Exosome

An exosome is classically defined as a small (roughly 30–150 nm diameter) membrane-enclosed extracellular vesicle formed when late endosomes invaginate their limiting membrane to produce intraluminal vesicles inside a multivesicular body (MVB); these vesicles are released as “exosomes” once the MVB fuses with the plasma membrane. Exosomes are commonly isolated from biofluids such as blood plasma or serum by ultracentrifugation or polymer-based precipitation methods, then characterized by nanoparticle tracking analysis, flow cytometry, or proteomics. Because most separation methods cannot confirm that isolated vesicles actually originated from the endosomal system, current field guidelines (MISEV2023) recommend using “exosome” only when subcellular origin has been demonstrated, and otherwise using the broader, biogenesis-agnostic term “extracellular vesicle” (or “small EV”).