Fig. 2. Exosome formation, secretion, and action on target cells. Formation of mesenchymal stem cells-derived exosomes (MSCs-Exos): ① Endocytosis: The extracellular component and cell surface protein are engulfed into the cells through the invagination of the plasma membrane to form an early endosome (EE); ② First exchange: The EE could initially exchange goods with the Golgi network and endoplasmic reticulum to form the late endosome (LE); ③ Secondary exchange: LE conducts cargo exchange again through the second plasma membrane invagination and finally forms intraluminal vesicles (ILVs), and the collection of multiple ILVs of different sizes is called a multivesicular body (MVB); ④ Plasma membrane budding: Some MVBs release exosomes extracellularly through plasma membrane budding; ⑤ Recycling: Some MVBs can be degraded by lysosomal fusion, and the degradation products can be recovered by cells. The structure of MSCs-Exos: Exosomes express tetraspanins (CD81, CD63, and CD9), heat shock proteins (HSP60, HSP70, and HSP90), ALG-2 interacting protein X (Alix), TSG101, integrins, flotillin, surface markers (CD44, CD73, and CD90) and adhesion molecules (CD29, CD44, and CD73). In addition, MSCs-Exos carry nucleic acids, proteins, and enzymes. Exosomes enter recipient cells in three ways: ⑥ receptor-mediated entry, ⑦ direct membrane fusion, and ⑧ endocytosis.