How extracellular vesicles are made
Biogenesis of exosomes and plasma membrane vesicles, why the two routes are hard to separate, and what that means for any product claim.
Extracellular vesicles reach the outside of a cell by at least two distinct routes. Exosomes are formed inside the cell by inward budding into a compartment called the multivesicular body, then released when that compartment fuses with the cell surface. Other vesicles bud directly outward from the plasma membrane and never pass through an internal compartment at all.
Both routes produce particles that overlap in size and in many of the markers used to detect them. This is the central practical problem of the field: the pathway defines the name, and the pathway is exactly what a routine isolation cannot demonstrate.
Two routes to the same destination
A cell has more than one way of putting a membrane-bound package outside itself, and the routes are genuinely different pieces of cell biology, not variants of one process.
The first route runs through the endosomal system. Material taken into the cell arrives in early endosomes. As those compartments mature, small regions of their limiting membrane bud inward, pinching off into the interior of the compartment and taking with them a selection of membrane proteins, lipids and cytoplasmic contents. The compartment now contains many small vesicles and is described as a multivesicular body. Its fate is a decision point. Fusion with a lysosome degrades the contents. Fusion with the plasma membrane releases them. The vesicles released by that second outcome are exosomes.
The second route is more direct. Regions of the plasma membrane bulge outward and pinch off, releasing a vesicle straight into the extracellular space. These particles are often called microvesicles or ectosomes. They are typically described as spanning a wider and larger size range than exosomes, but the ranges overlap at the small end, which is precisely the end that matters for products marketed on the exosome name.
The machinery, in outline
Inward budding at the endosome is associated with a set of protein complexes conventionally called the endosomal sorting complexes required for transport, along with accessory proteins. Additional mechanisms that do not require those complexes have also been described, including routes dependent on particular membrane lipids. The important reading point is that more than one mechanism operates, sometimes in the same cell, so vesicles from a single culture are not necessarily a single population even before isolation begins.
Cargo is not loaded at random. Sorting into an intraluminal vesicle involves recognition steps, and the composition of released vesicles differs from the composition of the parent cell. That selectivity is one of the more interesting findings in the field and one of the reasons the biology is taken seriously. It also means that the phrase "contains everything the cell contains" is wrong in both directions: vesicles concentrate some things and exclude others.
Why the two routes cannot be separated by centrifugation
Isolation methods sort particles by physical properties. Differential centrifugation sorts by sedimentation behaviour, which reflects size and density. Size exclusion chromatography sorts by hydrodynamic size. Filtration sorts by size. Precipitation reagents sort by solubility behaviour under altered conditions. None of these properties encodes biogenesis.
The consequence is unavoidable. A small extracellular vesicle preparation contains particles from both routes in proportions nobody has measured for that batch, unless the manufacturer went substantially beyond routine practice. Naming that preparation after one of the two routes is a choice of vocabulary, not a finding.
An isolation protocol based on size, density or precipitation yields a preparation of exosomes specifically.
- Proposed mechanism
- Exosomes are released into culture medium and concentrated by the chosen physical separation.
- What has been shown
- Physical separation methods enrich for particles within defined size or density bands. Published method comparisons consistently report that such bands contain vesicles of more than one biogenesis route together with non-vesicular material, and that the relative proportions differ substantially between methods applied to the same starting fluid.
- Highest level reached
- In vitro only
- Main confounders
- Method choice, medium composition, cell confluence, culture duration and clearing steps each alter what ends up in the final fraction. Serum-derived particles persist unless deliberately depleted.
GradeNOT SUPPORTED
What would change thisA separation step that selects on biogenesis rather than on physical properties, for example capture using a marker demonstrated to be specific to the endosomal route in that cell type, with the specificity established rather than assumed. Immunocapture using markers now known to be present on multiple vesicle types does not achieve this.
What changes the vesicles a cell releases
Vesicle release is not a fixed property of a cell line. It responds to conditions. Published work across many cell types reports that the quantity and composition of released vesicles shift with factors including oxygen tension, nutrient availability, cell density, mechanical environment, inflammatory signals and the age of the culture. Some of those shifts are large enough that two batches from the same cell bank grown under different protocols should not be assumed comparable.
This matters commercially for a reason rarely stated plainly. If a manufacturer changes a culture parameter to increase yield, the product may change as well as the amount. Yield and composition are not independent. A batch record that reports only particle count cannot detect this. We deal with the consequences in batch to batch variability.
| Property | Endosomal route (exosomes) | Plasma membrane route |
|---|---|---|
| Where formed | Inside the cell, budding into a multivesicular body | At the cell surface, budding outward |
| Release step | Fusion of the multivesicular body with the plasma membrane | Direct scission from the plasma membrane |
| Size | Small, conventionally tens of nanometres upward | Wider range, overlapping the small band at its lower end |
| Separable by size alone | No | No |
| Distinguished by common markers | Not reliably, several widely used markers appear on both | Not reliably |
| Named on most product labels | Yes | Rarely, though present in most preparations |
Apoptotic material, the category people forget
Dying cells release membrane-bound fragments as part of programmed cell death. These are a third category, generally larger, and generally removed by clearing steps. Generally is doing real work in that sentence. If a culture was stressed or harvested late, the proportion of material originating from dying cells rises, and the clearing steps that remove it are the same ones a manufacturer might soften to protect yield. A culture health record is therefore part of a meaningful specification, which is one reason we treat cell source and culture practice as inseparable in why the cell source matters.
What this section does and does not establish
It establishes that cells release membrane-bound particles by at least two well characterised routes, that cargo loading is selective, and that release responds to culture conditions. All of that is settled cell biology and none of it is controversial.
It does not establish that any particular preparation contains predominantly exosomes, that the particles survive any given handling regime intact, or that they reach any particular target tissue after application. Those are separate claims requiring separate evidence, and we treat them separately. The next step in the chain, what the particles carry, is covered in what is inside an extracellular vesicle. The step after that, whether a recipient cell does anything with them, is covered in how vesicles are taken up by cells.
Questions readers ask
What is the difference between an exosome and a microvesicle?
Their origin. An exosome forms inside the cell, budding into a multivesicular body, and is released when that compartment fuses with the cell surface. A microvesicle buds directly outward from the plasma membrane. They overlap in size and in several commonly used markers, so a routine preparation contains both.
Can a laboratory prove a preparation contains exosomes?
It can build a strong case using multiple orthogonal measures, including marker profiles, density, imaging and depletion controls. Proving biogenesis for particles already released is difficult, which is why consensus guidance in the field steers authors towards operational terms.
Do all cells release extracellular vesicles?
Release has been reported across a very wide range of cell types and organisms, and it is generally treated as a widely shared cell behaviour rather than a specialised one. The quantity and composition vary greatly between cell types and conditions.
Is vesicle cargo just a random sample of the cell?
No. Composition of released vesicles differs from the parent cell, with some molecules enriched and others excluded, which indicates active sorting. The mechanisms of that sorting are an active research area.
Does culture stress change what is released?
Published work reports that vesicle release and composition respond to conditions including oxygen tension, nutrient state, cell density and inflammatory signals. This is a reason to treat culture protocol as part of product identity rather than as a background detail.