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).