What exosomes are
A short course in the biology, written so that the rest of this site makes sense. No prior science needed.
The short version
An exosome is a tiny bubble of membrane that a cell releases to deliver a package of molecules to another cell.
Almost every cell in the body makes them. They are typically 30 to 150 nanometres across, which is far smaller than a cell and smaller than most bacteria. Under an electron microscope they look like cup-shaped spheres.
The word you will see in research papers is extracellular vesicles, or EVs. Exosomes are one type of EV, defined by how the cell makes them. In consumer products the word "exosome" is used loosely for any small vesicle preparation, so it helps to know what a label actually means.
How a cell makes one
This is the part that separates exosomes from other vesicles.
Cells constantly fold their outer membrane inward to bring material in. Some of those inward folds become a compartment called an endosome. Inside the endosome, the membrane folds inward a second time, pinching off small vesicles into its interior. At this stage the structure is called a multivesicular body.
The multivesicular body can then fuse with the outer membrane of the cell and release its contents outside. Those released inner vesicles are exosomes.
Because the membrane was folded twice, the outside of an exosome matches the outside of its parent cell. That is why exosomes carry surface proteins from the cell that made them, and why a receiving cell can recognise and take them in.
Other EVs form differently. Microvesicles bud directly off the cell surface and are usually larger. Apoptotic bodies are fragments of dying cells. When a product says "exosomes" without describing how the vesicles were isolated and characterised, it may contain a mix of all three.
What is inside
The cargo depends on the cell that made the exosome and the state that cell was in.
- Membrane
A lipid bilayer, rich in cholesterol and sphingolipids, that protects the cargo and lets the vesicle merge with a target cell.
- Surface proteins
Tetraspanins such as CD9, CD63 and CD81 are the most common markers, and labs use them to confirm that a preparation contains exosomes. Adhesion proteins on the surface influence which cells the vesicle binds to.
- Proteins
Enzymes, signalling proteins, growth factors and cytokines. The set varies widely by cell source.
- RNA
Mostly small RNAs, including microRNAs that can change which genes a receiving cell switches on or off. This is the mechanism that gets the most research attention.
- Lipids and metabolites
Signalling lipids and small molecules that reflect the parent cell's condition. This is one reason exosomes are being studied as diagnostic markers.
Exosomes, stem cells and growth factors
These three are often mentioned together and often confused.
| Stem cells | Exosomes | Growth factors | |
|---|---|---|---|
| What it is | A living cell that can divide and become other cell types | A membrane vesicle released by a cell; not alive, cannot divide | A single protein that signals cells to grow or change |
| Size | 10–30 micrometres | 30–150 nanometres | A few nanometres |
| Contains | Full genome, organelles | A selected mix of proteins, RNA and lipids | One molecule |
| In consumer skincare | Not used; live cells are not in cosmetics | Used, usually from plant, cultured human, or animal cells | Used, often recombinant |
| US regulatory status | Biologic; approved uses are narrow | Cosmetic if topical; unapproved if injected | Cosmetic if topical |
Terms you will see
A glossary for reading labels, clinic websites, and papers.
- EV
Extracellular vesicle. The umbrella term. Most careful labs say "EV" rather than "exosome" unless they have confirmed the origin.
- Secretome
Everything a cell secretes, including exosomes, free proteins and metabolites. "Conditioned media" is the liquid the cells were grown in and contains the secretome.
- MSC
Mesenchymal stem cell, or mesenchymal stromal cell. The most common human cell source for exosome products, often from umbilical cord, fat or bone marrow.
- Plant-derived
Vesicles isolated from plants such as ginger, grape or rose. These are structurally similar but are not the same as human exosomes and carry different cargo.
- Particle count
The number of vesicles per millilitre, usually measured by nanoparticle tracking analysis. It counts every particle of the right size, not only exosomes, so it is a rough guide rather than a purity measure.
- Lyophilised
Freeze-dried. Used to make exosome powders that are more stable at room temperature than liquid preparations.
- MISEV
Minimal Information for Studies of Extracellular Vesicles. Guidelines from the International Society for Extracellular Vesicles that describe how a preparation should be characterised. A product that cites MISEV-style testing is being more careful than one that does not.
- Xeno-free
Grown without animal-derived ingredients such as fetal bovine serum. Relevant for allergen and contamination concerns.