Evaluation of Blood-Brain Barrier Permeability: Tracer Extravasation, TEER Measurement, and Interpretation of In Vitro Model Results

Blood-brain barrier permeability evaluation is used to determine whether the cerebral microvascular endothelial barrier is damaged, whether drugs or tracer molecules can enter brain tissue, and whether inflammation, ischemia, tumors, or delivery systems alter cerebrovascular barrier function. Experimental design should not rely only on “increased signals in the brain.” Instead, tracer molecular weight, vascular perfusion, tissue background, TEER, Papp, and tight junction markers should be interpreted together.

Keywords: blood-brain barrier; BBB permeability; tracer extravasation; TEER; Papp; tight junction proteins; brain microvascular endothelial cells

1 Research Logic of Blood-Brain Barrier Permeability Evaluation

1.1 Structural basis of the blood-brain barrier

(1) Brain microvascular endothelial cells

Brain microvascular endothelial cells form the core cellular layer of the blood-brain barrier. They have continuous tight junctions, low transcytotic activity, and highly polarized transport systems. Permeability evaluation should first determine whether the endothelial barrier is disrupted, rather than simply interpreting tracer signals detected in brain tissue as “entry into the brain parenchyma.”

(2) Tight junctions and adherens junctions

Junctional molecules such as Claudin-5, Occludin, ZO-1, JAM-A, and VE-cadherin maintain endothelial barrier integrity. If small-molecule tracer extravasation increases together with decreased continuity of Claudin-5 or ZO-1, this more strongly supports opening of the paracellular pathway. If junctional protein morphology remains largely intact while entry of a specific drug increases, transporter activity, transcytosis, or carrier-mediated delivery mechanisms should be considered.

(3) Neurovascular unit

The blood-brain barrier is not a single endothelial structure. It also includes pericytes, astrocytic endfeet, basement membrane, microglia, and neuronal signal regulation. In inflammation, ischemia-reperfusion, brain tumors, and neurodegenerative diseases, permeability changes often occur together with cytokine responses, basement membrane remodeling, endothelial activation, and glial reactions.

1.2 Questions that permeability evaluation should answer

(1) Whether the barrier is disrupted

Barrier disruption is usually manifested by vascular tracers entering the brain parenchyma, plasma protein extravasation, decreased TEER, increased Papp, and interrupted tight junction continuity. The vascular location and tissue background of positive regions should be evaluated simultaneously to avoid misinterpreting intravascular residual signals as extravasation.

(2) Which class of molecules shows altered permeability

Different tracers reflect barrier opening at different molecular scales. Small-molecule sodium fluorescein can reflect early or mild increases in permeability. FITC-dextran at 4 kDa, 10 kDa, 40 kDa, or 70 kDa can be used to distinguish leakage across different molecular weight ranges. Evans blue binds to serum albumin and is therefore more suitable for evaluating macromolecular plasma-protein-like extravasation.

(3) Whether the permeability change has functional significance

Increased tracer signal in the brain does not necessarily indicate effective drug delivery, nor does it necessarily indicate severe barrier injury. Its biological significance should be interpreted together with neuroinflammation, brain edema, neurological function scores, brain/blood drug concentration ratio, transporter expression, and tissue localization.

Table 1 Core Detection Framework for BBB Permeability Evaluation

Evaluation Level	Common Indicators	Recommended Methods	Interpretation Focus
In vivo tracer extravasation	Evans blue, sodium fluorescein, FITC-dextran	Intravenous injection, perfusion, brain tissue fluorescence/absorbance detection, section imaging	Determine whether extravascular tracer signals increase
In vitro barrier integrity	TEER, Papp, transmembrane tracer amount	Transwell, co-culture model, microfluidic chip	Distinguish electrical resistance changes from actual molecular flux changes
Tight junction structure	Claudin-5, Occludin, ZO-1, JAM-A	IF, IHC, Western blot, qPCR	Evaluate junctional protein expression and cell-border continuity
Endothelial phenotype	CD31, VE-cadherin, GLUT1, P-gp	IF, flow cytometry, Western blot, transcriptomic analysis	Confirm whether the model maintains BBB endothelial features
Inflammation and injury	ICAM-1, VCAM-1, MMP-9, TNF-α, IL-1β	ELISA, Western blot, qPCR, IF	Explain causes of increased permeability
Drug delivery	Brain/blood drug concentration ratio, intracerebral spatial distribution	LC-MS/MS, fluorescence imaging, radiotracing	Distinguish barrier disruption from active delivery mechanisms

2 In Vivo Evaluation of Blood-Brain Barrier Permeability

2.1 Tracer extravasation experiments

(1) Evans blue extravasation

After binding to serum albumin, Evans blue mainly reflects albumin-like macromolecular extravasation and is suitable for evaluating relatively obvious blood-brain barrier disruption. Adequate cardiac perfusion before sacrifice is required to remove residual dye from the vascular lumen. If perfusion is insufficient, blue staining or fluorescence signals in brain tissue may mainly come from intravascular residual dye rather than true extravasation.

(2) Sodium fluorescein extravasation

Sodium fluorescein has a low molecular weight and is sensitive to mild changes in barrier permeability. It is suitable for evaluating early injury, inflammatory stimulation, or weak barrier opening after drug intervention. Because small molecules diffuse rapidly and are strongly affected by tissue background and renal clearance, injection dose, circulation time, perfusion method, and tissue sampling region should be standardized.

(3) FITC-dextran molecular weight gradient

FITC-dextran can be selected by molecular weight to determine the degree of barrier opening. Low-molecular-weight dextran is more likely to detect mild leakage, while high-molecular-weight dextran is more suitable for identifying severe barrier disruption or vascular structural injury. If a 4 kDa tracer increases but a 70 kDa tracer does not, this usually suggests mild barrier opening. If multiple molecular weights all show marked extravasation, more severe vascular barrier injury should be considered.

2.2 Tissue imaging and quantification

(1) Brain-region localization

Blood-brain barrier permeability differs significantly among brain regions. In ischemic models, local extravasation often occurs in the infarct border zone, cortex, or striatum. In brain tumor models, increased permeability is often associated with abnormal tumor vasculature. In inflammatory models, diffuse or region-selective changes may occur. ROI should be predefined during quantification to avoid bias caused by selecting only the strongest signal regions.

(2) Distinguishing intravascular and extravascular signals

Section imaging should be combined with vascular markers such as CD31, Lectin, or Collagen IV to determine whether tracer signals are located in the vascular lumen, around the vascular wall, or in brain parenchyma. Whole-brain homogenate quantification can improve throughput, but it cannot distinguish vascular residues from true cross-barrier extravasation. Therefore, it is best used together with tissue section localization.

(3) Signal normalization

In vivo extravasation results can be normalized to brain tissue weight, protein amount, injection dose, or plasma tracer concentration. If circulating blood tracer concentrations differ substantially among animals, simply comparing brain tissue fluorescence intensity may be inaccurate. In drug delivery studies, the brain/blood concentration ratio or brain tissue AUC is more suitable for reflecting cross-barrier exposure.

2.3 Dynamic imaging evaluation

(1) MRI contrast agent leakage

Dynamic contrast-enhanced MRI using gadolinium-containing contrast agents can be used for noninvasive evaluation of blood-brain barrier disruption and is suitable for longitudinal observation of brain tumors, cerebral ischemia, inflammation, and therapeutic intervention. This method provides spatial distribution and dynamic parameters, but it is sensitive to modeling, vascular input function, and tissue perfusion status.

(2) Two-photon and in vivo imaging

Two-photon microscopy can observe the process of tracer extravasation from cerebral microvessels into surrounding tissue in vivo. It is suitable for studying local microvascular permeability and dynamic barrier opening. This method has high spatial resolution but limited field of view and high technical requirements, so it is usually used for mechanistic studies rather than large-sample screening.

(3) Near-infrared or whole-body fluorescence imaging

Near-infrared probes or fluorescent nanomaterials can be used for whole-body imaging and drug delivery evaluation. Because scalp, skull, tissue scattering, and blood residues can affect signals, results should be confirmed by ex vivo brain imaging, tissue sections, or quantitative detection.

Table 2 Selection of In Vivo BBB Permeability Tracers

Tracer	Main Readout	Common Detection Methods	Suitable Scenarios	Notes
Evans blue	Albumin-like macromolecular extravasation	Formamide extraction from brain tissue, fluorescence/absorbance detection, section imaging	Obvious BBB disruption, brain edema, inflammatory injury	Adequate perfusion is required to avoid vascular residue
Sodium fluorescein	Increased small-molecule permeability	Fluorescence quantification, tissue sections	Mild or early BBB opening	Susceptible to clearance rate and tissue background
FITC-dextran 4 kDa	Small- to low-molecular-weight flux	Fluorescence imaging, tissue homogenate	Mild to moderate barrier opening	Circulation time and perfusion conditions must be fixed
FITC-dextran 40 kDa	Medium-molecular-weight leakage	Fluorescence imaging	Moderate vascular barrier injury	Sensitive to perfusion and tissue section quality
FITC-dextran 70 kDa	Larger-molecule leakage	Fluorescence imaging	Severe BBB disruption or vascular injury	May be insensitive in mild injury
HRP	Enzymatic or ultrastructural-level extravasation	DAB color development, electron microscopy, histochemistry	Ultrastructural and paracellular pathway studies	Technically complex; fixation and color development must be strictly controlled
Albumin/IgG extravasation	Endogenous plasma protein leakage	IF, IHC, Western blot	Barrier disruption and inflammatory models	Intravascular residue must be distinguished from parenchymal extravasation

3 In Vitro Blood-Brain Barrier Model Evaluation

3.1 Common model types

(1) Transwell monolayer model

The Transwell model uses brain microvascular endothelial cells to form a barrier layer and can be used for TEER, Papp, and drug transmembrane transport assays. This model is relatively standardized and suitable for screening and mechanistic studies. However, a single endothelial model cannot fully simulate the regulatory effects of astrocytes, pericytes, and shear stress on the BBB phenotype.

(2) Co-culture model

Co-culture of endothelial cells with astrocytes, pericytes, or neurons can more closely approximate the neurovascular unit environment. Co-culture usually improves tight junction maturation and barrier resistance, but cell ratio, seeding sequence, medium compatibility, and cell source can significantly affect results.

(3) Microfluidic BBB chip

Microfluidic chips can introduce fluid shear stress, three-dimensional structures, and multicellular interactions, making them more suitable for evaluating dynamic permeability, drug delivery, and inflammatory cell adhesion. This model has higher physiological relevance, but platforms vary widely, technical requirements are high, and additional correction is required when converting results into conventional Papp or TEER indicators.

3.2 TEER measurement

(1) Barrier resistance readout

TEER reflects the resistance of the cell layer to ion passage and is a common indicator for evaluating the integrity of in vitro BBB models. Increased TEER usually indicates tighter intercellular junctions, whereas decreased TEER suggests impaired intercellular barrier function, cell death, or loose junctional structures.

(2) Area correction

TEER should be calculated after subtracting blank membrane resistance and normalized to membrane area as Ω·cm². Different pore sizes, membrane materials, insert areas, and electrode types can affect absolute values. Therefore, uncorrected TEER values should not be directly compared across different models.

(3) Combined use with flux assays

TEER mainly reflects ionic permeability and cannot fully represent the transmembrane permeability of drugs or macromolecules. Some treatments may reduce TEER without significantly changing macromolecular flux, while others may not affect TEER but may alter transporter-mediated drug passage. Therefore, TEER should be evaluated together with fluorescent tracers or target-drug Papp.

3.3 Papp and tracer transmembrane assays

(1) Papp calculation

Papp is used to evaluate the apparent permeability of a compound or tracer across a cell barrier. It is usually calculated based on the concentration change on the receiver side, the initial concentration on the donor side, membrane area, and time. The experiment should ensure stable donor-side concentration, accurate receiver-side sampling volume, and flux within a linear time range.

(2) Tracer selection

Lucifer yellow, sodium fluorescein, FITC-dextran, and radiolabeled sucrose can be used for evaluating in vitro model permeability. Small-molecule tracers are suitable for detecting minor barrier defects, while dextran tracers are suitable for evaluating molecular-weight-dependent permeability. If drug delivery is being studied, the target drug itself should also be measured rather than using only tracers as substitutes.

(3) Recovery rate and mass balance

Papp experiments should record mass balance among the donor side, receiver side, and cell/membrane-bound fraction. If recovery is too low, compound adsorption, degradation, cellular uptake, or membrane-material binding may be present. Directly calculating permeability from receiver-side signals alone may underestimate true transport.

Table 3 Comparison of In Vitro BBB Models and Permeability Evaluation Methods

Method	Main Readouts	Advantages	Limitations	Recommended Uses
Transwell endothelial monolayer model	TEER, Papp, tracer flux	Simple operation, suitable for batch comparison	Lacks complex neurovascular unit environment	Drug screening, barrier integrity detection
Endothelial-astrocyte co-culture	TEER, Papp, tight junction proteins	More similar to BBB-inducing environment	More system variables and higher reproducibility requirements	Inflammation, glial regulation, barrier maturation studies
Endothelial-pericyte co-culture	TEER, transporters, junctional proteins	Suitable for studying pericyte regulation of the vascular barrier	Strongly affected by cell source and ratio	Vascular stability and neurovascular unit research
iPSC-derived BBB model	TEER, Papp, transporter function	High human relevance, suitable for disease models	Differentiation batch variation requires strict control	Genetic diseases, patient-derived models, drug screening
BBB microfluidic chip	Dynamic permeability, transport under flow	Can simulate shear stress and three-dimensional structure	Limited platform standardization and higher cost	Dynamic delivery, inflammatory cell adhesion, mechanistic studies
Cell-free membrane diffusion system	Compound membrane flux	Can exclude cellular factors	Cannot represent BBB barrier structure	Compound adsorption, membrane diffusion, and methodological controls

4 Tight Junction and Endothelial Phenotype Analysis

4.1 Tight junction markers

(1) Claudin-5

Claudin-5 is an important component of BBB endothelial tight junctions. Its continuity and border localization are more informative than total expression alone. If Claudin-5 changes from a continuous cell-border pattern to a fragmented, diffuse, or reduced pattern, barrier structural damage is usually suggested.

(2) Occludin

Occludin is commonly used to evaluate tight junction stability and barrier regulatory status. Inflammatory factors, oxidative stress, and ischemic injury can affect Occludin expression or localization. It should be interpreted together with ZO-1 and Claudin-5 rather than relying on a single marker.

(3) ZO-1

ZO-1 is an important scaffold protein connecting membrane proteins with the cytoskeleton. ZO-1 immunofluorescence can directly show cell-border continuity and is suitable for combined interpretation with TEER and Papp results. If TEER decreases together with fragmented ZO-1 borders, intercellular junction disruption is more strongly supported.

4.2 Endothelial and neurovascular unit markers

(1) Endothelial markers

CD31, VE-cadherin, vWF, and GLUT1 can be used to confirm the characteristics of brain microvascular endothelial cells. In in vitro models, if endothelial markers are insufficient or cell morphology is abnormal, even high short-term TEER does not necessarily indicate a stable BBB phenotype.

(2) Pericyte and astrocyte markers

PDGFRβ and NG2 can be used for pericyte analysis, while GFAP and AQP4 can be used to study astrocytic responses and endfoot-related changes. In in vivo experiments, disrupted AQP4 polarization or increased GFAP often suggests that glial responses participate in BBB changes.

(3) Basement membrane and inflammatory markers

Collagen IV, Laminin, MMP-2, and MMP-9 can be used to evaluate basement membrane structure and degradation. ICAM-1, VCAM-1, TNF-α, IL-1β, and IL-6 can reflect endothelial inflammatory activation. If increased permeability is accompanied by increased MMP-9 and tight junction disruption, inflammatory responses or matrix degradation are usually involved in barrier disruption.

Table 4 Common Molecular Markers for BBB Permeability Evaluation

Analysis Target	Recommended Indicators	Methods	Interpretation Focus
Tight junctions	Claudin-5, Occludin, ZO-1, JAM-A	IF, IHC, Western blot, qPCR	Prioritize observation of cell-border continuity and localization changes
Adherens junctions	VE-cadherin, β-catenin	IF, Western blot	Evaluate stability of endothelial intercellular adhesion
Endothelial cells	CD31, vWF, GLUT1	IF, IHC, flow cytometry	Confirm brain vascular endothelial structure and model phenotype
Pericytes	PDGFRβ, NG2, Desmin	IF, IHC	Evaluate vascular stability and pericyte coverage
Astrocytes	GFAP, AQP4	IF, IHC, Western blot	Evaluate glial response and AQP4 polarization
Basement membrane	Collagen IV, Laminin	IF, IHC	Analyze vascular basement membrane integrity
Inflammatory activation	ICAM-1, VCAM-1, TNF-α, IL-1β	qPCR, Western blot, ELISA, IF	Explain inflammatory mechanisms behind increased permeability
Matrix degradation	MMP-2, MMP-9	Western blot, ELISA, zymography	Assess risk of junctional disruption and basement membrane degradation

5 Permeability Evaluation Strategies in Different Research Scenarios

5.1 Cerebral ischemia and reperfusion

(1) Early permeability increase

BBB permeability after ischemia-reperfusion can change in a time-dependent manner. Early small-molecule tracer extravasation may reflect endothelial stress and tight junction regulation, while later macromolecular extravasation, brain edema, and plasma protein deposition more strongly suggest obvious barrier disruption. Tracer selection and sampling time points should be determined according to the model time window.

(2) Infarct region and penumbra

Permeability changes in ischemic models often show spatial heterogeneity. Severe vascular injury may occur in the infarct core, while the penumbra is more suitable for evaluating the protective effect of therapeutic interventions on the barrier. Quantification should be combined with TTC staining, brain-region dissection, or imaging localization to avoid diluting differences by mixing different regions.

(3) Neuroprotective interventions

If the intervention group shows reduced tracer extravasation together with improved Claudin-5/ZO-1 continuity, decreased MMP-9, and reduced brain edema, this more strongly supports a BBB-protective effect. If fluorescence signals decrease without improvement in structural or functional indicators, it should be checked whether the intervention affects tracer plasma concentration, systemic clearance, or tissue fluorescence background.

5.2 Neuroinflammation and infection

(1) Inflammatory factor induction

LPS, TNF-α, IL-1β, IL-6, and IFN-γ can induce endothelial activation, increase ICAM-1, VCAM-1, and chemokine expression, and affect tight junction stability. In in vitro models, inflammatory factor stimulation can be followed by TEER, Papp, and junctional protein localization assays to establish an inflammatory BBB injury model.

(2) Immune cell transmigration

In neuroinflammation studies, permeability evaluation should not focus only on molecular extravasation. Leukocyte adhesion and transendothelial migration should also be considered. Endothelial adhesion molecules, chemokines, immune cell markers, and tissue localization can be combined to determine whether barrier opening is accompanied by immune cell entry into the central nervous system.

(3) Cell viability control

Excessively strong inflammatory stimulation can cause endothelial cell death, leading to a sharp decrease in TEER and massive tracer permeability. Such results are closer to cytotoxic injury and do not necessarily represent physiological barrier regulation. Cell viability, apoptosis, or membrane integrity should be detected in parallel.

5.3 Brain tumors and drug delivery

(1) Blood-tumor barrier

Brain tumor regions often contain abnormal vessels and a blood-tumor barrier, but permeability is not uniform. The vascular structures in the tumor core, tumor margin, and surrounding brain tissue differ. Entry of drugs or nanoparticles into tumor tissue cannot be simply extrapolated to mean that they can cross the normal BBB.

(2) Delivery system evaluation

Studies involving nanoparticles, liposomes, exosomes, receptor-mediated transport systems, or focused-ultrasound-mediated BBB opening should evaluate both delivery efficiency and barrier safety. The ideal result is not simply “the higher the permeability, the better,” but effective intracerebral delivery while controlling plasma protein extravasation, inflammatory responses, and neurotoxicity.

(3) Distinguishing targeting mechanisms

If the intracerebral signal of a delivery carrier increases, blood-brain barrier disruption, receptor-mediated transcytosis, intravascular retention, and cellular uptake should be distinguished. Perfusion, vascular co-localization, receptor blocking, parenchymal localization, and brain/blood concentration ratio should be evaluated together.

6 Experimental Design and Quality Control

6.1 Control points for in vivo experiments

(1) Adequacy of perfusion

Perfusion is a key step in in vivo permeability experiments. Insufficient perfusion leaves residual tracer in the vascular lumen, causing false elevation of brain tissue signals. Perfusion solution type, perfusion volume, perfusion speed, and endpoint criteria should be standardized. When necessary, vascular markers or residual hemoglobin detection can be used to evaluate perfusion efficiency.

(2) Circulation time

Different tracers differ in plasma half-life, distribution speed, and tissue extravasation rate. If the circulation time is too short, permeability may be underestimated. If it is too long, clearance differences and tissue diffusion background may increase. An appropriate time window should be determined by pilot experiments based on tracer molecular weight and model injury severity.

(3) Brain-region sampling

Whole-brain homogenates are suitable for overall quantification, but they may mask local permeability changes. Regional disease models should separate cortex, hippocampus, striatum, tumor region, or injured/contralateral tissue, and imaging localization should be combined to confirm signal sources.

6.2 Control points for in vitro experiments

(1) Integrity of the cell monolayer

Before conducting permeability assays, it should be confirmed that cells have reached confluence and formed a stable barrier. If the cell layer contains holes, edge detachment, or uneven density, increased Papp may simply reflect model failure. Microscopic morphology and baseline TEER should be recorded before the experiment.

(2) Tracer concentration and sampling time

Excessively high tracer concentration may increase background or alter osmotic pressure. Excessively long sampling time may enter a nonlinear phase. A time-flux curve should be established first, and Papp should be calculated using time points within the linear range.

(3) Cytotoxicity screening

Drug treatments or stimulation conditions may directly damage endothelial cells. If TEER decreases and Papp increases while cell viability decreases markedly, the result should be interpreted as cell-injury-associated barrier disruption rather than specific regulation of BBB permeability.

6.3 Misinterpretations in data analysis

(1) Relying on a single tracer

A single tracer can only reflect a specific molecular weight or readout window. If the degree of barrier opening is being studied, small-molecule and macromolecular tracers should preferably be combined. If drug delivery is being studied, the target drug itself should be measured, with tracers used as references for barrier status.

(2) Relying only on total protein expression

Unchanged total tight junction protein expression does not indicate intact barrier structure. Displacement, fragmentation, or internalization of junctional proteins from the cell border can also increase permeability. Therefore, localization information from IF or multiplex imaging is usually more informative than Western blot alone for explaining barrier structural changes.

(3) Equating brain signals with brain parenchymal entry

Brain tissue signals may come from intravascular residues, vascular wall binding, meninges, choroid plexus, ependymal regions, or nonspecific tissue adsorption. Drug delivery and nanomaterial studies especially require perfusion, vascular co-localization, and cellular localization to confirm brain parenchymal distribution.

Table 5 Common Abnormal Results in BBB Permeability Experiments and Optimization Directions

Abnormal Result	Common Causes	Priority Checks	Optimization Suggestions
High fluorescence in control-group brain tissue	Insufficient perfusion, vascular residue, excessive tracer dose	Perfusion efficiency, vascular co-localization, blank tissue background	Standardize perfusion and reduce tracer dose
TEER decreases in treatment group but Papp does not change	Ion permeability changes but macromolecular flux remains unchanged	Tracer molecular weight, sampling time, cell morphology	Add small-molecule tracers or extend linear sampling
Papp increases but cell viability decreases	Drug or stimulus causes cytotoxicity	Viability assay, LDH, morphology observation	Reduce treatment concentration and distinguish toxic injury
Total tight junction protein is unchanged but permeability increases	Protein localization changes or junctions are disrupted	IF localization, cell-border continuity	Combine ZO-1/Claudin-5 imaging analysis
FITC-dextran extravasation is not obvious	Molecular weight is too large or injury is mild	Switch to lower-molecular-weight tracer	Combine sodium fluorescein or 4 kDa dextran
Evans blue variation is too large	Perfusion, circulation time, and extraction conditions are inconsistent	Perfusion volume, extraction time, tissue-weight correction	Fix workflow and increase sample size
Brain drug signal increases but vascular co-localization is obvious	Intravascular retention or endothelial binding	CD31/Lectin co-staining, imaging after perfusion	Distinguish vascular signal from brain parenchymal distribution

7 Reagent and Material Selection for Blood-Brain Barrier Permeability Evaluation

Table 6 Common Tracers, Control Reagents, and Detection Materials

Product Type	Representative Product	CAS No.	Application Positioning	Selection Notes
Macromolecular extravasation tracer	Evans blue	314-13-6	Assessment of albumin-like plasma protein extravasation	Suitable for models with pronounced BBB disruption, cerebral edema, and inflammatory injury; thorough perfusion is required
Small-molecule tracer	Sodium fluorescein	518-47-8	Assessment of mild or early-stage BBB permeability increase	High sensitivity, suitable for slight barrier opening; circulation time and tissue background correction should be standardized
Fluorescent dye	Fluorescein	2321-07-5	Fluorescent labeling, tracer control, and methodological validation	pH, tissue autofluorescence, and detection channel selection should be considered
Fluorescent labeling reagent	FITC	3326-32-7	Labeling of proteins, polysaccharides, or tracer molecules	Labeling degree, removal of free dye, and molecular weight changes after labeling should be controlled
Red fluorescent labeling reagent	Rhodamine B	81-88-9	Red fluorescence tracing, tissue imaging, and channel compatibility	Suitable for dual-channel comparison with green tracers; background fluorescence should be controlled
Red fluorescent labeling reagent	Tetramethylrhodamine isothiocyanate (TRITC)	36877-69-7	Red fluorescent labeling of proteins or polysaccharides	Can be used to construct red dextran or protein tracer systems; free dye should be removed
Near-infrared fluorescent dye	Indocyanine green (ICG)	3599-32-4	Near-infrared imaging, whole-body fluorescence tracing, and delivery system evaluation	Suitable for in vivo imaging, but plasma protein binding and hepatobiliary clearance should be considered
Low-permeability small-molecule control	Sucrose	57-50-1	Low-permeability reference for paracellular pathways or alternative strategy for radiotracer studies	Commonly used in in vitro barrier paracellular permeability studies
High-permeability small-molecule control	Urea	57-13-6	High-permeability small-molecule reference	Can be used for methodological comparison, but should not be directly equated with BBB disruption
Permeability modulator	D-Mannitol	69-65-8	Hyperosmotic BBB opening or positive treatment reference	Concentration, administration route, exposure time, and safety window should be controlled
Inflammatory stimulus	Histamine dihydrochloride	56-92-8	Positive stimulus reference for vascular permeability	Suitable as a positive control for barrier opening; dosage should be optimized according to in vivo or in vitro models
Inflammatory stimulus	Lipopolysaccharide (LPS)	93572-42-0	Construction of inflammatory BBB injury models	Can induce endothelial inflammatory activation and tight junction alterations; cell viability should be measured in parallel
Oxidative stress inducer	Hydrogen peroxide	7722-84-1	Construction of oxidative stress-related BBB injury models	Suitable for positive treatment in in vitro endothelial injury models; concentration and exposure time must be strictly controlled
Hypoxia/chemical hypoxia modeling reagent	Cobalt chloride	7646-79-9	Simulation of hypoxia-like stress and HIF-related responses	Can be used in barrier stress studies; cell viability and inflammatory markers should be evaluated together
Ischemic injury auxiliary reagent	TTC	298-96-4	Localization of infarct regions in ischemic models	Can be used to match BBB extravasation results with specific brain regions
Vascular perfusion auxiliary reagent	Heparin sodium	9041-08-1	Reduction of blood coagulation and vascular residue during perfusion	Suitable for pretreatment before in vivo tracer extravasation experiments
Tissue fixation reagent	Paraformaldehyde	30525-89-4	Tissue fixation, cellular immunofluorescence, and section imaging	May affect fluorescence retention, antigen exposure, and tissue background
Tissue fixation reagent	Formaldehyde solution	50-00-0	Routine tissue fixation	Suitable for pretreatment before histology and immunostaining; fixation time should be controlled
Clearing reagent	Xylene	1330-20-7	Clearing treatment for paraffin sections	Suitable for histological workflows; use cautiously with fluorescent samples
Blocking protein	Bovine serum albumin (BSA)	9048-46-8	Immunostaining blocking, protein standard, and binding model	Batch variation may affect background; Evans blue-albumin binding experiments can serve as a reference system
Extracellular matrix coating material	Collagen I	9007-34-5	Transwell, chip-based, or endothelial cell culture coating	Affects cell adhesion, barrier formation, and TEER baseline
Extracellular matrix coating material	Fibronectin	86088-83-7	Endothelial cell adhesion and Transwell coating	Can improve cell spreading; coating concentration and batch consistency should be controlled
Extracellular matrix coating material	Gelatin	9000-70-8	Cell adhesion and basic coating	Suitable for routine endothelial cell culture, but has limited BBB specificity
Cell viability assay	MTT	298-93-1	Assessment of the effect of treatment conditions on endothelial cell viability	Used with TEER/Papp assays to distinguish barrier regulation from cytotoxicity
Cell viability assay	CCK-8/WST-8	193149-74-5	Cell viability detection in in vitro models	Suitable for cytotoxicity assessment after drug treatment
Nuclear staining reagent	DAPI	28718-90-3	Nuclear counterstaining and tissue localization	Can be combined with vascular, tight junction, and tracer imaging

8 Common Questions

8.1 Should Evans blue or FITC-dextran be prioritized for BBB permeability evaluation?

It depends on the research question. Evans blue is more suitable for evaluating albumin-like macromolecular extravasation and relatively obvious BBB disruption. FITC-dextran can be selected at different molecular weights to assess the scale of barrier opening. If mild or early permeability increase is being studied, sodium fluorescein or low-molecular-weight FITC-dextran can be prioritized.

8.2 Why is perfusion necessary in in vivo BBB extravasation experiments?

Perfusion removes residual tracer from the vascular lumen. If perfusion is insufficient, brain tissue fluorescence or dye extraction signals will be significantly amplified by intravascular residue, resulting in false-positive results. In permeability studies, perfusion standardization is usually as important as tracer selection.

8.3 Does decreased TEER necessarily indicate increased blood-brain barrier permeability?

Not necessarily. TEER mainly reflects ionic permeability and cell-layer resistance, and it cannot fully represent macromolecular or drug flux. Decreased TEER should be interpreted together with Papp, tracer transmembrane flux, tight junction imaging, and cell viability assays.

8.4 Does increased Papp mean that a drug can effectively enter brain tissue?

Not directly. Papp reflects transmembrane permeability in an in vitro model, but intracerebral delivery is also affected by blood flow, plasma protein binding, transporter efflux, metabolism, diffusion in brain parenchyma, and tissue binding. Drug delivery studies require further measurement of brain/blood drug concentration ratios and intracerebral spatial distribution.

8.5 Why can permeability still increase when total Claudin-5 or ZO-1 protein does not decrease?

Tight junction function depends not only on total protein amount but also on cell-border localization, continuity, phosphorylation status, and linkage to the cytoskeleton. If total protein remains unchanged but immunofluorescence shows border fragmentation or diffusion, permeability may still increase.

8.6 Is a higher TEER value always better in an in vitro BBB model?

High TEER usually indicates a tighter barrier, but an excessively high or unusually stable value does not necessarily represent true BBB function. Endothelial phenotype, tight junction localization, low tracer permeability, cell viability, and transporter function should also be confirmed to avoid evaluating model quality using TEER alone.

8.7 How can increased intracerebral fluorescent tracer signal be distinguished from intravascular residue?

Adequate perfusion, vascular marker co-staining, tissue section localization, and three-dimensional imaging analysis should be combined. If tracer signals mainly overlap with CD31- or Lectin-positive vessels, they are more likely to represent intravascular residue or vascular wall binding. If signals are located outside vessels and diffuse into surrounding parenchyma, cross-BBB extravasation is more strongly supported.

8.8 Is tracer extravasation alone sufficient for BBB drug delivery studies?

No. Tracer extravasation only indicates barrier status or model permeability changes and cannot replace target-drug measurement. Delivery studies should also measure target-drug brain concentration, plasma concentration, brain-region distribution, cellular localization, and safety indicators, while distinguishing active delivery, barrier disruption, and vascular retention.

Blood-brain barrier permeability evaluation should be designed across multiple levels, including tracer selection, vascular perfusion, in vitro barrier models, tight junction structure, and data correction. Reliable results are not based on a single increase in fluorescence signal, but on consistent support from extravasation localization, TEER/Papp changes, tight junction remodeling, and sample matrix control.

For more related articles, please see below:

[1] In vitro modeling of the blood-brain barrier

[2] Blood-brain barrier permeability assay