The mechanism of vesicle formation.

Phospholipid molecules exhibit amphiphilic properties and therefore they aggregate either in their crystalline state or in polar solvents into ordered structures with typical lyotropic liquid crystalline symmetries (de Gennes, 1974). At high lipid concentrations in water these are predominantly lamellar phases (Luzzati, 1968), while in aqueous solutions phospholipid molecules normally form self-closed spherical or oval structures where one or several phospholipid bilayers entrap(s) part of the solvent in its/their interior (Bangham & Home, 1964; Papahadjopoulos, 1978). In the case of one bilayer separating the internal and external solvent these structures are called, with respect to their size, small or large unilamellar vesicles (SUV and LUV, respectively) while the term multilamellar vesicles (MLV) is used in the case of many bilayers entrapping some of the solvent. The terms giant vesicle (GV) and large or small oligolamellar vesicles (LOV, SOV) are also used for very large vesicles and structures where several bilayers surround the entrapped solvent. Sometimes, especially in the case of technological applications, the term liposome is a homonym for SUV, LUV and MLV while in the older literature terms liposome and MLV are often synonyms. Vesicles are very important in many different areas of science and technology. In basic research they serve as models for cell membranes and their fusion, transport studies and investigations of membrane proteins that can be reconstituted in vesicles (i.e. 'proteoliposomes'). They also serve as delivery vehicles for drugs, genetic material, enzymes and other (macro)molecules into living cells and through other hydrophobic barriers in pharmacology, medicine, genetic engineering, cosmetic industry and food industry (Gregoriadis, 1984). They are also used as