4.7. Trans-Endothelial Electrical Resistance (TEER) Assay

To evaluate the integrity of the bEnd.3 cell monolayer, the TEER value was measured with a Millipore Millicell ERS system equipped with chopstick electrodes (Millipore, Billerica, MA, USA) as previously described with modification [27]. Briefly, the cells were seeded into the upper ‘apical’ chamber of transwell inserts (3460; Corning, NY, USA) with the density of 2 × 10⁵ cells/well in 0.5 mL DMEM normal culture medium, while 1.5 mL of the same medium was added to the lower chamber to prevent the formation of an oncotic pressure gradient. The TEER values were measured every day with regular intervals until the bEnd.3 cells reached a peak resistance. The values were standardized by the area of the culture inserts and shown as Ω·cm² by subtracting the resistance of blank filters (i.e., no cells) from that of the sample resistance.