cylindrical micelles with an aspect ratio RilOOO. Their diameter equals the thickness of a lipid bilayer ( 4 nm) and their length reaches up to several micrometers, with a persistence length on the order of millimeters, (c) An inset of B, demonstrating the thickness of the nanorod white arrow heads point out a thickness of rj4 nm (approximate bilayer thickness, identical for the spherical vesicle and the nanorods), (d) Schematic of a MVLBisG2/DOPC sphere-rod diblock liposome. Reprinted with permission from [58]. Copyright 2008 American Chemical Society... 

New theory will be required to describe the phase diagram of block liposomes. In particular, theories have to break new ground in explaining why nanorods and nanotubes stay attached to spherical vesicles. All current theories of lipid self-assemblies (based on Helffich s theory of membranes [98]), in contrast, predict spherical, tubular, and micellar shaped liposomes but only as separate objects. In our experiments, not a single instance of an isolated rod- or tube-shaped liposome (i.e., not connected to a sphere- or pear-shaped vesicle) was found. 

Figure 4.14 Micellar structures, (a) Spherical (anionic) micelle. This is the usual shape at surfactant concentrations below about 40 per cent, (b) Spherical vesicle bilayer structure (see also Figure 4.28), which is representative of the living cell, (c) and (d) Hexagonal and lamellar phases formed from cylindrical and laminar micelles, respectively. These, and other structures, exist in highly concentrated surfactant solutions...

This is an interconnected network of flattened or spherical vesicles and tubules found in the cytoplasm of eukaryotic cells. These structures are enveloped by a membrane that separates the endoplasmic reticulum cavities or cisternae. The cisternae constitute a network of channels that go through the cytoplasm and regulate the transport of various cell products, generally to the exterior environment. In some cells the cisternae also serve as a storage area. There are two types of endoplasmic reticulum granular (or rough) and smooth. 

The Golgi has distinct forms in different cell types. The most characteristic arrangement is a stack of circular flattened vesicles, with variable sizes, each one held by a single membrane in which can be found smaller spherical vesicles that bud off from the larger ones. In many cells, the Golgi complex is situated near the cell nucleus but in other cells, it is dispersed in the cytoplasm. 

Bozic and Svetina [36] analysed a different situation, where addition of membrane constituents happens from the external milieu, and there is no metabolism inside, but there is limited permeability. They supposed that the membrane assumes spontaneous membrane curvature. This is non-zero if the properties of the inside and outside solutions differ, or if the two layers of a bilayer membrane differ in composition, or if some membrane-embedded constituents are asymmetrically shaped. They were able to show that under these assumptions membrane division is possible provided TLkC4 > 1.85, where T is the time taken to double the membrane area, L is the hydraulic permeability of the membrane, k is the bending modulus, and C is the spontaneous membrane curvature. In this model growing vesicles first retain spherical shape, then are distorted to a dumbbell, then to a pair of asymmetric vesicles coupled by a narrow neck, and finally to a pair of spherical vesicles linked by a narrow neck. Separation of the two daughter vesicles occurs as a result of mechanical agitation in the solution. 

Liposomal formulations may also be used to deliver TLR4 agonists. Consisting of spherical vesicles formed by the self-assembly of phospholipid bilayers, liposomes are a versatile, biocompatible vaccine adjuvant formulation. The wide variety of available phospholipid molecules that are employed to make liposomes may have significant effects on the structure and biological activity of the adjuvant. 

Negative-staining electron micrographs of our PSI-enriched particles (Fig. 2) show more-or-less spherical vesicles of 200 100 nm in diameter. 

Fragments of endoplasmic reticulum are transformed from lipid bilayer sheets, with attached ribosomes, into spherical vesicles. This is a result of the homogenization used in preparing the samples and also the tendency of lipid bilayers (Fig. 1-4) to spontaneously reseal. 

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