Blog highlights:

• “Exosomes” are gaining visibility in cosmetics, wellness, and clinical research, but the term is sometimes used more broadly than precisely.
• In cell biology, exosomes refer to a narrowly defined class of extracellular vesicles, distinct from other particles secreted by cells.
• Distinguishing exosomes from other extracellular particles requires multiple markers, orthogonal assays, and careful reporting, leading many experts to favor the broader terms “EVs” (extracellular vesicles) or “EPs” (extracellular particles)
• As this field matures, transparency in naming, sourcing, and characterization will be key, bridging semantics to real therapeutic translation with platforms like RoosterBio’s suite of EV analytics, bioprocess development services, and products.

“Cottleston Cottleston Cottleston Pie,
A fly can’t bird, but a bird can fly.
Ask me a riddle and I reply
Cottleston Cottleston Cottleston Pie.”

-Winnie the Pooh

Like birds in the park, exosomes are “floccing” together, practically inescapable these days! And as Winnie the Pooh once noted (quote above), they are wrapped in riddles in how we might speak about them. You can find them promoted in skincare clinics [1] or human trials [2] as the “secret sauce” for microneedling. They’re blended into cosmetic serums for use on the epidermis, or pitched to physicians and patients [3] under U.S. federal “right-to-try” or expanded access therapies, or in places like Florida or Utah. Meanwhile, investors chase the gold rush of extracellular-vesicle (EV) startups, [4] and clinicians at translational centers are now asking how to source ready-to-use, qualified raw materials for first-in-human trials. [5, 6] Since their discovery, academic labs have probed exosomes’ underlying biology, including therapeutic mechanisms such as immune modulation via CD73. [7, 8] Yet blended with the word “exosome” is an enigma of stringent scientific nomenclature, marketing shorthand, and muddy semantic landscape that leaves even the most engaged audiences baffled: what exactly are we talking about? [9]

Getting Technical with Exosomes/EVs & Extracellular Particles (EPs)

Bioscientists use the word exosome in a much tighter sense than many popular uses. [10, 11] In a cell biology laboratory, exosomes are defined by their inclusion within a highly specialized subset of extracellular vesicles, i.e., small, lipid–bound particles released from cells (see Figure 1, below).  The MISEV Consortium does not want to stifle innovation and research into extracellular vesicles, but does suggest “In general, ISEV recommends use of the generic term ‘EV’ and operational extensions of this term instead of inconsistently defined and sometimes misleading terms such as ‘exosomes’ and ‘ectosomes’ that are associated with biogenesis pathways that are difficult to establish.”  [12]

But WAIT!! [*cue vinyl record scratch effect] …It’s important to also clarify here that when we talk about exosomes in this context, we mean the vesicle, not the “exosome complex” of quality control enzymes that degrade RNA inside the cytoplasm. [13] The Complex is a protein machine involved in RNA turnover, while the “other exosomes” carry lipids, proteins, and nucleic acids out of and between cells. Yet both kinds of exosomes may be critical for the recycling of cellular raw materials.

Figure 1. Suggested Venn diagram to depict the hierarchical classification of different species of extracellular particles (EPs), enveloped (EVs) and non-enveloped (NVEPs).  Technically speaking, “exosomes” are a highly specialized category of lipid vesicle, notable by their biogenesis pathway.

Exosomes are distinguished foremost by their origin story. [11] Lipid compartments first invaginate into nascent intraluminal vesicles (ILVs) via the membranes of specialized, late endosome multivesicular bodies (MVBs). Picture a bubble pinching itself off within a larger bubble (the cell). Now imagine that smaller bubble (the endosome) swallowing parts of its own surface, creating lots of smaller bubbles within, i.e., as an MVB. When this MVB later fuses with the plasma membrane, the tiny “grandkid” ILV vesicles are then released into the extracellular space as new exosomes. They exhibit the opposite topology of an endocytic vesicle, since their prior budding events preserve plasma membrane orientation where the exosome’s outer surface corresponds to the cell surface. Thus, an exosome lumen contains a dram of cell cytosol, like a “mini-cell,” only virus-sized.

The lower pH environment of an MVB is favorable to the activation of proteases that are adapted for MHC Class II antigen processing in professional antigen presenting cells (APCs) like dendritic cells. [14] This subcellular compartmentalization can impact prospective immunotherapies that harvest such secreted exosomes into a drug product. [15, 16, 17, 18] Nevertheless, exosomes are biosynthesized and secreted by virtually all cell types. Their generic function is still a matter of debate, however. Hypotheses range from the elegant as per intercellular communication [19, 20] to the mundane, e.g., cellular garbage disposal. [21] Perhaps both are true in different contexts? [22]

The intraluminal vesicle trafficking process is powered by the cell’s endosomal sorting complexes required for transport (ESCRT), a set of proteins that sculpt and pinch off the vesicles, with help from accessory molecules such as ALIX and syntenin. Small Rab-family GTPases, particularly Rab27a and Rab27b, then signal the multivesicular body toward the plasma membrane and orchestrate its fusion. Instead of being degraded in a lysosome to recycling of cellular raw materials within, the cargo-filled body with its proteins, lipids, and RNA opens to the outside world, releasing its collection of intraluminal cargoes as exosomes. [11, 23]

Retrotransposons and/or retroviruses, including HIV, can exploit the same machinery (i.e., by non-canonical, yet ESCRT-dependent), budding from cells in a manner that’s sometimes challenging to distinguish from exosome or ectosome release. [24, 25, 26] Because they cloak themselves in the host membranes’ “self” proteins, these viruses can more insidiously evade the immune system. This phenomenon can be described as borrowing nature’s “stealth technology” in what has been called the Trojan exosome hypothesis. [27]

Markers of Exosomes – or Disguises of EVs?

Given that exosomes are partly defined by the means of their biogenesis, labs worldwide who discovered, characterized, and staked their careers in exosome biology don’t consider it “splitting hairs” to be precise with the evolving terminology. [28, 29, 30] Yet, once exosomes are released beyond the plasma membrane, how can you tell them apart from these other species of the cell secretome? [31] Here is a hint: It isn’t easy.

Researchers have converged on a set of enrichment markers that, while not 100% exclusive, are usually associated with exosomes. The tetraspanins CD9, CD63, and CD81 are commonly found on their surface and serve as reliable hallmarks in immunoassays. Inside, exosomes carry endosomal sorting proteins such as TSG101, ALIX, and syntenin, reflecting their multivesicular body origins. Their membranes are enriched in unusual lipids like lysobisphosphatidic acid (LBPA) and ceramides, which contribute to the inward budding process and help distinguish them from plasma membrane–derived vesicles. Further, exosomes often package a distinct repertoire of RNAs and cytosolic proteins, from microRNAs to heat-shock proteins. This constellation of molecules yields characteristic cargoes that can differ (by a matter of degree) from microvesicles, apoptotic bodies, or non-vesicular extracellular particles.

Another feature often cited is size. Exosomes are generally described as small vesicles with diameters in the range of about 30 to 150 nanometers. This makes them smaller than most apoptotic bodies (which can span hundreds to thousands of nanometers) and many microvesicles (often 100–1000 nm). But size alone is a blunt instrument: some “small ectosomes” can also be as tiny as exosomes, and the isolation method (whether ultracentrifugation, filtration, or size-exclusion chromatography) can bias which populations are recovered. For this reason, EV-experts treat size (e.g., as measurable by nanoparticle tracking analysis, NTA) as a supportive characteristic, not a definitive marker.

To truly show that a preparation contains exosomes and not just a grab bag of extracellular particles, researchers look for a combination of evidence. These vesicles should carry endosomal markers like CD63, TSG101, or ALIX, lack contaminants such as lipoproteins, and display a characteristic lipid fingerprint. Transmission electron microscopy (TEM) helps confirm their small, membrane-bound morphology, while functional assays (e.g., blocking the Rab27/ESCRT machinery and/or specific biotin labeling of the external cell surface) demonstrate that they were released from multivesicular bodies rather than budding directly from the cell surface. Yet, only by layering these orthogonal approaches can one credibly distinguish exosomes from the rest of the secretome menagerie. Hence, seasoned cell biologists tend to be guarded when drawing conclusions from even fastidious data figures, lest their publication drafts run aground in the review process. It’s often much more prudent to refer to these “exosome-ish” populations of vesicles, now, as “small EVs,” and to openly acknowledge the caveats. [12, 29]

Exosomes Amidst the Flock of Extracellular Vesicles (EVs) & Particles (EPs)

By analogy, just as we might typically denote any generic facial tissue as “Kleenex®,” clinic-minded investigators and developers have historically referred to EVs as “exosomes.” Although purists in the cell science community might begin to take issue (…or say “tissue”), many incoming groups are isolating extracellular particles (EPs) from the secretomes of human cells, cow’s milk, mammalian cells, plant tissues, eukaryotic cells, or bacteria. For simplicity, many continue to call their products “exosomes.”

Could these novel preparations work as cosmetics, therapeutics, and treatments? Some most certainly will, so long as these products are meticulously defined and continuously validated according to their quality attributes, manufactured consistently, and tested through rigorous trials for safety and efficacy. It might even turn out that, in some instances, more complex combinations of vesicle species could outperform simpler formulations, based on cryptic synergies. We’ll find out soon enough!

Now that we have a clearer explanation of exosomes in the stringent sense of the word, let’s look at all the discrete subcellular products that resemble them. Below is a table (Table 1) describing many of the other, non-exosome items that might comprise or hitchhike “along for the ride” with exosome preparations, either as residuals or necessary co-ingredients.

Table 1

Species / Type	Size Range	Biogenesis / Origin	Distinguishing Features / Relevance	Potential Applications	Category
Exosomes	~30–150 nm	ILVs of MVBs, released upon fusion with plasma membrane	Tetraspanins (CD63, CD9, CD81); ESCRT proteins; LBPA; RNA/protein cargo; central to therapy & biomarkers	Prospective drug delivery, regenerative medicine, liquid biopsy diagnostics	EV
Apoptotic Bodies	500 nm–5 µm	Blebbing during programmed cell death	Organelle / nuclear fragments; apoptosis marker	Apoptosis biomarkers, disease diagnostics	EV
ERV‑derived Particles	~80–120 nm	Budding via non-canonical ESCRT, & via ERV genes	Noninfectious; syncytin functions; placental roles	Placental biology, cell–cell fusion studies, aging & cellular senescence, possible therapeutic vaccines	EV-like / ERV
Enveloped Virus Particles	~80–120 nm	Bud via non-canonical ESCRT or PM	Trojan exosome mimicry; shared machinery	Vaccine platforms, antiviral drug discovery	EV-like / Pathogen
Microvesicles (Ectosomes)	~100–1000 nm	Bud directly from plasma membrane	Annexin+, integrins/ARF6; signaling and immune roles	Immune modulation, thrombosis biomarkers, inflammation studies	EV, Ectosome
Exophers	1–10 µm	Bulk elimination of damaged contents	Waste expulsion from neurons/muscle	Neurodegeneration research, models of cellular quality control	EV, Ectosome
Oncosomes	1–10 µm	Tumor plasma membrane shedding	Carry oncogenic cargo; biomarker potential	Cancer biomarkers, monitoring tumor progression	EV, Ectosome
Migrasomes	~500–3000 nm	Form during cell migration	Carry mitochondria and signals; developmental roles	Developmental biology, signaling pathway research	EV, Ectosome
Small Ectosomes (ARMMs)	~40–150 nm	Plasma membrane budding via ARRDC1–TSG101 (ESCRT)	Overlap in size with exosomes; signaling-specific	Therapeutic signaling, engineered delivery vectors	EV, Ectosome, Microvesicle
Secretory Granules	100–1000+ nm	Exocytosis in immune, endocrine, neuro-endocrine, and other cells	Contain cytotoxic or hormonal cargos	Immunotherapy, targeted cell killing (e.g., CTLs), hormone release studies	NVEP
Exomeres	~35-45 nm	Non-vesicular particles	Enzymes & RNA-rich; confound exosome studies	Biomarker discovery, metabolic profiling	NVEP
Supermeres	~25–30 nm	Distinct fraction post-exomeres	Disease-associated proteins, brain uptake	Biomarker enrichment, disease monitoring (esp. neurological)	NVEP
Lipoproteins (HDL, LDL, VLDL…)	~10–90 nm	Secreted lipid–protein complexes	ApoA1/B markers; common EV prep contaminants	Cardiovascular disease risk diagnostics, confounders in EV studies	NVEP
Vault / RNP Particles	~70nm x 40nm	Cytoplasmic ribonucleoprotein complexes	Barrel shape, uniform symmetry; carry vRNAs; mysterious roles	Potential for RNA/drug delivery, nanomedicine engineering	NVEP
Non‑enveloped Virus Particles	~20–100 nm	Cytoplasmic assembly	May hitch vesicles; NVEP-like	Viral diagnostics, studying vesicle-mediated viral spread	NVEP-like / Pathogen

Table 1. Distinguishing features of different kinds of extracellular particles (EPs), enveloped (EVs), and non-enveloped (NVEPs), based on outstanding reviews by Jeppesen et al. (2023), [23] Dixson et al. (2023), [32] and others (see reference section).

Are the Semantics Getting Frantic?

“You can know the name of a bird in all the languages of the world, but when you’re finished, you’ll know absolutely nothing whatever about the bird… So let’s look at the bird and see what it’s doing— that’s what counts.”

-Richard Feynman

Science seeks increasingly intelligible models and vocabulary to explain the complex phenomena of nature. It’s under constant revision—an uncaged, fluid, gliding flight, surfing and dodging currents of (sometimes hot) air, en route to better vantage points. Thus, so long as our language strives to illuminate in open dialogue, where’s the harm in freedom of speech? As nomenclature evolves with broader usage, knowledge can travel more widely. Such drift inevitably sparks fresh questions, new perspectives, and the invention and adoption of qualifying terms. In the end, as Feynman reminds us, naming alone is never enough. It’s in watching what the “bird” (or exosome) is actually doing that our shifting words find their true value.

Who can deny that word “exosome” sounds far snazzier on a skincare bottle label than “plant cell extracellular vesicle supernatant?” On that note, it’s doubtful that even the most dedicated cell biologist will ever be able to wrestle “exosome” back into a precise and “safe” semantic domain. This scientist may also be increasingly aware of the uphill battle it would take to “prove” that their “exosome” preparation of, say, 10e11 particles per mL is confidently comprised of “95% pure” exosomes? Better to call it an “exosome-enriched” extracellular particle mixture? …Or simply, “EVs?” …Short for extracellular vesicles? That unfortunately cuts into the lingo tango with electric vehicles . “Exosome”—however it is to be defined in its context-dependent way—will probably remain in the discriminating eye of the beholder.

So, “when is an exosome not an exosome?” The answer is…complicated. Going forward, perhaps it would ultimately be much easier to name each new product specifically, based on its source, the means of how it was isolated and manipulated, and moreover, the fit within its overarching target product profile (TPP). Finally, let it be supported by the necessary documentation and definitions based on methodical, reliable characterization. [29] Regardless of where you stand in this perplexing battle of words, the science will surely remain thrilling for many years to come, spawning myriad novel therapeutic concepts. [33] Importantly, there will be the need for a well-designed CMC package, backed by valid drug master files (DMF) and qualified assays to monitor potency, stability, safety, and purity.

At RoosterBio, we’ve built the tools to help turn this evolving language into real-world therapies. Our cGMP-ready EV and MSC platforms, backed by Type II Master Files with the FDA, [34] come paired with optimized, scalable processes and qualified analytics assays for monitoring potency (e.g., to gauge CD73 activity), [7, 8] stability, safety, and purity. [35] The result is that the therapeutic programs we enable can navigate our partners’ path to the clinic in under a year, [36] bridging the gap between semantics and translation.

 

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