ABSTRACT

The currently used classification of vesicles is mainly based on the cell type of origin or the presumed subcellular origin of the vesicle. This has resulted in a rather confusing nomenclature including terms such as “prostasomes,” “exosomes,” “microvesicles,” “microparticles,” “ectosomes,” and “shedding vesicles.” Although in theory these different vesicle types can be quite well defined on the basis of their (sub)cellular origin, once released in the extracellular space this classification is very difficult. First, endosomally derived vesicles and plasma membrane-derived vesicles can overlap in size and in buoyant density. Second, up to now there are no markers described that are exclusively present on one vesicle type. Consequently, it is not yet possible to phenotypically classify a released vesicle on the basis of its subcellular origin. Classification based on function is also complicated. The molecular composition of a released vesicle is dynamic and depends not only on the subcellular origin of the vesicle but also on the producing cell type and the activation status of these cells. Hence, the total population of released vesicles is heterogeneous and probably consists of a mixture of different vesicle types.12,13 Extracellular vesicles can affect their target cells by signaling via receptor-ligand interactions,14 via modulatory lipids,15 or through transfer of regulatory (small) RNAs.16 Besides the molecular composition of the vesicles, the status of the target cell determines the functional outcome of the crosstalk between vesicles and target cells. Altogether, these different factors might explain the pleiotropic biological functions that have been described for extracellular vesicles. 17.1.1 Methods to Characterize Individual Vesicles and

Vesicle PopulationsThe majority of the vesicles released by cells is smaller than 300 nm.17,18 As a result, high-resolution imaging techniques, such as electron microscopy (EM) or atomic force microscopy (AFM) (see Chapter 9), are required to visualize individual nanosized vesicles. These high-resolution imaging techniques, however, are not suitable to study large numbers of vesicles needed to reliably characterize vesicle populations that are largely heterogeneous.