We thank Quanding Yan et al. for their valuable comments regarding our recent article identifying the finding that adipose tissue-derived extracellular vesicles isolated through a neo-mechanical protocol promote enhanced angiogenesis and adipogenesis, and could serve as a promising biomaterial for adipose tissue engineering when combined with adipose tissue extracellular matrix hydrogel.1
Quanding Yan et al. want to know what the biological activity of adipose tissue–derived extracellular vesicles (ATEVs) was after decellularized adipose tissue hydrogel injection. This is a limitation of our study, because tracking extracellular vesicles in vivo is a technical difficulty. We tried to trace extracellular vesicles in vivo using traditional tracing methods, such as labeling vesicles with PKH26, but we neither clearly observed the location of the vesicles nor confirmed whether they were internalized by the cells under a confocal fluorescence microscope. As far as we know, extracellular vesicles are essentially different from cells in their biological characteristics. Extracellular vesicles are nanoscale complexes wrapped in a double-layer phospholipid membrane that includes proteins and genetic material, but not nuclei and organelles.2 Thus, the biological activity of extracellular vesicles has a certain time limit, although few researchers have confirmed action time. It would be more convincing if the biological activity of ATEVs was verified. Further studies will be conducted to address this point.
Next, Quanding Yan et al. asked, if most ATEVs survive, whether it shows that decellularized adipose tissue hydrogel possesses biological activity, such as promoting adipogenesis, stimulating ATEV proliferation, or something else. Further study of biological mechanisms may be needed. As we discussed above, extracellular vesicles do not possess nuclei and organelles. Therefore, ATEVs cannot proliferate or “survive” because they are just a kind of complex of proteins and RNAs. In addition, the potential to promote adipogenesis has been verified by other studies.3–5 However, there are still limitations, as the controlled-release ability of decellularized adipose tissue hydrogel for ATEVs has not been verified.
Finally, the authors would like to discuss the optimal method of separating ATEVs. The ultracentrifugation technique was used for ATEV isolation, but it is too time-consuming for extracellular vesicle isolation. We believe that the tangential flow filtration technique is more suitable for the isolation of ATEVs because of its high-efficiency and large-volume filtration system.6,7 Furthermore, use of the tangential flow filtration technique to isolate ATEVs may effectively avoid the rupture and aggregation of extracellular vesicles caused by excessive ultracentrifugation.6,7
In conclusion, several points we discuss above still need to be addressed before the clinical application of ATEVs can be achieved. For the following research, we also have an innovative hypothesis. In addition to stem cells, a variety of cytokines can also be extracted from adipose.8 If free growth factors in adipose liquid extract could be separated and concentrated together with ATEVs, a stronger product, including both ATEVs and growth factors, could be made for tissue repair and regeneration.
The authors have no financial interest or conflicts of interest to declare in relation to the content of this communication. The authors received no funding support for this work.
Yuan-zheng Zhu, M.D.
Yang-yan Yi, M.D.
Department of Plastic Surgery
The Second Affiliated Hospital of Nanchang University
Nanchang, Jiangxi, People’s Republic of China
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