Cells actively secrete nano-vesicles called exosomes via an endocytic pathway. Exosomes are found in the extracellular space and all body fluids, including blood, urine, and saliva. The molecular content of exosomes is derived from the cells that release them. The frequency and amount released depends on the cell type, such as normal or cancerous, and environment, such as the level of dissolved oxygen and pH. They carry small non-coding RNAs, though the number of microRNA (miRNA) molecules enveloped inside each exosome may vary. Some reports suggest that the majority of miRNAs in the blood are contained within exosomes. By fusing with recipient cells and releasing their RNA and other cargo, exosomes are thought to play a role in short and long-range intercellular signaling.
Exosomes can be differentiated from other circulating vesicles by the markers of the endosomal pathway and their small size that distinguishes them from other microvesicles. Though an important differentiator, their reported size and shape vary substantially, which likely reflects the difference in cells that secreted them, utilized isolation techniques and the methods used in their characterization. We analyzed the influence of the characterization method on the measured size and shape of hydrated and desiccated exosomes. It was found that hydrated exosomes are close to spherical with a hydrodynamic size that is significantly larger than the geometric size. In addition, surface fixation prior to analysis in liquid that is required for techniques such as atomic force microscopy significantly impacts the size and shape of exosomes. For desiccated exosomes, it was found that the shape and sizing are influenced by the manner in which drying occurred. Isotropic desiccation in aerosol preserves the near spherical shape of the exosomes, while drying on a surface likely distorts their shapes and influences sizing results.
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