What are the functions of exosomes?

What are exosomes? Exosomes are 40–150 nm extracellular vesicles (EVs) released by all cell types (1–3). The definition of the term exosomes has evolved with time (2), and in this review, exosomes are defined as small EVs with a multivesicular endosomal origin (2). This definition is challenged by the experimental difficulties associated with the study of exosomes, as the enriched exosome preparations that are obtained using current methodologies may not strictly discriminate from microvesicles shed from the budding of the plasma membrane, independent of intracellular multivesicular endosomes (2, 4, 5). The definition of the term exosomes will likely continue to evolve over time in the scientific literature with ongoing efforts to better track their endocytic origin and distinguish them from other microvesicles (6). Exosomes are highly heterogeneous (6) and likely reflect the phenotypic state of the cell that generates them (Figure 1). Similar to cells, exosomes are composed of a lipid bilayer and, at any given point, can contain all known molecular constituents of a cell, including proteins, RNA, and DNA (7–9). Notably, as a result of repeated invagination of the lipid bilayer membrane during their biogenesis, the orientation of the lipid bilayer of exosomes mirrors that of the cells they originate from.