The electrical resistance of a cellular monolayer, measured in ohms, is a quantitative measure of the barrier integrity.26 (link) The classical setup for measurement of TEER, as shown in Figure 1 , consists of a cellular monolayer cultured on a semipermeable filter insert which defines a partition for apical (or upper) and basolateral (or lower) compartments. For electrical measurements, two electrodes are used, with one electrode placed in the upper compartment and the other in the lower compartment and the electrodes are separated by the cellular monolayer. In theory, the ohmic resistance can be determined by applying a direct current (DC) voltage to the electrodes and measuring the resulting current. The ohmic resistance is calculated based on Ohm's law as the ratio of the voltage and current. However, DC currents can damage both the cells and the electrodes. To overcome this issue, an alternating current (AC) voltage signal with a square waveform is applied. In the widely used and commercially available TEER measurement system known as an Epithelial Voltohmmeter (EVOM),27 an AC square wave at a frequency of 12.5 Hz is used to avoid any charging effects on the electrodes and the cell layer. The EVOM system has a measurement range of 1-9,999 Ω with a 1Ω resolution and uses a pair of electrodes popularly known as a STX2/“chopstick” electrode pair. Each stick of the electrode pair (4 mm wide and 1 mm thick) contains a silver/silver chloride pellet for measuring voltage and a silver electrode for passing current. The measurement procedure includes measuring the blank resistance (RBLANK) of the semipermeable membrane only (without cells) and measuring the resistance across the cell layer on the semipermeable membrane (RTOTAL) The cell specific resistance (RTISSUE) in units of Ω, can be obtained as:
Where resistance is inversely proportional28 (link) to the effective area of the semipermeable membrane (MAREA) which is reported in units of cm2.
TEER values are typically28 (link)-29 reported (TEERREPORTED) in units of Ω.cm2 and calculated as:
The TEER readings with EVOM2 are highly dependent on the electrode positions and a careful handling of the electrodes is required while introducing them into the well under test to avoid any disturbance to the cells. The uniformity of the current density generated by the electrodes across the cell layer has a significant effect on the TEER measurements. The STX2/chopstick electrode cannot deliver a uniform current density over a relatively large membrane,27 such as the ones used in 24 mm diameter tissue culture inserts, and leads to an overestimation of the TEER value. As an alternative to STX2/chopstick electrodes, an EndOhm chamber30 which allows the cups from culture wells to be inserted can be used. In an EndOhm chamber, both the chamber and the cap contain a pair of concentric electrodes: a voltage-sensing silver/silver chloride pellet in the center plus an annular current electrode. The symmetrical arrangement of circular disc electrodes on both sides of the membrane in an EndOhm chamber generates a more uniform current density across the membrane when compared to STX2/chopstick electrodes. Also, with Endohm's fixed electrode geometry, variation of measurements on a given sample is reduced30 to 1-2 Ω when compared to 10-30 Ω with STX2 electrodes. Apart from the EVOM/STX2/EndOhm systems, a few of the other commercial systems available for TEER measurements include Electric Cell-Substrate Impedance Sensing (ECIS)31 (link) (Applied BioPhysics Inc., Troy, NY), REMS AutoSampler (World Precision Instruments, Sarasota, FL), Millicell-ERS system10 (link), 32 (link) (Millipore Corp., Bedford, USA) and Ussing Chamber Systems33 (link) (Warner Instruments, Hamden, CT).
Where resistance is inversely proportional28 (link) to the effective area of the semipermeable membrane (MAREA) which is reported in units of cm2.
TEER values are typically28 (link)-29 reported (TEERREPORTED) in units of Ω.cm2 and calculated as:
The TEER readings with EVOM2 are highly dependent on the electrode positions and a careful handling of the electrodes is required while introducing them into the well under test to avoid any disturbance to the cells. The uniformity of the current density generated by the electrodes across the cell layer has a significant effect on the TEER measurements. The STX2/chopstick electrode cannot deliver a uniform current density over a relatively large membrane,27 such as the ones used in 24 mm diameter tissue culture inserts, and leads to an overestimation of the TEER value. As an alternative to STX2/chopstick electrodes, an EndOhm chamber30 which allows the cups from culture wells to be inserted can be used. In an EndOhm chamber, both the chamber and the cap contain a pair of concentric electrodes: a voltage-sensing silver/silver chloride pellet in the center plus an annular current electrode. The symmetrical arrangement of circular disc electrodes on both sides of the membrane in an EndOhm chamber generates a more uniform current density across the membrane when compared to STX2/chopstick electrodes. Also, with Endohm's fixed electrode geometry, variation of measurements on a given sample is reduced30 to 1-2 Ω when compared to 10-30 Ω with STX2 electrodes. Apart from the EVOM/STX2/EndOhm systems, a few of the other commercial systems available for TEER measurements include Electric Cell-Substrate Impedance Sensing (ECIS)31 (link) (Applied BioPhysics Inc., Troy, NY), REMS AutoSampler (World Precision Instruments, Sarasota, FL), Millicell-ERS system10 (link), 32 (link) (Millipore Corp., Bedford, USA) and Ussing Chamber Systems33 (link) (Warner Instruments, Hamden, CT).
