Exosomes are the smallest Extracellular Vesicles with sizes between 30 and 150 nm which are generated through the cells’ endolysosomal pathway (as shown in the figure). This pathway begins at the plasma membrane, where the membrane folds inward and then pinches off an empty IntraLuminal Vesicle. The process of endocytosis continues on the membrane of the intraluminal vesicle, creating smaller vesicles commonly known as Exosomes. The empty IntraLuminal Vesicle then matures into a MultiVesicular Body, where the Exosomes accumulate. The MultiVesicular Body can either fuse with the cell’s plasma membrane and release those Exosomes or can further mature into a lysosome. This endolysosomal pathway leading to the generation and release of Exosomes is regulated and the Exosomes encapsulate cell proteins, DNA, RNA and other metabolites.

Exosomes and other Extracellular Vessicles are present in tissues and biological fluids, including blood, urine, cerebrospinal fluid, etc. Exosomes were originally thought to function solely as a mechanism to dispose of unwanted proteins and other molecules, but it is now well established and generally accepted that Exosomes mediate cell to cell communications, for example in immune responses, angiogenesis, proliferation and cell differentiation.