What is the definition of a vesicle?

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Definition of a Vesicle

A vesicle is a membrane-bound structure enclosed by a lipid bilayer that is released from cells and cannot self-replicate. 1

Core Structural Characteristics

Vesicles are microscopic sacs that enclose a volume with a molecularly thin membrane, consisting of amphiphilic molecules with dual hydrophilic-hydrophobic character. 2 The defining feature is the lipid bilayer envelope that distinguishes true vesicles from non-vesicular particles. 1

Key Defining Features:

  • Membrane-bound structure: All vesicles possess a phospholipid bilayer enclosing their contents 1, 3
  • Cell-derived origin: Released naturally from both eukaryotic and prokaryotic cells 4, 5
  • Incapable of self-replication: This distinguishes them from cellular organisms 1
  • Size range: Typically 30 nm to several micrometers depending on vesicle type 1

Classification by Size and Origin

The International Society for Extracellular Vesicles (ISEV) provides operational terminology based on measurable characteristics: 1

Small Extracellular Vesicles (sEVs)

  • Diameter: <200 nm, often 30-150 nm 1
  • Origin: Formed within multivesicular endosomes (exosomes) or by plasma membrane budding 1, 3
  • Biogenesis: Exosomes are assembled in the multi-vesicular endosomal system and released upon fusion with the cell membrane 1

Large Extracellular Vesicles (lEVs)

  • Diameter: >200 nm, ranging from 100-1000 nm or larger 1
  • Origin: Pinched off directly from the plasma membrane (microvesicles/ectosomes) 1
  • Special category: Apoptotic bodies (50-5000 nm) released by dying cells 1

Molecular Composition

Vesicles contain a complex cargo of biological molecules that reflects their cell of origin: 1

  • Proteins: Including membrane-associated and cytosolic proteins 1, 4
  • Nucleic acids: DNA, messenger RNA, microRNA, and other RNA species 1, 3
  • Lipids and metabolites: Reflecting the parent cell's lipid composition 1
  • Organelle fragments: In some vesicle types 1

Functional Significance

Vesicles serve as intercellular communication vehicles, delivering their molecular cargo to recipient cells and modulating biological functions. 4, 5, 3 They are present in all body fluids tested, including blood, urine, and other biological fluids, where they represent a proxy for the tissues from which they originate. 1

Critical Distinction from Non-Vesicular Particles

The presence of a lipid bilayer membrane is the defining criterion that separates true vesicles from non-vesicular extracellular particles (NVEPs). 1 NVEPs are multimolecular assemblies released from cells without a lipid bilayer, such as exomeres (approximately 35 nm non-membranous nanoparticles). 1

Common Pitfall to Avoid:

Do not assume all extracellular particles are vesicles—confirmation of lipid bilayer presence is required to classify a structure as a vesicle rather than a non-vesicular particle. 1 The term "extracellular particles" (EPs) should be used when vesicle identity cannot be confirmed according to ISEV guidelines. 1

References

1
Guideline

Guideline Directed Topic Overview

Dr.Oracle Medical Advisory Board & Editors, 2025

2
Research

Polymer vesicles.

Science (New York, N.Y.), 2002

3
Research

Shedding light on the cell biology of extracellular vesicles.

Nature reviews. Molecular cell biology, 2018

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Professional Medical Disclaimer

This information is intended for healthcare professionals. Any medical decision-making should rely on clinical judgment and independently verified information. The content provided herein does not replace professional discretion and should be considered supplementary to established clinical guidelines. Healthcare providers should verify all information against primary literature and current practice standards before application in patient care. Dr.Oracle assumes no liability for clinical decisions based on this content.

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