Fig. 1

Exosome biogenesis, composition and targeted uptake. Exosome formation begins with the formation of early endosomal vesicles through plasma membrane invagination, and early endosomal vesicles invaginate to form late endosomes, which then regulate the accumulation of mRNA and proteins, lipids, and other substances through a variety of complex mechanisms to form multivesicular bodies. Multivesicular bodies can be fused with lysosomes to be degraded, and can also release exosomes through fusion with the plasma membrane. Exosomes are surrounded by a phospholipid bilayer and contain different types of cell surface proteins, intracellular proteins and DNA. Several molecules are used as exosome markers (CD9, CD63, CD81 and ALIX). Exosomes can also be taken up by target cells through direct membrane fusion, membrane invagination to form an endosomal envelope that fuses with the membrane, membrane protrusion phagocytosis, and receptor-ligand binding