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Exosomes differ from other types of primary extracellular vesicles (e.g., apoptotic bodies and microvesicles) in terms of size, content, and production mechanism. The most popularly accepted mechanism of exosome formation, i.e., an endosomal route, is as follows. The initial endosomes are produced by cell membrane invagination during which the bioactive substances begin to accumulate within the early sorting endosomes. The late sorting endosomes then form
multivesicular bodies (MVBs) after a second indentation. Finally, the MVBs fuse with the cell membrane, releasing the carried exosomes to the outside. Non-endosomal route of exosome biogenesis, such as plasma membrane budding, has also been reported.

As the three major exosome databases (i.e., ExoCarta, Vesiclepedia, and EVpedia) summarize, exosomes contain numerous molecules, including proteins, glycoconjugates, lipids, nucleic acids,metabolites, and other bioactive substances. The examples of each category and the corresponding functions have been thoroughly reviewed elsewhere. On the one hand, exosomes comprise a complex protein network including external proteins (e.g., tetraspanins, antigen-presenting complexes, and adhesion molecules) and internal proteins (e.g., heat shock proteins, ESCRT machinery, cytokines and chemokines, and membrane transporters). On the other hand, as the most abundant in human exosomal nucleic acids, microRNA (mRNA) could participate in hematopoiesis, exocytosis, and nerve and vascular regeneration through exosome-mediated cellular communication. 

There are various uptake mechanisms once exosomes reach the recipient cell, all of which can be categorized into membrane fusion, receptor interaction, and internalization. Finally, the exosomal cargos are released into the cytoplasm, the process of which depends on the source of the exosome, nature of the cargo, and the metabolic state of the recipient cell. The entire lifecycle from exosome biogenesis to uptake and intracellular signaling can be tracked using fluorescent,
luminescent, and radioactive techniques. 

     (文節錄自：Tan, F., Li, X., Wang, Z. et al. Clinical applications of stem cell-derived exosomes. Sig
Transduct Target Ther 9, 17 (2024). https://doi.org/10.1038/s41392-023-01704-0)