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Trans-epithelial electrical resistance, or TEER, is a well-established method of evaluating and monitoring the barrier integrity of in vitro epithelial or endothelial tissue in a non-destructive assay. TEER is often used with epithelial and endothelial cells in a monolayer as a strong indicator of cell barrier integrity and permeability. These indicators are often used in vitro to evaluate transport of drug chemicals or perform drug screening assays.

Epithelial and endothelial cells form barriers in the body via intercellular junctions that separate distinct fluid compartments. The integrity of the barrier is dependent on the tight junctions that limit permeability in between the cells of the layer. Such cell layers are selectively permeable, with passage across the cell layer controlled via the tight junctions and transport processes across the cell membranes. Importantly, many such cell barriers in the body dynamically modulate passage between distinct compartments, such as the blood-brain barrier.

Cell barriers may be modeled in vitro, with the cells cultured on a permeable membrane providing access to the basal and apical compartments. Some cells, either primary cells or cell lines, will develop tight junctions when cultured in vitro, allowing the study of barrier integrity for applications in modeling human disease or drug delivery. It was discovered that the permeability across an in vitro cell layer was correlated with the electrical resistance across the membrane, as measured by passing current between two electrodes positioned on either side of the cell layer. The electrical resistance measured across the membrane, or trans-epithelial electrical resistance (TEER), provides a fast and simple means of assessing the integrity of a cell barrier.

Basic Technique
TEER measurements utilize electrodes, positioned on either side of a cell membrane, to measure the electrical resistance of cell barrier. Typically, TEER measurements are performed using a permeable filter insert positioned within a cell culture dish. The insert rests within the cell culture medium and divides the chamber into an upper and lower compartment. The cells are cultured on the insert until the cell monolayer is confluent, covering the area of the insert. Then, electrodes may be positioned in the upper and lower compartments to measure the electrical resistance of the cell membrane.

The electrical resistance measurement may be performed by applying a DC voltage across the two electrodes and measuring the resulting current. Using Ohm’s Law, the resistance of the cell layer may then be calculated. However, in practice, there are several disadvantages to using a DC voltage, such as damage to the electrodes or to the cell monolayer. More often, an AC voltage is applied between the two electrodes to derive the electrical resistance of the cell membrane at a particular frequency.

In some cases, the cell membrane may be cultured at an air-liquid interface to better mimic physiological conditions, such as for lung cells. In this configuration, the lower compartment will be filled with media, but the upper compartment will have the media removed. In this way, the cells may be cultured at the interface between air and liquid. However, to perform a TEER measurement, media must later be added to the upper compartment to complete the electrical connection between the two electrodes.

Variations
There exist multiple configurations for making TEER measurements, some of which will be described in this article. However, in principle, all configurations require a cell monolayer, a conductive medium on either side of the cell membrane, and two opposing electrodes.

Chopstick Electrodes
A simple approach for measuring TEER utilizes so-called chopstick electrodes. Two rods, with electrodes at the tips, are manually positioned into the upper and lower compartments on either side of the cell membrane. Typically, these electrodes are used with specially designed cell culture filter inserts containing built-in access points to insert the electrode into the lower compartment.

Once the electrodes are positioned, a device is connected to the electrodes and makes the TEER measurement at a fixed frequency. Such measurements may be influenced by the exact positioning of the electrodes, or through the disturbances to the cell membranes when removed from the cell culture incubator. Also, it is important to measure the TEER across a filter insert without cells positioned in an identical cell culture dish, in an effort to control for the electrical resistance of the insert and the electrodes themselves.

Finally, TEER measurements are typically reported as normalized to the area of the filter insert, and thus the corresponding area of the cell membrane, to control for differences in the TEER measurement that arise due to the insert area.

Embedded Microelectrodes
For screening applications, where the reliability and throughput of the assay are critical, measurements of electrical resistance may be made using multiwell plates with electrodes embedded in the substrate at the bottom of each well. In this case, the cells are seeded into each well and grown directly on top of the electrodes. The electrical resistance measured between the electrodes is related to the resistance underneath the cell layer and through the cell layer, and correlates well with TEER measurements made from chopstick electrodes.

The electrode geometries are precisely defined and consistent across the wells in a standard microtiter cell culture plate, ensuring reliable measurements across biological replicates. Such plates are available in multi-well plate formats for significantly higher throughput than a typical chopstick electrode setup. The measurement devices often allow the plate to remain in a standard cell culture incubator or themselves produce a controlled environment for temperature and CO2, such that continuous monitoring of barrier integrity is possible over hours to days to weeks.

Also, in addition to measurements of barrier integrity, the embedded microelectrodes can be used to quantify cell coverage, or confluency, in each well as the culture is established. These measurements are performed using multiple frequencies of AC voltage simultaneously, such that aspects of cell coverage and barrier integrity may be quantified in a single measurement.

Unlike experiments with filter inserts, these plates do not allow fluidic access to both sides of the cell membrane.

Applications
TEER measurements have two broad uses: 1) quality control assessment of cell membrane barrier integrity and 2) endpoint assay for evaluating changes in barrier integrity.

Experiments interested in quantifying the transport of certain chemical entities across a cell membrane rely on a consistent and fully developed cell membrane in each test sample. However, this can be difficult to achieve without interfering with the chemical transport experiment of interest. So, TEER measurements are often used to assess the integrity of the cell membrane and used as a quality control criterion before a transport experiment begins.

TEER measurements also have many applications as the primary assay endpoint in various cell culture models and screening assays. Such assays and model systems include:
 * Blood-brain barrier
 * Gastrointestinal tract
 * Pulmonary models
 * Organ-on-a-chip