Detection of extracellular vesicles: size does matter

Cells release small sacks filled with fluid, which are called "extracellular vesicles". The diameter of extracellular vesicles (EV) typically ranges from 30 nm to 1 µm. Because cells release EV into their environment, our body fluids contain numerous EV. Cells release EV to remove waste and to transport cargo to other cells. Since the size, concentration, cellular origin, and composition of EV in body fluids change during disease, EV have promising clinical applications, such as diagnosis of cancer. However, clinical applications of EV are not realized yet, because currently used detection techniques lack the sensitivity to detect the majority of EV. The aim of this thesis is to improve the detection of EV by (1) obtaining insights into physical properties of EV, and (2) gaining a profound understanding of techniques to detect EV. This thesis explains why the reported concentrations of EV in human blood plasma differ 100,000,000-fold. In addition, this thesis shows that flow cytometry detects large vesicles and swarms of smaller vesicles, which both are counted as a single event signal. The relationship between light scattering and the diameter of EV is modelled using Mie theory to demonstrate that a currently widely applied standardization procedure for EV detection selects EV and cells with a diameter of 800-2,400 nm instead of the envisioned 500-900 nm. Furthermore, a method based on nanoparticle tracking analysis is developed to determine the size and refractive index of single EV and other nanoparticles.