Sources, Biological Functions, Isolation/Characterization, and Emerging Applications of Exosomes

Exosomes are small membrane-bound vesicles (typically ~30–150 nm in diameter) generated via the endocytosis–multivesicular body (MVB) pathway and represent an important subtype within the extracellular vesicle (EV) system. By selectively loading proteins, nucleic acids, and lipids, exosomes mediate intercellular communication across diverse physiological and pathological processes, and they show broad potential in disease biomarker discovery, drug delivery, and regenerative medicine. At the same time, their pronounced heterogeneity and methodological biases pose substantial challenges for data interpretation and clinical translation.

I. Positioning of Exosomes Within the Extracellular Vesicle Lineage

1.1 Overview of Extracellular Vesicle (EV) Classification

Extracellular vesicles are a collective term for nano- to micron-scale particles actively released by cells into the extracellular space and enclosed by a lipid bilayer. They are commonly categorized—based on biogenesis and size—into exosomes, microvesicles (ectosomes), and apoptotic bodies. Exosomes mainly originate from the endosomal–MVB pathway and are typically ~30–150 nm; microvesicles form by direct outward budding from the plasma membrane and are generally larger with a broader size distribution; apoptotic bodies are larger vesicular structures produced during apoptosis. Importantly, in real-world samples, size alone is often insufficient to fully separate these vesicle classes. Consequently, an increasing number of studies and guidelines recommend describing EV preparations using “biogenesis context + isolation workflow + multi-parameter characterization,” rather than relying solely on the single label “exosomes.”

1.2 Operational Definition of Exosomes

In mainstream experimental practice, “exosomes” are often operationally defined as a population of small EVs obtained via a specified isolation workflow, enriched in the ~30–150 nm range, and showing enrichment of endosome-associated marker proteins. This definition is fundamentally method-driven: exosomes are not a single, fully homogeneous particle population, but rather a collection of multiple subpopulations. Cell source, culture conditions, and isolation procedures can all substantially affect composition and properties. Therefore, clearly reporting the isolation method and supporting identification evidence is particularly critical in study design and result interpretation.

II. Biogenesis and Secretion Mechanisms of Exosomes

2.1 Biogenetic Pathway

Exosome formation generally proceeds through three major stages: endocytosis to generate early endosomes, inward budding of the endosomal membrane to form intraluminal vesicles (ILVs), and fusion of MVBs with the plasma membrane. The plasma membrane first undergoes endocytosis to form small vesicles that fuse into early endosomes. The endosomal membrane then invaginates inward, encapsulating selected proteins and nucleic acids to form ILVs, driving maturation into MVBs. MVBs subsequently face two principal fates: (i) fusion with lysosomes, resulting in degradation of their contents; or (ii) fusion with the plasma membrane, releasing ILVs into the extracellular space—these released ILVs are exosomes.

2.2 Molecular Regulatory Mechanisms

Exosome biogenesis and secretion are coordinately regulated by multiple molecular mechanisms. The ESCRT machinery and associated proteins can mediate membrane invagination and cargo sorting, contributing to ILV formation. Lipids such as ceramide and members of the tetraspanin family represent ESCRT-independent routes, participating in membrane domain organization and selective cargo loading. Rab family small GTPases (e.g., Rab27, Rab11, Rab35) regulate MVB trafficking and fusion with the plasma membrane, while SNARE complexes directly mediate membrane fusion events. Cell type, activation state, stress conditions, and culture environment can modulate these pathways, thereby altering both exosome yield and cargo profiles.

III. Composition and Molecular Cargo of Exosomes

3.1 Lipid Composition

Exosome membranes are enriched in cholesterol, sphingomyelin, phosphatidylserine, and related components, and often contain lipid raft–like microdomains. These lipids help maintain membrane stability, influence interactions with recipient cell membranes, and in some cases participate directly in signal transduction. Compared with the parent cell membrane, exosomal membranes frequently display biased enrichment patterns in lipid composition, supporting the concept of exosomes as “selectively loaded structures.”

3.2 Protein Cargo

Exosomal proteins show strong selectivity and enrichment. Common membrane and endosome-associated components include tetraspanins such as CD9, CD63, and CD81, as well as endosomal pathway–related factors such as TSG101 and ALIX. Exosomes may also contain chaperones (e.g., Hsp70, Hsp90), cytoskeletal proteins, metabolic enzymes, adhesion molecules, and selected receptors or ligands. Notably, these proteins are better regarded as supportive features rather than universally specific markers across all samples; actual profiles vary with cellular origin and physiological state.

3.3 Nucleic Acids and Small Molecules

Exosomes can encapsulate diverse nucleic acids, including miRNAs, lncRNAs, circRNAs, and mRNA fragments. After uptake by recipient cells, these nucleic acids may contribute to gene expression regulation and remodeling of signaling networks. DNA has also been reported in exosomes, but its origin and function remain controversial and are strongly influenced by isolation methods and the rigor of contamination control. Exosomes may additionally carry metabolic small molecules, signaling lipids, and glycosylation-related components, which are closely linked to processes such as tumor metabolism and immune microenvironment regulation.

IV. Modes of Action and Biological Functions of Exosomes

4.1 Interaction With Recipient Cells

Exosomes interact with recipient cells primarily via two modes: membrane-surface recognition and endocytic uptake. On the one hand, exosomal surface receptors, adhesion molecules, and glycan structures can bind ligands or receptors on recipient cell membranes and may trigger signaling even without internalization. On the other hand, exosomes can enter cells through clathrin-mediated endocytosis, macropinocytosis, or phagocytosis, followed by interactions with endosomal/lysosomal systems. Under certain conditions, exosomal membranes can fuse with endosomal membranes, releasing internal cargo into the cytosol and thereby modulating gene expression and functional states in target cells.

4.2 Immune Regulation and the Tumor Microenvironment

Exosomes derived from immune cells and tumor cells play key roles in immune regulation and remodeling of the tumor microenvironment. Exosomes secreted by antigen-presenting cells can carry MHC–peptide complexes and co-stimulatory molecules, contributing to T-cell activation and amplification of immune responses. In contrast, tumor-derived exosomes may carry immunosuppressive factors and regulatory miRNAs that inhibit effector T cells and promote accumulation of regulatory cell populations, thereby shaping an immune milieu favorable for tumor growth and metastasis. Exosomes can also participate in angiogenesis, epithelial–mesenchymal transition, and formation of pre-metastatic niches.

4.3 Tissue Injury and Regeneration

Exosomes from mesenchymal stem cells and other tissue stem cells show pro-repair effects in multiple injury and regeneration models. For example, in contexts such as myocardial injury, liver fibrosis, and skin wound repair, exosomal growth factors, miRNAs, and signaling proteins can modulate apoptosis, inflammation, angiogenesis, and matrix remodeling. These effects typically arise from coordinated actions of multiple molecules and pathways; in many models, the precise “key components” and their targets remain to be fully elucidated.

V. Methods for Exosome Isolation and Purification

5.1 Differential Centrifugation and Ultracentrifugation

Differential centrifugation combined with ultracentrifugation is among the earliest and most widely used strategies for exosome isolation. By progressively increasing centrifugal force to remove cells, debris, and larger vesicles, small vesicles are ultimately pelleted at high speed. This approach is suitable for samples such as cell culture supernatants, benefits from broad equipment availability, and can yield vesicle populations with relatively focused size distributions. However, the workflow is time-consuming, demands specialized equipment and technical proficiency, and can co-pellet protein aggregates, lipoproteins, and other contaminants. Prolonged high shear may also affect vesicle structure and function.

5.2 Density Gradient Centrifugation

Introducing sucrose or iodixanol density gradients on top of ultracentrifugation enables separation based on buoyant density, improving discrimination between vesicles and certain contaminants and yielding higher-purity fractions. This method offers high resolution and is suited for mechanism studies or omics analyses requiring higher purity, but it is operationally complex, typically has lower recovery, and remains limited in standardization and throughput—more often used for confirmatory experiments than routine high-throughput preparation.

5.3 Size-Exclusion Chromatography and Filtration Technologies

Size-exclusion chromatography (SEC) fractionates sample components through porous media, enabling effective removal of free proteins and small-molecule impurities while preserving vesicle integrity—particularly suitable for body fluids such as plasma and serum. It offers good reproducibility and relatively minimal impact on vesicle function, but has limited ability to separate lipoproteins and other components with sizes similar to exosomes. Ultrafiltration or tangential flow filtration is often used for concentration and partial size selection and can be combined with SEC or gradient centrifugation; these approaches have advantages in scale-up and preclinical preparation.

5.4 Immunoaffinity Enrichment and Polymer Precipitation

Immunoaffinity capture leverages antigen–antibody recognition (e.g., antibodies against CD63 or CD81) to selectively enrich particular exosome subpopulations, increasing compositional “specificity.” However, these methods introduce strong bias toward marker-positive subpopulations and are constrained by cost and scalability. Polymer precipitation (e.g., PEG-based kits) is convenient and requires minimal equipment, making it suitable for screening or applications with lower purity requirements, but it often co-precipitates substantial amounts of proteins and lipoproteins; interpretation of downstream omics and functional assays therefore requires particular caution.

5.5 Comparison of Isolation Methods

Method	Principle	Advantages	Limitations
Ultracentrifugation	Differences in density and sedimentation coefficient	High purity; classical and reliable	Time-consuming; requires specialized equipment; may damage vesicles
Density gradient centrifugation	Precise separation with sucrose/iodixanol gradients	Highest purity	Complex; low recovery
Immunoaffinity capture	Antigen–antibody specific binding (e.g., anti-CD63)	High specificity; enriches target exosomes	High cost; limited yield
Size-exclusion chromatography	Size-based (volume-exclusion) separation in porous resin; larger vesicles elute first, while small molecules and soluble proteins elute later	Gentle; preserves vesicle integrity	Moderate purity; may require additional purification
Commercial kits	Combination of precipitants (e.g., PEG) and centrifugation	Fast and convenient; suitable for high throughput	Lower purity; protein contamination common

VI. Identification and Quality Control of Exosomes

6.1 Particle Size and Concentration Measurement

Nanoparticle tracking analysis (NTA), dynamic light scattering (DLS), and tunable resistive pulse sensing (TRPS) are commonly used to assess particle size distributions and relative concentrations. NTA estimates size and concentration by tracking Brownian motion and is widely used, but it is sensitive to particle refractive index and aggregation. DLS is highly sensitive to small numbers of large particles and can be strongly influenced by aggregates; it is therefore more appropriate as supportive information rather than a standalone criterion.

6.2 Morphological Characterization

Transmission electron microscopy (TEM) and cryo-electron microscopy (cryo-EM) directly visualize vesicle morphology and membrane structures, providing critical evidence for vesicular identity. Conventional negative-stain TEM often shows a “cup-shaped” appearance, reflecting dehydration and staining artifacts to some extent; cryo-EM better preserves native structures but requires higher technical expertise and incurs higher costs.

6.3 Marker Proteins and Contamination Assessment

According to the MISEV guidelines proposed by the International Society for Extracellular Vesicles, it is recommended to assess multiple classes of markers and evaluate potential contaminants. Common “positive” supportive markers include tetraspanins CD9/CD63/CD81 and endosome-associated proteins such as TSG101 and ALIX, whereas organelle markers such as endoplasmic reticulum proteins are often used as “negative” indicators to flag cellular debris contamination. For body-fluid samples such as plasma, albumin and apolipoproteins should also be monitored to assess interference from soluble proteins and lipoproteins.

VII. Biomedical Application Directions for Exosomes

7.1 Liquid Biopsy and Disease Biomarkers

Exosomes are widely distributed across body fluids including blood, urine, and cerebrospinal fluid. Their nucleic acid and protein components can, to an extent, reflect tissue and cellular states, making them a widely explored vehicle for liquid biopsy. In oncology, candidate biomarkers derived from plasma exosomal miRNAs and proteins, as well as fragments of circulating tumor DNA, have been investigated for early screening, subtyping, and therapeutic monitoring. In neurodegenerative and cardiovascular diseases, vesicular components in body fluids—originating from the central nervous system or myocardium—are also being explored as actionable early diagnostic indicators.

7.2 Platforms for Drug and Nucleic Acid Delivery

Owing to their bilayer lipid membrane, biocompatibility, and certain capacities for transport across tissue barriers, exosomes are regarded as potential therapeutic delivery vehicles. Through cell engineering or ex vivo loading, small-molecule drugs, siRNA/miRNA, mRNA, or proteins can be loaded into exosomes or conjugated to their surfaces, and targeting ligands can be used to improve enrichment at specific tissues or lesion sites. Current research on exosome delivery focuses largely on tumor therapy, inflammatory diseases, and genetic disorder interventions, but major challenges remain in loading efficiency, targeting specificity, in vivo distribution and clearance mechanisms, and large-scale GMP-grade manufacturing.

7.3 Regenerative Medicine and Immunomodulatory Applications

Exosomes from mesenchymal stem cells and related sources have demonstrated tissue-protective and repair-promoting effects in models of myocardial infarction, cerebral ischemic injury, hepatorenal injury, and chronic wound repair, suggesting potential roles as alternatives or adjuncts to cell therapy for certain indications. Exosome-mediated immunomodulation has also attracted attention in therapeutic research for graft-versus-host disease and autoimmune disorders. Looking forward, exosomes in regenerative medicine are more likely to be developed as “standardized, processed vesicle formulations,” rather than simple mixtures of primary vesicles.

VIII. Aladdin-Related Products

Catalog No.	Product Name	Grade and Purity	Application
E778163	Exosome Isolation Kit (Magnetic Particles)	BioReagent, sterile	For rapid isolation/enrichment of exosomes (EVs) from cell culture supernatants and common body-fluid samples; products are suitable for downstream NTA/size analysis, WB, qPCR/sequencing, proteomics/lipidomics, and functional studies.
E771324	Exosome Isolation Kit (for cell culture media or urine)	BioReagent	For isolating exosomes from cell culture supernatant or urine; suitable for exosome biomarker studies, method development, and downstream molecular assays (RNA/protein/lipid).
E771451	Exosome Isolation and Purification Kit (Serum/Plasma)	BioReagent	For isolating exosome fractions from serum/plasma for detection; commonly used for liquid biopsy/biomarker research and sample preparation prior to exosome omics and functional experiments.
E778166	Exosome Assay Kit (ELISA Sandwich Method)	BioReagent,sterile	For sandwich ELISA-based quantification/relative quantification of exosomes in samples; suitable for yield evaluation, batch-to-batch consistency/QC, and comparative assessment of how treatments affect exosome secretion.
C778164	Cell Culture Media/Urine Exosome Purification Kit (SEC)	BioReagent,sterile	For SEC purification of exosomes from cell supernatant/urine to reduce free-protein contamination; suitable for functional uptake assays and omics analyses requiring higher purity.
E778168	Exosome Decorating Kit (Sortase A)	BioReagent,sterile	For site-specific, mild enzymatic modification of exosome surfaces (e.g., conjugation of peptides/proteins/probes); suitable for targeted engineering, fluorescent/biotin labeling, delivery-system construction, and tracing studies.
E778169	Exosome-Free FBS	sterile,BioReagent,for cell culture	For exosome-related cell culture to reduce background interference from serum-derived exosomes; suitable for exosome collection, secretion-mechanism studies, and downstream functional evaluation.
E778171	Exosome-Specific Serum-Free Culture Medium	BioReagent,sterile,Animal Free, for cell culture	For cell culture intended for exosome collection/production (serum-free, animal-free), reducing exogenous vesicle and protein background; suitable for exosome preparation, purification, and functional/omics studies.
E778183	239T Exosomes	BioReagent, sterile, ≥90%, 1E+10 Particles/EA	As a standard sample/positive control for cell-derived exosomes; suitable for validating isolation/identification workflows, method development, uptake/tracing experiments, and reference for functional studies.
E778186	HeLa Exosomes	BioReagent, sterile, ≥90%, 1E+10 Particles/EA	As tumor cell–derived exosome material; suitable for tumor-related exosome functional studies, uptake/delivery experiments, and assay development and QC controls.
B778185	BMSCs Exosomes	BioReagent, sterile, ≥90%, 1E+10 Particles/EA	As MSC-derived exosome material; suitable for mechanisms of tissue repair/immunoregulation, intercellular communication and delivery studies, and in vitro functional assays.
M778179	Milk Exosomes	BioReagent, sterile, ≥90%, 1E+12 Particles/EA	As natural milk-derived nanovesicle material; suitable for oral delivery/drug loading, uptake mechanism studies, analysis of natural vesicle composition and function, and methodological controls.
C778181	Coconut Exosomes from Fructification	BioReagent, sterile, ≥90%, 1E+10 Particles/EA	As plant-derived extracellular vesicle material; suitable for natural vesicle delivery/uptake mechanism studies, in vitro functional evaluation, and method development for characterization.
H778182	Houttuynia cordata Exosomes from Rhizome	BioReagent, sterile, ≥90%, 1E+10 Particles/EA	As plant-derived extracellular vesicle material; suitable for natural vesicle component analysis, cellular uptake/tracing, delivery-system exploration, and functional evaluation.
E778180	Dendrobium Officinale Exosomes from Stem Stalk	BioReagent, sterile, ≥90%, 1E+10 Particles/EA	As plant-derived extracellular vesicle material; suitable for delivery/targeting engineering studies, in vitro functional assays, and establishment of characterization workflows.
A778178	Aloe vera Exosomes from Leaves	BioReagent, sterile, ≥90%, 1E+10 Particles/EA	As plant-derived extracellular vesicle material; suitable for cellular uptake and functional evaluation, delivery system development, and validation of related analytical methodologies.
Lib412092	Exosome Secretion Related Compound Library		For small-molecule screening (HTS/HCS) related to exosome biogenesis/secretion regulation, supporting mechanism studies and lead discovery, and enabling subsequent SAR studies and validation.

Exosomes are an important component of the extracellular vesicle system. By selectively loading and transporting multiple classes of biomolecules, they participate in complex intercellular communication and show broad application prospects in biomarker discovery, drug delivery, and regenerative medicine. Meanwhile, high heterogeneity, challenges in isolation and identification, and insufficient standardization constrain reproducibility and clinical translation. In the future, with continued advances in isolation and purification technologies, quality-control frameworks, and engineering strategies, exosome-related research is expected to progressively enable verifiable and standardized translational applications in selected diagnostic and therapeutic directions, while maintaining scientific rigor.

Aladdin: https://www.aladdinsci.com/