Fig. 3.

Exosome and microvesicle-mediated transfer of biologically active material into recipient cells. Importantly, both exosomes (31) and microvesicles (12) carry functional molecules able to modify the phenotype of recipient cells. Exosomes are preferentially endocytosed and may release their contents by fusion with the recipient endosomal membrane by a process called back-fusion (8). This may be mediated by the BMP (highlighted in red) present both on the exosome and endosome membrane, because BMP is fusogenic in an acidic pH environment (23). Even though some microvesicles are internalized by receptor-mediated endocytosis (61), the microvesicle membrane is devoid of BMP, and how microvesicles transfer their contents inside target cells is unclear. Instead, the fusion of large microvesicles with the peripheral cell membrane has been observed (97) and fusion between microvesicles and the cellular plasma membrane might represent the preferential transfer mechanism of material from microvesicles to recipient cells.