MDCK Monolayer Integrity: How to Validate Your Barrier Before Transport Assays

You’ve grown your Madin-Darby Canine Kidney (MDCK) cells, the plate looks confluent, and you’re ready to start screening compounds. But pause on this question: is the monolayer actually ready?

Confluent-looking cells and a trustworthy epithelial barrier are not the same thing. 

Running transport compounds across a leaky or inconsistent monolayer can produce data that looks plausible but reflects leakage between the cells rather than genuine transcellular transport. If that happens, your apparent permeability (P_app) values (and the absorption rankings based on them) may be wrong without you realizing it.

This article walks through the ways to determine MDCK monolayer integrity, including how to select an appropriate seeding density and culture duration, and how to use transepithelial electrical resistance (TEER) and lucifer yellow (LY) passage as diagnostic tools to tell when the monolayer is ready to use.

Why MDCK cells for drug absorption screening

MDCK cells form tight, polarized monolayers on membrane supports and have been used for decades to screen drug permeability. 

In unmodified MDCK cells, the monolayer acts as an epithelial barrier for measuring a compound’s apparent permeability, primarily reflecting passive transcellular diffusion.  MDCK cells are also used as an alternative to Caco-2-based absorption models.

How the Millicell® 96-well system supports the workflow

The Millicell® 96-well cell culture plate system consists of several components that work together to support polarized cell culture and barrier measurements.

The core assembly consists of:

• a 96-well filter plate with a 0.4 µm polycarbonate membrane
• a feeder plate that supplies medium to the basolateral face during culture
• a transport analysis plate used during the assay
• a lid 

The membrane divides each well into an apical compartment (where cells grow and compounds are applied) and a basolateral compartment (where transported compounds accumulate).

The membrane surface area is 0.11 cm² per well, which is important because this is the figure you will use in the P_app calculation. Basolateral access ports and an apical assist channel allow pipette tips to reach each compartment without contacting the monolayer (Figure 1).

Figure 1. The components of the Millicell® 96-well cell culture plate: 96-well filter plate; single-well feeder plate; transport analysis plate; and lid.

The drug permeability screening workflow follows a fixed sequence (Figure 2): 

1. Grow MDCK cells
2. Seed MDCK cells onto the filter plate 
3. Culture the cells with regular medium exchange
4. Verify barrier integrity using TEER and LY
5. Add your compound of interest and measure the absorption rate 

Step 4 is the focus of this article and is where the decision about monolayer readiness is made.

Figure 2. Workflow for drug transport studies using MDCK cells on the Millicell® system: grow the cells, seed the cells onto the filter plate, culture and exchange the medium, verify barrier integrity, then run the transport assay.

Seeding density determines the time to confluency

You can grow MDCK monolayers to confluence on the Millicell® 96-well plate in anywhere from 3 to 7 days, depending on your initial seeding density.

A higher seeding density will typically give you faster confluence, and a lower seeding density means your culture period will be longer. You should consider your workflow when deciding the approach to take.

A previously reported Millipore® seeding optimization experiment used MDCK cells on the Millicell® 96-well cell culture plate over a 4-day culture period. In this study, scientists compared five seeding densities ranging from 25,000 to 45,000 cells/well (approximately 227,000–409,000 cells/cm²). 

The best-performing condition was 35,000 cells/well, which is a useful starting point for your workflow.

Culture conditions throughout:

• 37°C, 5% CO₂, 95% relative humidity
• Media exchange every 48–72 hours, starting no earlier than 48 hours after initial plating
• Dulbecco’s MEM with high glucose, 10% FBS, 1× NEAA, 10 mM HEPES, 100 units penicillin, 0.1 mg/mL streptomycin, 4 mM L-glutamine

Bear in mind that different passage numbers, handling conditions, or incubator setups can affect the growth of your monolayer, so it is important to optimize the seeding density for your own assay.

A simple confluency assessment was not the defining factor in Millipore®’s determination of the optimal seeding density for its drug transport assay. The monolayers also performed well in two barrier-integrity checks: TEER and LY passage. Before looking at the results, let’s define what the barrier-integrity checks tell you.

Assessing MDCK monolayer integrity: TEER as a diagnostic tool

TEER measures ionic conductance across the cell monolayer and is used as an indicator of barrier integrity. This non-destructive technique allows you to measure your monolayer integrity before using it in your transport assay.

A high TEER is consistent with a well-formed, functionally tight barrier. A low TEER is often a warning signal that the monolayer may not be ready for transport work.

However, a low TEER reading doesn’t always mean the monolayer has failed. It can also reflect measurement issues such as temperature equilibration, electrode positioning, or residual medium differences, so you should assess the context of the well before excluding it.

How to measure TEER

You need to remove the plate from the incubator and allow it to equilibrate to room temperature for 15–30 minutes. Using the Millicell® ERS 3.0 Digital Voltohmmeter with the 96-well electrode, position the probe so one end is in the medium in the filter well (apical compartment) and the other passes through the basolateral access port (Figure 3). The probe should sit flat on the plate when correctly positioned.

You must record the resistance of the cultured MDCK cells in every well. Wells meeting your target threshold can proceed to the transport assay; wells that fall short are typically excluded.

Figure 3. Patented design of the Millicell® 96-well cell culture plate showing the apical assist channel, apical access port, basolateral access port, and receiver tray. The basolateral access port is used to position the TEER electrode probe.

Lucifer yellow: catching what TEER misses

TEER alone is not sufficient to confirm MDCK monolayer integrity. A monolayer can show adequate resistance yet still exhibit localized paracellular leakage that TEER won’t detect.

LY passage gives you the complementary check. LY is a small fluorescent dye that crosses cell monolayers primarily by the paracellular route, which means through the gaps between cells rather than through the cells themselves. If your monolayer has paracellular leaks, you will detect LY in the basolateral compartment.

The LY test is typically run after your transport experiment, but you can also include a subset of control wells to monitor integrity during the assay itself.

Running the LY test

• Rinse the monolayer three times with HBSS (100 µL/well)
• Add 75 µL of lucifer yellow at 100 µg/mL to the apical compartment (filter well)
• Add 250 µL HBSS to the basolateral compartments of a fresh transport analysis plate
• Assemble the plates and incubate for 1 hour at 37°C
• Read fluorescence at 485 nm excitation / 535 nm emission

Calculating % LY passage

Prepare an equilibrium reference by diluting your starting LY solution (100 µg/mL × 0.075 mL) into the total assay volume (0.325 mL) to give approximately 23 µg/mL.

If the relative fluorescence Units (RFU) of your sample match the RFU of your equilibrium reference, LY passage was 100%.

You should use the formula below to calculate % LY passage:

% LY passage = [RFU(test) − RFU(blank)] / [RFU(equilibrium) − RFU(blank)] × 100

For example, if RFU(test) = 2,000, RFU(blank) = 1,000, and RFU(equilibrium) = 300,000, then % LY passage = (2,000 − 1,000) / (300,000 − 1,000) × 100 = 0.3%.

What the numbers of the seeding density experiment look like

Now, let’s go back to the seeding density experiment. At 35,000 cells/well after 4 days in culture, TEER was 996 ± 19 Ω (raw resistance across the well, as reported by the Millicell® ERS 3.0 system). Across all five seeding densities tested, the range was 991–1055 Ω. 

The % LY passage was 0.3 ± 0.03%. Across all tested densities, the range was 0.2–0.3% (Figure 4).

These values are benchmarks for the described Millicell® 96-well MDCK workflow and should only be used as a starting point. You should determine your own acceptance thresholds based on the cell source and history, passage range, handling conditions, replicate variability, and assay-specific requirements.

Figure 4. TEER (Ω) and % lucifer yellow passage across five MDCK seeding densities (25,000–45,000 cells/well) after 4 days in culture on the Millicell® 96-well plate. In summary, the seeding density you choose should give the highest average TEER with the lowest variability, and the lowest LY passage.

Running the transport assay

Once the monolayer has passed barrier integrity checks, you can move forward with your transport assay.

Setting up the experiment

First, wash the monolayer 3 times with HBSS, then transfer the filter plate to a fresh transport analysis plate.

The next step depends on the setup of your assay.

Apical-to-basolateral (A→B) direction:

• Add 75 µL of test compound (10–200 µM in HBSS, pH 7.4) to the filter well 
• Add 250 µL HBSS to the transport analysis plate wells

Basolateral-to-apical (B→A) direction:

• Add 250 µL test compound to the transport analysis plate
• Add 75 µL buffer to the filter well

Incubate the plate at 37°C for 1–2 hours with or without a rotary shaker set at 60 rpm. When the transport period ends, you should remove a sample of, typically, 50 µL from both the apical and basolateral compartments via the basolateral access ports. You should transfer your acquired samples to a fresh transport analysis plate for LC/MS quantification.

Calculating Apparent Permeability

P_app reflects the combined transport resistance across the full system, including the membrane, aqueous boundary layers, and paracellular pathways, rather than membrane permeability alone.

For a substantial proportion of compounds under standard assay conditions, the measured P_app is limited by aqueous boundary layer diffusion rather than the cell membrane itself.

For highly lipophilic compounds or those beyond Lipinski’s Rule of 5, intracellular binding can delay the time to steady state, and standard 1–2 hour incubations may underestimate permeability. These are assay-level factors that apply regardless of monolayer quality and should be considered when interpreting P_app for compounds outside the assay’s typical operating range.

For MDCK drug transport workflows, P_app is calculated using the following endpoint equation:

P_app =[V_A / (Area x Time)] x [[drug]_acceptor /[drug]_{(initial, donor)}]

Where V_A is the volume in the acceptor well (mL), Area is the membrane surface area (0.11 cm² for the Millicell® 96-well plate), Time is the total transport period in seconds, and [drug] is the concentration of the drug.

Example results

A drug transport experiment with 4-day MDCK cultures at 35,000 cells/well (TEER: 996 ± 19 Ω; % LY passage: 0.3 ± 0.03) produced the following P_app values for three reference compounds with known high permeability:

The failure modes worth knowing

Most MDCK transport assay problems are introduced upstream of the assay itself. Important and avoidable issues are listed below.

Non-uniform cell suspension at seeding

If cells settle or clump in the tube while you are seeding multiple plates, the distribution across wells will be uneven. This produces inconsistent monolayers that fail TEER in a scattered, unpredictable pattern. You should frequently mix your cells throughout seeding to prevent this.

Edge effects from humidity

If wells on the perimeter of the plate show lower TEER or higher LY passage than interior wells, your incubator is not providing adequate humidity. Using an incubator with electronic humidity control is the cleanest fix. If that is not available, reducing how often the incubator is opened during the culture period helps.

Monolayer drying during media exchange

This is the most common source of unexpected well failures. When the filter plate is separated from the feeder plate to exchange medium, the exposed monolayer can dry out within seconds. Two practices you can use to avoid this are:

• Keeping the separation period under one minute
• Positioning the vacuum manifold on the opposite side of the apical assist port during aspiration. This leaves a slightly higher residual volume in the wells and reduces the risk of drying (Figure 5).

Alternatively, you could exchange the medium without disassembling the plates by aspirating through the basolateral access ports.

Figure 5. Manifold placement near the apical assist for maximum media removal (left) versus opposite the apical assist to leave higher residual volume and reduce drying risk (right).

Probe positioning for TEER measurement

You must sit the 96-well electrode flat on the plate with the thin collared end correctly placed in the basolateral access port. An incorrectly positioned probe produces inconsistent resistance readings across wells and is a common source of apparent TEER variability.

The takeaway

Monolayer quality is one of the earliest controllable sources of data quality in an MDCK transport assay. If the barrier is leaky, inconsistent, or poorly differentiated, the resulting P_app values may appear credible but may reflect monolayer failure rather than true compound permeability.

That is why barrier integrity checks should be treated as decision points, not protocol formalities.

TEER gives you a non-destructive readout of electrical resistance across the monolayer, while LY passage provides a complementary check for paracellular leakage.

Used together, they help you decide whether a well is suitable for transport measurements before you commit valuable compounds and analytical time.

The optimal seeding density, culture duration, and acceptance thresholds will depend on your cells, passage history, handling conditions, plate setup, and assay requirements. A previously optimized workflow can provide a useful starting point, but you should establish your own assay-specific criteria for MDCK monolayer integrity.

In short: before asking what crossed the barrier, first confirm that the barrier itself is fit for purpose.

References:

1. Merck KGaA. Optimization of MDCK Cell Growth and Differentiation for Drug Transport Assay Studies Using Millicell® 96-Well Cell Culture Plates. Application Note MK_AN15026EN Ver. 1.0; 2025.
2. Cui, Y.; Desevaux, C.; Truebenbach, I.; Sieger, P.; Klinder, K.; Long, A.; Sauer, A. A Bidirectional Permeability Assay for beyond Rule of 5 Compounds. Pharmaceutics 2021, 13(8), 1146. https://doi.org/10.3390/pharmaceutics13081146.
3. Ebert, A.; Dahley, C.; Goss, K.-U. Pitfalls in Evaluating Permeability Experiments with Caco-2/MDCK Cell Monolayers. European Journal of Pharmaceutical Sciences 2024, 194, 106699. https://doi.org/10.1016/j.ejps.2024.106699.