What an exosome actually is, and how the word is used commercially

The word has a strict meaning and a loose one

Two people can use the word exosome in the same sentence and mean different things. In cell biology it names a particle defined by how it was made. In commercial aesthetics it usually names a particle defined by where it was found, which is to say in the fluid a cell culture was grown in. Those are not the same claim, and the difference is not pedantry. It determines what you can reasonably infer about the contents of a vial.

The strict definition rests on biogenesis. A cell takes part of its own membrane inward to form an internal compartment. Inside that compartment, further inward budding produces small vesicles sitting in the lumen. The compartment, now full of them, is called a multivesicular body. If it fuses with the outer cell membrane rather than being sent for degradation, its contents are released into the space outside the cell. Those released intraluminal vesicles are exosomes.

The loose definition is a description of a procedure. Grow cells. Collect the fluid. Remove cells and large debris. Concentrate what remains. Call the result exosomes. Everything in that sequence can be done competently and still not entitle anyone to the word, because the procedure does not distinguish an intraluminal vesicle released from a multivesicular body from a vesicle that budded directly outward from the cell surface, nor from a lipoprotein particle, nor from a protein aggregate of similar size.

Why the field moved to a broader term

The International Society for Extracellular Vesicles publishes a consensus document, the Minimal Information for Studies of Extracellular Vesicles, updated periodically, which sets out what a study should report before it uses a specific subtype name. The direction of travel in those guidelines is consistent: unless you can show the pathway, use the generic term extracellular vesicle and describe the preparation by the operational properties you actually measured, such as size range, density, and the markers present.

That is a discipline about honouring uncertainty, not a rhetorical retreat. If a paper says small extracellular vesicles, it is telling you something narrower and more reliable than a paper that says exosomes without evidence. Read the methods and you can usually tell which kind of paper you have within a paragraph.

What is actually in a preparation

A preparation from conditioned culture medium is a mixture. The proportions depend on the cell type, the culture conditions, the medium used, how long the cells were left, and above all the isolation method. The categories that turn up are well described in the literature:

• Vesicles from the multivesicular body route. The particles the word exosome properly names.
• Vesicles budded from the plasma membrane. Formed by a different mechanism, overlapping in size with the first group, and not separable from them by size alone.
• Larger vesicles and fragments. Removed to varying degrees depending on how aggressively the preparation was cleared.
• Non-vesicular material. Lipoprotein particles, protein complexes, nucleic acid bound to protein rather than packaged inside a membrane, and residues of the culture medium itself, including serum components if serum was used.

The last category is the one that gets least attention in commercial material and most attention in careful papers, because it is the category that can carry biological activity while having nothing to do with vesicles at all. If an effect is observed and attributed to exosomes but the preparation was never depleted of free protein, the attribution is not established. It might still be right. It is not established.

A product labelled as containing exosomes contains particles formed by the exosome biogenesis pathway.

Proposed mechanism

Cells release intraluminal vesicles from multivesicular bodies into culture medium, from which they are concentrated by the manufacturer.

What has been shown

That cultured cells release such vesicles is well established in cell biology. That any specific commercial preparation is enriched for them, rather than for a mixture of vesicle types plus non-vesicular material, is a manufacturing claim that depends entirely on the isolation and characterisation performed for that product.

Highest level reached

In vitro only

Main confounders

Isolation method determines composition more than cell type does. Size-based methods cannot separate biogenesis pathways. Serum in the culture medium contributes vesicles and lipoproteins of its own unless specifically depleted.

GradeNOT ASSESSABLE

What would change thisProduct-specific characterisation published against the current consensus reporting framework, including marker profiles for both expected and unexpected populations, particle to protein ratio, and evidence that non-vesicular material was depleted rather than merely diluted. Without that, the claim is not a scientific statement about the product, it is a name for it.

Size does not settle it

Reported diameters for small extracellular vesicles overlap heavily between subtypes, and the commonly quoted range is a convention rather than a boundary in nature. Two consequences follow that matter when reading a specification sheet.

First, a particle size distribution alone tells you the preparation contains particles in a size band. It does not tell you they are vesicles. Nanoparticle tracking instruments count light scattering events; a protein aggregate scatters light too. Second, an average diameter compresses a distribution that may be wide, multimodal, or dominated by a population nobody intended to isolate. A single number in a marketing document is the least informative form the data can take.

Why the distinction has practical weight

If the active principle in a preparation is a vesicle, then membrane integrity, storage temperature, freeze and thaw cycles and route of application all matter enormously, because a ruptured vesicle is no longer a vesicle. If the active principle is a soluble protein that happened to be co-isolated, most of those handling concerns fall away and different ones appear. You cannot design sensible handling rules for a product whose active principle has never been identified.

This is also why the regulatory question is difficult in a way that has nothing to do with bureaucratic caution. A regulator asked to assess a product needs to know what the product is, what varies between batches, and what the intended biological action is. A preparation defined by its isolation procedure rather than by its contents makes each of those questions harder to answer. We set out the current position in the UK position on exosome products.

What a careful reader does with the word

Treat exosome as a flag, not a fact. When you encounter it, ask three questions in order. What cell released the material? By what method was it isolated? What characterisation was done, and against what reference framework? A supplier that can answer all three is telling you about a product. A supplier that can answer none is telling you about a category.

None of this means the biology is uninteresting. Cells clearly do release membrane-bound packages carrying protein, lipid and nucleic acid, those packages are clearly taken up by other cells, and the consequences are an active and legitimate research field. The gap sits between that biology and the specific claim that a given vial will produce a given result in skin. We look at how that gap is normally argued in paracrine signalling explained, and at how to judge the studies offered to close it in in vitro, in vivo, in human.

Questions readers ask

Is exosome the wrong word to use?

It is not wrong when biogenesis has been demonstrated. It is unsupported when it is applied to any preparation isolated from culture medium without that demonstration. The safer general term is small extracellular vesicle, which describes what was measured without asserting how the particles were formed.

Are exosomes cells or stem cells?

Neither. An exosome is a particle released by a cell. It has no nucleus and cannot divide or replicate. Products described as cell free are describing exactly this: material derived from cells, without the cells themselves. That distinction carries regulatory weight as well as biological weight.

Does the size of the particles tell you what they are?

No. Size ranges overlap between vesicle subtypes, and non-vesicular material such as lipoprotein particles and protein aggregates falls within the same band. Size distribution is one input to characterisation and is not sufficient on its own.

Why do some papers say small extracellular vesicles instead?

Because that phrase claims only what the method can support. Consensus reporting guidance in the field discourages subtype names unless the study demonstrates the biogenesis route, so careful authors default to the operational term.

Does any of this mean exosome products do not work?

No. It means the label does not by itself tell you what is in the vial or what it will do. Whether a specific preparation produces a specific clinical effect is a separate question, answered by clinical evidence rather than by nomenclature. We grade those claims individually.