User:Xorx/sandbox/Transepithelial electrical resistance

The Transepithelial/Transendothelial Electrical Resistance (TEER) is the electrical impedance of cell cultures measured with alternating current in the SI unit ohm (Ω). If the frequency of the current is not provided 12.5 Hz are assumed. A frequency of 1000 Hz is also typically used.

Ohm's Law
An alternating current (AC) is applied through the cell culture to determine the resistance and thus the integrity of a biological barrier. To determine the resistance measurement of a monolayer, a recorded reading of a blank Transwell insert without cells is subtracted from a measured reading of an insert with the grown monolayer. TEER measurements are typically inversely proportional to the effective area of the monolayer in the insert, meaning that TEER measurements become higher as the effective area decreases and vice versa. This inversely proportional relationship between TEER value of a tissue (R) and effective area of the membrane (M) can be shown through the following equation:

$$R_{tissue}(\Omega) \quad \alpha \quad \frac{1}{M_{Area}(cm^2)}$$

Applications/Examples in Biological Modeling
TEER Assays have been used in various research studies to determine the integrity of cell junctions in-vitro for the following:

Blood-Brain Barrier (BBB)
TEER Assays are developed for the BBB since in-vivo models from different species, such as rat and mice, don't translate very well to humans. Therefore, researchers have been looking into a reliable in-vitro model to measure the barrier integrity of the BBB. Two challenges associated with creating a proper human BBB model is the lack of human BBB cells and poor barrier formation when attempting to grow human endothelial cells. Currently, there has been development of a human BBB model for TEER Assay using a combination of pluripotent stem cells and chemical treatment.

Gastrointestinal (GI) Tract
TEER Assays are attempted to be applied to GI tract since a proper model could give insight into how nutrients and drugs are delivered through the GI tract's epithelial layer junctions. Some research studies have developed Caco-2 endothelial cells from the colon or rectum to imitate GI epithelial cells and its integrity has shown to be properly measured by the TEER assay.

Pulmonary Models
Lung epithelial cells are characterized by tight junctions that are covered by a lung surfactant. TEER Assays are attempted to be applied to pulmonary airway models since the integrity of lung epithelial cells can be easily damaged from shear and/or mechanical stresses. Pulmonary models have been replicated using each using cells that have been individually isolated from rat, dog, bovine, horse, and human. These endothelial cells have been grown using an air-liquid interface where cell medium is placed beneath the growing cells, but the top layer is only exposed to air instead of cell medium.

Organ-on-Chips
Since there are many external environmental factors that could affect TEER measurements, TEER Assays have been commonly implemented into organ-on-chip models that use small electrodes and a small effective area of an organ to create consistent results that would be unaffected by factors such as temperature, medium formulation, and human error that could occur from manually handling the probes. TEER Assays have been implemented to chips that have been used to model the function of a human placenta.

Notes and References
Category:Monolayers