Figure 1.

A schematic representation of exosome biogenesis and secretion. Exosomes are formed through the endocytic pathway. Invagination of the plasma membrane during endocytosis leads to the formation of early endosomes; exosome precursors called intraluminal vesicles (ILVs) are then formed by inward budding of late endosomes (cell RNAs, proteins, and lipid are incorporated at this step). The process of ILV biogenesis can be ESCRT-dependent or independent. Endosomes with an accumulation of ILVs are termed multivesicular bodies (MVBs); the fusion of MVBs with the plasma membrane releases the ILVs into the extracellular space by exocytosis, and these ILVs therefore become exosomes. The fusion of MVBs with the plasma membrane requires several crucial factors, such as Rab GTPases and SNARE complexes. In some instances, depending on the function and content of MVBs, they may fuse with the cell membrane and release exosomes or fuse with lysosomes for content degradation. Exosomes can directly interact with receptors on the target cell, fuse with the plasma membrane of the target cell, or enter into the target cell by endocytosis, macropinocytosis, or phagocytosis.