Endothelial cells release microvesicles that harbour multivesicular bodies and secrete exosomes

Extracellular vesicles (EVs) released by endothelial cells support vascular homeostasis. To better understand cell EV biogenesis, we examined cultured human umbilical vein (HUVECs) prepared rapid freezing, freeze-substitution, and serial thin section electron microscopy (EM). Thin sections of HUVECs revealed clusters membrane protrusions on the otherwise smooth surface. The contained membrane-bound organelles, including multivesicular bodies (MVBs), appeared to be verge pinching off form microvesicles. Beyond peripheries, with internal MVBs were observed, confirmed that they not connected cells. These observations are consistent notion these multi-compartmented microvesicles (MCMVs) pinch-off from protrusions. Remarkably, omega figures formed fusion MCMV limiting directly apparently releasing exosomes MCMV. In summary, MCMVs a novel bud HUVEC surface, contain release exosomes. suggest can harboured within transiting after departure parent cell, constituting new site exosome biogenesis occurring potentially additional types.