Exosome (vesicle)

Exosomes are cell-derived vesicles that are present in many and perhaps all eukaryotic fluids, including blood, urine, and cultured medium of cell cultures. The reported diameter of exosomes is between 30 and 100 nm, which is larger than low-density lipoproteins (LDL) but much smaller than, for example, red blood cells. Exosomes are either released from the cell when multivesicular bodies fuse with the plasma membrane or released directly from the plasma membrane. Evidence is accumulating that exosomes have specialized functions and play a key role in processes such as coagulation, intercellular signaling, and waste management. Consequently, there is a growing interest in the clinical applications of exosomes. Exosomes can potentially be used for prognosis, for therapy, and as biomarkers for health and disease.

First discovered in the maturing mammalian reticulocyte (immature red blood cell) , exosomes were shown to participate in selective removal of many plasma membrane proteins as the reticulocyte becomes a mature red blood cell (erythrocyte). In the reticulocyte, as in most mammalian cells, portions of the plasma membrane are regularly internalized as endosomes, with 50 to 180% of the plasma membrane being recycled every hour. In turn, parts of the membranes of some endosomes are subsequently internalized as smaller vesicles. Such endosomes are called multivesicular bodies because of their appearance, with many small vesicles, or "intralumenal endosomal vesicles," inside the larger body. The intralumenal endosomal vesicles become exosomes if the multivesicular body merges with the cell membrane, releasing the internal vesicles into the extracellular space.

Exosomes contain various molecular constituents of their cell of origin, including proteins and RNA. Although the exosomal protein composition varies with the cell and tissue of origin, most exosomes contain an evolutionarily-conserved common set of protein molecules. The protein content of a single exosome, given certain assumptions of protein size and configuration, and packing parameters, can be about 20,000 molecules. The cargo of mRNA and miRNA in exosomes was first discovered at the University of Gothenburg in Sweden. In that study, the differences in cellular and exosomal mRNA and miRNA content was described, as well as the functionality of the exosomal mRNA cargo. Exosomes have also been shown to carry double-stranded DNA.

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