Figure 1.

Biogenesis, release, content and uptake of exosomes. Early endosome is formed from the plasma membrane via endocytic pathway. MVB can be formed by the invagination of endosomal membrane. Dependent on the function and content, MVB then can be directed to fuse with plasma membrane and release to the extracellular space as exosomes. During the biogenesis of exosomes and prior to their secretion, proteins (e.g., tetraspanin, cytosolic proteins, receptor), nucleic acids (e.g., mRNA, miRNA, DNA), and lipids (e.g., sphingomyelin, cholesterol) are uploaded to exosomes. Cells appear to take up exosomes via several ways: (a) receptor-/lipidraft medated endocytosis, (b) phagocytosis, (c) macropinocytosis, (d) fusion with the plasma membrane of the target cell. MVB: multivesicular body.