Exosomes
Exosomes are membrane-bound vesicles ranging from 30-120 nm in diameter, and are released upon fusion of multi-vesicular bodies with the cell membrane (64,65,66). The contents of exosomes include proteins, nucleic acids, lipids and sugars, and they are thought to be involved in intercellular communication (67,68,69,70,71). Exosomes are endosome derived extracellular vesicles. Early endosomes are formed from internal budding of the cell membrane with accompanying membrane proteins and lipids into the cytoplasm. As the endosome matures, there is transport of cytosolic, golgi and nuclear cargo such as nucleic acids, proteins and other metabolites into the lumen of the endosome. There is subsequently intravesicular budding of the endosomal membrane, forming multivesicular bodies (MVB). The MVB fuses with the cell membrane releasing its vesicles as exosomes into the extracellular space. Involvement of the endosomal pathway differentiates exosomes from microvesicles and apoptotic bodies which are the other extracellular vesicles. Microvesicles and apoptotic bodies are larger vesicles formed from outward budding of the cell membranes from living and dying cells respectively, into the extracellular space (72)
Mechanisms by which recipient cells take up exosomes include macropinocytosis, receptor or lipid raft mediated endocytosis, phagocytosis, or direct fusion with the recipient cell membrane (73). Exosomes are found in many body fluids including blood, urine, cerebrospinal fluid, amniotic fluid, saliva, breast milk, peritoneal fluid, etc. (74).