## 4 MSC-Derived Extracellular Vesicles  
An important fraction of the MSC secretome is composed of EV. They are defined as nano-sized particles surrounded by a lipid bilayer, without the capacity to replicate [44]. They can be released by the majority of cell types, and can be found in a variety of fluids [38]. There are three subtypes of EV: microvesicles, exosomes and apoptotic bodies. They are differentiated according to their size, biogenesis, content, release pathway and function [45]. Exosomes measure between 30 and 150 nm. They have an endosomal origin, through the fusion of plasma membranes with multivesicular bodies [46]. Microvesicles measure between 100 nm and 1 \(\upmu\)m, and are formed by the evagination of the plasma membrane.  
Finally, apoptotic bodies range from 50 nm up to 5000 nm in diameter. They are released by dying cells, due to the increase in hydrostatic pressure. Thus, cell contractions cause the separation of plasma membranes from cytoskeleton [45]. For more information about the biogenesis of EV, readers can consult some reviews [45; 47].  
Likewise, EV may contain active molecules like proteins, peptides, carbohydrates, lipids, and nucleic acids (miRNA, mRNA, non-coding RNA, double-stranded DNA, and mtDNA) that can be delivered into neighbor and even distant cells, modulating different biological process. These cargos can be different, depending on parent cells, biogenesis as well as physiological conditions at the time of formation [45; 46; 48]. They are intensively explored, because not only play an important role in the normal physiological process, but also in the development and progression of diseases [42; 49]. Indeed, EV are involved in certain processes such as immune response, homeostasis maintenance, coagulation, inflammation, cancer progression, angiogenesis and antigen presentation [38].  
In fact, in recent years, interest in MSC-derived EV (MSC-EV), as a regenerative therapeutic tool, has increased. Their effects have been studied in several important diseases involving the kidney, liver, cardiovascular system, wound healing, as well as neurological and neurodegenerative ones, among others [38; 49]. Their use is considered a cell free-based therapy. Therefore, they overcome the safety limitations of whole-cell therapies. Among these limitations and undesirable effects xeno-contaminations caused by components of culture medium, including viruses. Likewise, unwanted physiological and differentiation changes may occur during in vitro expansion, prior to application. Also, the possibility of producing thromboembolism and fibrosis in the tissues where they are inoculated [50]. Conversely, MSC-EV offer the following advantages: (i) since they are not living and proliferative cell populations, problems related to immune compatibility, tumorigenicity, emboli formation and transmission of cell-borne infections are avoided; (ii) dosage and potency might be evaluated, such as conventional pharmaceutical agents; (iii) are stable and could be stored for long periods of time, without significant efficacy loss; (iv) collection processes are non-invasive, and different factors can be obtained changing culture media; (v) could be produced in large scales, using specific cell lines, in controlled conditions; (vi) could be engineered for carrying specific molecules, and labeled with certain surface proteins, to reach target cells; and (vii) show easy intravenous circulation, being even capable of crossing the brain barrier [17; 51; 52; 53].