Biogenesis of exosomes. A: At the limiting membrane of MVBs, several mechanism act jointly to allow specific sorting of transmembrane, chaperones, membrane associated and cytosolic proteins on the forming ILVs. B: Presence of several sorting mechanisms may induce heterogeneity in the population of ILVs in single MVBs by acting separately on different domains of the limiting membrane. C: Receiving lipids and proteins from the endocytic and the biosynthetic pathway, different subpopulations of MVBs may be generated whose composition confers them different fate: (1) back fusion of the ILVs with the limiting membrane. During this process molecules previously sequestered on the ILVs are recycled to the limiting membrane and to the cytosol. Change in the composition of the limiting membrane may be responsible for the tubulation allowing plasma membrane expression of endosomal proteins. (2) Unknown mechanism may lead MVBs toward the plasma membrane where proteins such as SNAREs and synaptogamins would allow their fusion and the consequent release of the ILVs in the extracellular medium as exosomes. (3) Similarly, the composition of the limiting membrane would preferentially induce fusion of MVBs with lysosomes leading to the degradation of the molecules sorted on ILVs. Reproduced from ref. 1 with the permission of Oxford University Press.