Vesicular structures

Liposomes are members of a family of vesicular structures which can vary widely in their physicochemical properties. Basically, a liposome is built of one or more lipid bilayers surrounding an aqueous core. The backbone of the bilayer consists of phospholipids the major phospholipid is usually phosphatidylcholine (PC), a neutral lipid. Size, number of bilayers, bilayer charge, and bilayer rigidity are critical parameters controlling the fate of liposomes in vitro and in vivo. Dependent on the preparation procedure unilamellar or multilamellar vesicles can be produced. The diameter of these vesicles can range from 25 nm up to 50 ym—a 2000-fold size difference. 

Note that in equilibria (2) the subscripts per and cyt are omitted where substrate S is concerned. This is obvious when the binding is measured to a solubilized transport protein, but also in the case where the enzyme is embedded in the membrane of closed vesicular structures, internal and external substrate will have equal concentrations at equilibrium (see Eig. 5). Consequently, the binding is independent of the orientation of the enzyme in the membrane. 

Fig. 7 Different types of vesicular structures of liposomes. (From Ref. 134.)...

One of the characteristic features of Reg soils is the vesicular nature of the uppermost soil horizon. The size distribution of the vesicles is up to a few mm in diameter. Similar vesicular structures were also observed in lithosols and takyr-like alluvial soils and were always associated with the presence of stones or thin, hard crusts that sealed the soil surface. It forms mostly through accumulation of aeolian dust (McFadden et al., 1998). 

The multilamellar bilayer structures that form spontaneously on adding water to solid- or liquid-phase phospholipids can be dispersed to form vesicular structures called liposomes. These are often employed in studies of bilayer properties and may be combined with membrane proteins to reconstitute functional membrane systems. A valuable technique for studying the properties of proteins inserted into bilayers employs a single bilayer lamella, also termed a black lipid membrane, formed across a small aperture in a thin partition between two aqueous compartments. Because pristine lipid bilayers have very low ion conductivities, the modifications of ion-conducting... 

In model systems for bilayers, one typically considers systems which are composed of one type of phospholipid. In these systems, vesicles very often are observed. The size of vesicles may depend on their preparation history, and can vary from approximately 50 nm (small unilamellar vesicles or SUVs) up to many pm (large unilamellar or LUV). Also one may find multilamellar vesicular structures with more, and often many more than, one bilayer separating the inside from the outside. Indeed, usually it is necessary to follow special recipes to obtain unilamellar vesicles. A systematic way to produce such vesicles is to expose the systems to a series of freeze-thaw cycles [20]. In this process, the vesicles are repeatedly broken into fragments when they are deeply frozen to liquid nitrogen temperatures, but reseal to closed vesicles upon thawing. This procedure helps the equilibration process and, because well-defined vesicles form, it is now believed that such vesicles represent (close to) equilibrium structures. If this is the case then we need to understand the physics of thermodynamically stable vesicles. 

Rod-coil copolymers are good candidates for the formation of rodlike or vesicular structures and can be built from polypeptide-block-containing copolymers, as previously discussed in Sect. 6. 

Since the discovery of vesicular structures, termed liposomes, by Alec Bangham, a tremendous amount of work on applications of liposomes has emerged. The use of small unilamellar liposomes as carriers of drugs for therapeutic applications has become one of the major fields in liposome research. The majority of these applications are based on the encapsulation of water-soluble molecules within the trapped volume of the liposomes. Long circulating poly(ethylene glycol) (PEG) modified liposomes with cytotoxic drugs doxorubicin, paclitaxel, vincristine, and lurtotecan are examples of clinically applied chemotherapeutic liposome formulations (1,2). 

Be sure to remove the extraembryonic membrane. Failure to do this may result in the lower colorimetric detection due to the reduced penetration of the probe. If technically difficult, the extraembryonic membrane should be at least partially torn. Additionally, be sure to puncture the vesicular structures such as brain, heart, and otic vesicles to prevent the false-positive staining caused by trapping of the probe. 

Figure 9.29 Membrane formation by meteoritic amphiphilic compounds (courtesy of David Deamer). A sample of the Murchison meteorite was extracted with the chloroform-methanol-water solvent described by Deamer and Pashley, 1989. Amphiphilic compounds were isolated chromatographically on thin-layer chromatography plates (fraction 1), and a small aliquot ( 1 p,g) was dried on a glass microscope slide. Alkaline carbonate buffer (15 p,l, 10 mM, pH 9.0) was added to the dried sample, followed by a cover slip, and the interaction of the aqueous phase with the sample was followed by phase-contrast and fluorescence microscopy, (a) The sample-buffer interface was 1 min. The aqueous phase penetrated the viscous sample, causing spherical structures to appear at the interface and fall away into the medium, (b) After 30 min, large numbers of vesicular structures are produced as the buffer further penetrates the sample, (c) The vesicular nature of the structures in (b) is clearly demonstrated by fluorescence microscopy. Original magnification in (a) is x 160 in (b) and (c) x 400.

Phospholipids, when dispersed in water, form spherical vesicular structures, an observation first made by Alex Bangham and Robert Home in 1952. An interesting and humorous account of the early work on liposomes has been published by Bangham as an introduction to the book by Ostro (1983) which is recommended reading for the interested student. 

At low concentrations, a hollow vesicle results with usually just one double layer and, as the concentration is increased, the number of double layers can increase in a transition from unilamellar vesicles to multilamellar structures. Since the hydro-plilic head groups are exposed on the inside as well as the outside of the vesicular structure this provides an opportunity to entrap hydrophilic guest drug molecules both inside the center of the vesicle and, if multilamellar, between the phospholipid bilayers as well. On the other hand, hydrophobic molecules can become incorporated in the hydrophobic regions of the bilayers where the hydrophobic tails overlap. 

Fig. 4 Serial sectioning of the hydrogenosomes of Neocallimastix sp. L2. A-D Bar = crometer h, hydrogenosomes r, ribosome globules (Munn et al. 1988). Asterisk, vesicular structures. From Voncken et al. 2002a, modified...

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