Correlative light and electron microscopy is a powerful tool to study interactions of extracellular vesicles with recipient cells.

Extracellular vesicles (EVs) and their interactions with recipient cells constitute a rapidly growing research area. However, due to the limitations in current methodologies, the mechanisms of these interactions are still unclear. Microscopic studies of EVs are challenging, because their typical diameter is near the resolution limit of light microscopy, and electron microscopy has restricted possibilities for protein labelling. The objective of this study was to combine these two techniques to demonstrate in detail the interactions of EVs by recipient cells. Hyaluronan synthase 3 (HAS3) is an integral transmembrane protein that is enriched in EVs. In this work, GFP-HAS3 was utilized to study the interactions of EVs with the recipient cells. Surprisingly, confocal analysis correlation with scanning electron microscopy (SEM) revealed that most of the EVs were indeed lying on the recipient cell's plasma membrane, while the level of EV-derived intracellular signal was low. Immunoelectron microscopy supported this finding. Furthermore, hyaluronan oligosaccharides decreased the numbers of bound EVs, suggesting that CD44 participates in the regulation of their binding. This study indicates that correlative light and electron microscopy is a reliable method to analyze EV interactions with recipient cells. Detailed 3D confocal imaging of EV carrying a GFP-label on their plasma membrane combined with high-resolution electron microscopy provides significantly more information than either of the techniques alone. In the future studies it is crucial to utilize these techniques and their combinations to solve in detail the ambiguous fate of EV in target cells. Furthermore, live cell imaging at high resolution will be required to obtain definite answers on the detailed mechanisms of binding, fusion and endocytosis of EVs.