The endothelium is a thin layer of squamous cells that acts as a semipermeable barrier regulating vascular permeability to let molecules pass through it thereby maintaining tissue fluid homeostasis. Physiological increase in endothelial or vascular permeability is transient, transpired by post-tissue injury during the initial phases of healing, whereas pathological permeability is persistent commonly witnessed in conditions such as atherosclerosis, chronic inflammation, tumor growth, and diabetic retinopathy. The in vivo or in situ use of animal models in the assessment of permeability not only raises inevitable ethical concerns but also confers difficulty to apply to high-throughput screening. Therefore, there is an ever-increasing dependency on in vitro studies to assess drug permeability, and various research programs have suffered to develop appropriate in vitro assays for measurement and prediction. In vitro models that both mimic in vivo microvascular endothelium and can be utilized to record changes in endothelial permeability are vital in delineating the mechanisms involved in the prevention and treatment of disorders related to vascular permeability. The Transwell® and the electric cell-substrate impedance sensing (ECIS) assays are extensively used to assess the trans-endothelial permeability of solutes such as albumin, dextrans, and sucrose across endothelial monolayers and based on electrical resistance, etc. These models have several advantages such as the ease to perform and avoid the complexities of using a live animal.
Keywords: Cell barrier; Endothelial cells; In vitro permeability; Transwell® assay.