Fig. 2: Serum-starved human endothelial cells display large vacuolar networks containing membrane vesicles.

A Electron micrographs of normal (N) HUVECs representative of two independent experiments. B Representative electron micrographs of HUVEC exposed to SS for 1, 2, or 3 h showing autophagosome formation (A; arrow), fusion between lysosomes (L) and autophagosomes (A) with multivesicular bodies (MVB), and large autolysosome (AL) formation containing late multilamellar autophagosomes (left panel, arrows), degraded materials, and ApoExo-size vesicles (right panel arrow) (representative of two independent experiments). C Quantification of the cytoplasmic surface occupied by autophagosomes, lysosomes, multivesicular bodies, and autolysosomes on the cell surface in HUVECs exposed to N or SS for 1, 2, or 3 h. Autophagosomes are characterized by a double membrane encapsulating cytoplasmic material. Multivesicular bodies are characterized by the internal budding of the endosomal membrane forming intraluminal vesicles. Lysosomes are characterized by a single membrane containing electron-dense material. Autolysosomes, formed by the fusion between an autophagosome or an MVB with a lysosome, are enclosed by a single membrane and contain cytoplasmic material and organelles in various stages of degradation. Here, autolysosomes were also found to contain an array of small membrane vesicles. Fusion events between autophagosomes or MVBs and lysosomes were observed, resulting in the formation of large autolysosomes. All values are expressed as the mean ± SEM. P values were obtained by the Kruskal–Wallis test (*P < 0.05; **P < 0.01; ***P < 0.001). n = 50 cell profiles from two independent experiments. Scale bar = 500 nm.