PF-derived exosomes were labeled using Exo-Glow based on carboxyfluorescein succinimidyl diacetate ester (CFSE) chemistry (System Biosciences) according to the manufacturer’s recommendations. Human umbilical vein ECs (HUVECs) (Lonza) were seeded at a density of 5 × 104 cells/well on a 24-well plate coverslip, and 10 μg/ml of labeled PF-derived exosomes were added to target ECs in culture for 24 hr at 37°C. Cells were washed twice with PBS and fixed with 4% buffered PFA (Sigma) in PBS for 20 min at RT. Nuclei were stained by DAPI staining while actin filaments were labeled using Rhodamine Phalloidin (Thermo Fisher Scientific). To assess the uptake of exosomes by ECs, confocal images were acquired with a Leica SP5-AOBS confocal laser scanning microscope attached to a Leica DM I6000 inverted epifluorescence microscope. All images were collected using a 63× NA 1.4 oil immersion lens objective. The excitation signals for Exo-Glow and Rhodamine Phalloidin were 494 and 540 nm, respectively. The fluorescence emitted from the cells was recorded at 521 nm for Exo-Glow and 565 nm for Rhodamine Phalloidin. In all cases, z-stack images were obtained covering the entire cell volume. Three-dimensional reconstruction of the confocal image z stacks confirmed the cytoplasmatic localization of internalized exosomes.