What is inside an extracellular vesicle

The three cargo classes

Descriptions of vesicle contents fall into three broad classes, and each has a different evidential status when a company presents it.

Proteins

Vesicles carry membrane proteins embedded in the bilayer, proteins associated with the membrane surface, and soluble proteins in the lumen. Several protein families appear so consistently across preparations that they became conventional markers, including certain tetraspanins and cytoskeletal and heat shock proteins. Their usefulness as markers is real but limited, because consistent appearance across vesicle types is exactly what makes a marker poor at distinguishing between vesicle types.

Lipids

The vesicle membrane is not simply a sample of the plasma membrane. Published lipidomic work reports characteristic enrichment of particular lipid classes relative to the parent cell. Lipid composition affects membrane rigidity and stability, which in turn affects how a preparation behaves during storage, freezing and application. This is the cargo class least discussed in commercial material and arguably the most relevant to whether a product survives its own supply chain.

Nucleic acids

Vesicle preparations contain RNA, including short regulatory species such as microRNA, along with fragments of longer transcripts. This is the cargo class that generates the most excitement, because a regulatory RNA delivered into a recipient cell offers a clean and satisfying mechanism for changing that cell's behaviour. It is also the class where the gap between detection and function is widest.

Why detection and delivery are different claims

Suppose a preparation is analysed and a set of microRNA species is reported. What has been established is that those molecules are present in the sample. Four questions remain open, and each requires separate work.

1. Are they inside vesicles? Nucleic acid can be bound to protein outside a vesicle. The test is whether the signal survives treatment with an enzyme that degrades free nucleic acid, and disappears when the membrane is disrupted first. Without that control, the location is assumed.
2. Are they delivered? Uptake of a vesicle by a cell does not guarantee release of contents into the cytoplasm. Material can be routed to degradation.
3. Are they functional on arrival? A molecule can be present and inactive.
4. Are they present in sufficient copies? This is the quietest and most serious issue. Published analyses have raised the question of how many copies of a given regulatory RNA are carried per vesicle on average, and whether typical numbers are compatible with the effects attributed to them. We are not able to source a specific figure and will not reproduce one, but the question is live in the literature and any careful reader should know it exists.

Regulatory RNA carried by extracellular vesicles is delivered into recipient cells in quantities sufficient to change gene expression in skin.

Proposed mechanism

Vesicles fuse with or are taken up by recipient cells, releasing microRNA that suppresses translation of target messenger RNA.

What has been shown

Transfer of vesicle-associated nucleic acid to recipient cells has been demonstrated in laboratory systems, typically using labelled material or reporter constructs at high vesicle to cell ratios. Whether the copy numbers involved at physiological or product-realistic doses are sufficient to produce a functional change is contested in the literature.

Highest level reached

In vitro only

Main confounders

High doses used in vitro relative to any plausible in vivo exposure. Label transfer can occur without functional cargo delivery. Co-isolated free protein and RNA can produce effects attributed to vesicles.

GradeCONTESTED

What would change thisQuantitative work reporting copies of a specified RNA per vesicle, the fraction of vesicles carrying it, and functional readouts at doses matched to what a recipient tissue would plausibly encounter, with depletion controls showing the effect disappears when the specific cargo is removed.

Cargo lists are cheap

Modern analytical platforms will return long lists of detected proteins and RNA species from a small sample. Producing such a list is now routine. It is worth understanding what that means for marketing material: a document showing hundreds of detected molecules represents a straightforward analytical run, not an unusual scientific achievement, and it says nothing about function.

The same is true of growth factor panels. If a preparation is assayed for a set of growth factors and several are detected, the result establishes presence in the sample. It does not establish that they are inside vesicles, and if they are outside, the product is functionally a conditioned medium concentrate with vesicles in it. That is a legitimate thing to be. It is a different thing from what the label usually says. We look at that category directly in conditioned media and the secretome.

Dose, the unglamorous problem

Any mechanism that depends on delivering molecules faces an arithmetic constraint. The number of molecules that arrive is the product of how many vesicles arrive, how many copies each carries, and what fraction of contents reaches the right compartment. Each of those three terms is uncertain, and multiplying three uncertain numbers produces a very uncertain result.

This is not an argument that nothing happens. It is an argument that the observed effects in laboratory systems, where vesicle to cell ratios are chosen by the experimenter and can be extreme, should not be read across to a product applied to skin without an explicit dose argument. We set out why counts are not doses in why nanoparticle count is not a dose.

What the cargo picture supports

Taken together, the cargo literature supports a reasonably strong general statement: vesicles are compositionally distinct from their parent cells, carry material capable in principle of affecting recipient cells, and differ between cell sources and culture conditions. That is a real and useful finding.

It does not support product-level claims about which molecule produces which clinical effect. When a supplier attributes an outcome to a named growth factor or a named microRNA in its preparation, the correct response is to ask which study established that specific attribution, in what system, at what dose, and with what depletion control. In our experience of reading this material, the attribution is usually inferred from a cargo list rather than demonstrated.

Questions readers ask

Do exosomes contain DNA?

Nucleic acid found in vesicle preparations includes RNA species and DNA fragments in some reports. As with all cargo, detection in a preparation does not by itself establish that the material is enclosed within vesicles rather than associated with their surface or co-isolated separately.

Are growth factors inside the vesicles?

Some may be. Growth factors detected in a preparation may also be free in solution, having been secreted by the cells conventionally. Distinguishing the two requires separation or protection experiments, which are not always reported.

Is a long cargo list a sign of a better product?

No. Detection lists are routine to generate and their length reflects analytical sensitivity as much as biology. A short, well controlled characterisation with location and quantity established is more informative than a long list of detections.

Does the lipid composition matter?

It affects membrane stability, which affects how a preparation survives freezing, thawing, storage and application. It is the cargo class least often discussed commercially and it bears directly on whether a product arrives intact.

Can vesicle cargo be engineered?

Loading vesicles with chosen molecules is an active research area in drug delivery. Whether that is achievable at scale, and how such a product would be regulated, are separate questions from anything currently marketed in aesthetics.