# Origin and Composition of Exosomes as Crucial Factors in Designing Drug Delivery Systems
## 5 Exosomes Natural Cargo and Structure  
Exosomes are generally recognized as cell-to-cell signaling molecules containing numerous proteins, nucleic acids, cytokines, transcription factors, and other cell-derived particles (Figure 2). Exosomes may deliver information not only to the local environment but also to greatly distanced cells and tissues. This may serve as a general signaling and communication pathway, but may also take part in oncogenes spread and further tumor progression and malignancy increase [62]. To this date, exosomes presence was confirmed in body fluids such as amniotic liquid, blood serum, breast milk, epididymal fluid, saliva, urine, effusions of ascites and pleural, bronchoalveolar lavage fluid, synovial fluid, and cell culture supernatant in vitro [63; 64; 65; 66]. Due to these facts, exosomes are proposed as novel therapeutic and diagnostic (theranostics) devices [6]. The excreted exosomes also serve as a way of unnecessary proteins and nucleic acids removal, e.g., during cell maturation (reticulocytes) or excretory system (guts and tubules of the kidney) [67; 68; 69]. For proper usage of exosomes as a DDS, we first have to consider their natural cargo and the routes of their formation. Then, the characteristics of exosomes affinity and cell uptake should also be described. Exosomal cargo exhibit different purposes and functions, thanks to exosomes being basic transporters presenting outstanding properties themselves. This can be utilized in cell targeting and also as an additional enhancement of drugs. To date, ExoCarta estimates about 10,000 proteins, 3500 mRNA, and over 1100 distinct lipids found in exosomes. For the systematic review, many great databases arise such as Vesiclepedia and ExoCarta were exosome isolation procedures and sources are described along with identified cargo [70; 71]. The most important proteins for DDS design are those influencing exosomes transport and cell intake. Proteins characteristic for exosomes are membrane-bound tetraspannins (CD9, CD63, CD81, and CD82), as well as EpCAM and Rab5, used routinely for isolation. Other commonly recognized proteins are receptors (CD46 and CD55), heat shock proteins (HSP; Hsc70, Hsp70, and Hsp90), proteins taking part in exosomes formation (Alix, TSG101), and membrane proteins responsible for fusion and transport (GTPases, annexins, and flotillin; ATP7A, ATP7B, MRP2, SLC1A4, SLC16A1, and CLIC1) [6; 44]. Detection of these markers with ELISA is used for confirmation of exosomes presence. There are also reports about exosomes containing other particles such as cholesterol (B lymphocytes derived), sphingolipids, phosphoglycerides, and ceramide. The presence of the latter also distinguishes the exosomes from the lysosomes [72]. Exosomes also transport morphogens (Hedgehog, Wingless, and Wingless-like) taking part in tissue patterning development described for _Drosophila melanogaster_[64]. Nucleic acids are another major group of particles found in exosomes, namely DNA and RNA, which onward can take the functional role in cells. About \(\sim\)1300 mRNA, 121 miRNA, and \(>\)100 small RNA were found in exosomes derived from mouse and human mast cells, while no DNA and rRNA were detected [73]. Other miRNAs found in exosomes were lin-4 and let-7, as well as miR-181a and miR-155 in breast milk [73; 74]. Some of the exosomal miRNAs can be used as biomarkers in diagnostics (Table 2).