Extracellular vesicles (EVs) are a cluster of nano-sized vesicles of different sizes, cargo, and surface markers that are secreted into the extracellular environment through a variety of mechanisms. They carry various components of the cytoplasm and cell membrane that are selectively loaded into these vesicles. They are secreted by all forms of living cells and play essential roles in different physiological functions and pathological processes. They also have been utilized as Diagnostic markers and therapeutic tools in several conditions.
Three types of EVs are biologically distinguishable from one another via the distinct processes through which they are released by the cell. However, the experimental classification of these vesicles is less clear as there is no consensus on what criteria to use for their differentiation. The first type is the microvesicles (MVs), or the ectosomes. These vesicles arise by direct budding through the cell membrane to the outside of the cell. The second type is the exosomes, which are first formed by budding into the endosomes to create what is called the multivesicular bodies (MVBs). These MVBs either fuse with lysosomes, resulting in content digestion, or fuse with the cell membrane, which results in the content released as exosomes. The third type arises from the cells during the process of apoptosis, hence its name apoptotic bodies (APBs). APBs emerge either by separation of the membrane blebs or from apoptopodia that arise during the process of apoptosis. There are two types of apoptopodia: non-beaded and beaded. The distinction in the course and the roles of these different routes is still under investigation.
EVs are secreted by all ranges of organisms, from prokaryotes to unicellular eukaryotes - like protozoa, yeasts, and fungi - to mammals. They can be isolated from all bodily fluids, including blood, plasma, serum, saliva, urine, sweat, cerebrospinal fluid, breast milk, and semen. In humans, these EVs have been reported to be secreted by nearly all types of cells in our physique, including but not limited to immune cells, endothelial cells, red blood cells, erythrocytes, liver cells, and epithelial cells.
After their isolation, it is necessary to confirm that the required population of EVs has been obtained. Scientists utilize several techniques to do this confirmation. First is the electron microscope. The regular scanning electron microscope shows EVs as irregular shaped membrane-enclosed vesicles. Alternatively, cryo-electron microscopy can be utilized. Despite being time-consuming and expensive, it gives more accurate information about the shape and size of the isolated vesicles. The second method of characterization is nano-track analysis, which provides a general idea about the concentration and size distribution of the isolated vesicles. The third method is a western blot, which is used to recognize protein markers associated with the membrane of the EVs. Despite some distinction of specific proteins on the surfaces of different EVs, there is still no consensus on which proteins can be used reliably as markers for the different types. Recently, flow cytometry procedures have been developed to characterize different types of EVs based on their surface markers.
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