Imaging extracelluar vesicles by transmission electron microscopy: Coping with technical hurdles and morphological interpretation

Extracellular vesicles (EVs) are cell-derived nanometric particles governing the complex interactions among cells through their bioactive cargo. Interest in EVs is rapidly increasing due to their extensive involvement in physiological and pathological conditions, their potential employment as diagnostic and therapeutic tools and their prospective use as bio-carriers of exogenous molecules. Given their nanometric size, transmission electron microscopy (TEM) provides significant contributions to assess EV presence and purity in a sample and to study morphological features.In this review, TEM methods for EV imaging are compared with respect to their applications, benefits and drawbacks. A critical evaluation of the actual contribution of TEM to the study of EVs is also provided and the most common artifacts encountered in the literature are discussed.TEM techniques are powerful tools for the investigation of EVs and have the potential to reveal sample purity, ultrastructure and molecular composition. However, technical challenges, procedural errors in sample processing or misinterpretations may result in a variety of different morphologies and artifacts.The last decades have seen exponential technological progress in EV imaging by TEM. Nevertheless, protocols have not been standardized yet and sample preparation remains a critical step. An optimized, standardized and integrated protocol of different techniques could minimize artifacts and interpretative errors that could significantly improve the quality and reliability of downstream studies.