In the years following these pioneer studies, the release mechanism of these vesicular structures has been investigated in different cell types, and common intercellular features in terms of shedding mechanisms and material composition, have been soon identified. In fact, exosomes secreted by various cell types have similarities such as the size, the endosomal origin [4] and the presence of identical molecules. However, there are also clear differences in their protein composition, as revealed by proteomic studies [5] and supposed function depending GSI-IX cell line on the physiology of the considered cell. The exosomes detectable in the extracellular compartment can

be visualized only by electron microscopy, revealing them as “cup shaped” membrane vesicles with a diameter of ±50–100 nm [6]. However, they are acknowledged to represent a heterogeneous population, with smaller vesicles often observed in the same preparation. As a general concept, cells are known to secrete a large array of vesicular structures, ranging from membrane vesicles to

apoptotic bodies [7]. Research groups focusing on exosomes have proposed various classifications mainly based on the different dimensions of these organelles as well as on density properties. A classification was also achieved by searching ABT-263 mw for reliable markers of endosomal origin. Nowadays, studies dealing with exosomes require standard visualization by electronmicroscopy, density gradient centrifugation

as well as characterization experiments involving purity assessment of isolated fractions together with expression of CD63, CD81 and other exosome-associated tetraspanins [8] and [9]. others The achievement of such standard requirements has greatly contributed to the reliability of exosome science. Since their discovery in the 1980s, many years had to pass until exosomes gained some visibility in the scientific community. In 2005, Jennifer Couzin, a journalist of Science Magazine, appropriately described the first encounter of cell biologists with these particles as “stumbling across the particles in their experiments” [10]. Subsequently, a great effort has been devoted by an ever growing number of investigating groups to dissect the world behind these small organelles, at first dismissed as cellular “debris”, secreted into the extracellular space. Like most non-transformed cells, also tumor cells release exosomes whose composition can vary depending upon nature and conditions of each individual cell. Exosome secretion is constitutive and exacerbated in cancer cells, although it may still be modulated by microenvironmental milieu, influenced for instance by growth factors [11], heat shock and stress conditions [12], pH variations [13], and therapy [14].