Understanding Extracellular Vesicles

Extracellular vesicle terminology is used loosely, including by people who should know better. The distinctions are not pedantic — they describe different biogenesis pathways, different cargo-loading mechanisms and different isolation behaviour.

Three populations, three origins

Exosomes (roughly 30–150 nm) originate inside the cell. Inward budding of the endosomal membrane forms intraluminal vesicles within a multivesicular body; when that body fuses with the plasma membrane, its contents are released. This endosomal origin is why exosomes are enriched in tetraspanins such as CD9, CD63 and CD81, and in ESCRT-associated proteins.

Microvesicles (roughly 100–1000 nm) bud directly outward from the plasma membrane and consequently carry a surface composition closer to it. Apoptotic bodies (1–5 µm) form during programmed cell death and are largely a disposal mechanism.

Because these size ranges overlap, size alone cannot assign origin. This is why MISEV recommends the operational term 'extracellular vesicle' with declared size and marker characterisation, unless biogenesis has actually been demonstrated.

Cargo is selected, not sampled

Vesicles do not contain a random sample of cytoplasm. Their cargo — proteins, lipids, mRNA, microRNA and other non-coding RNA — is selectively enriched relative to the parent cell, with specific microRNAs concentrated well above their intracellular abundance. Sorting is directed by ESCRT machinery, tetraspanin-enriched microdomains and ceramide-dependent pathways.

The demonstration that vesicle-delivered mRNA is translated in recipient cells, and that delivered microRNA silences target transcripts, established that this is functional intercellular communication rather than debris clearance.

Isolation determines what you get

Differential ultracentrifugation is the historical reference method: scalable in principle, but co-pelleting protein aggregates and lipoproteins and capable of damaging vesicles. Size exclusion chromatography better preserves integrity and separates soluble protein, at lower concentration. Density gradient separation offers the highest purity and the lowest yield. Precipitation kits are fast and convenient but co-precipitate substantial contaminant. Immunoaffinity capture is highly specific but selects for the marker chosen.

No method is universally correct, and every method biases the population recovered. What matters is that the method is declared, because a preparation cannot be interpreted without it.

The minimum that must be reported

Particle concentration and size distribution by a stated method. Presence of transmembrane and cytosolic markers. Absence of expected negative markers. Source cell identity and culture conditions. Isolation method and storage conditions. A preparation lacking these has been produced but not characterised — and the difference is the entire basis on which a physician can evaluate it.