a brain-like phenotype. To confirm the existence of tight junctions between ciBECs, we analyzed the cells by transmission electron microscopy. ciBECs co-cultured with astrocytes clearly formed tight junctions (Figure 5D). To further validate the barrier function in our iPSC-derived BBB model, we investigated the permeability of fluorescein-Na (FN, 376 Da) and fluorescein isothiocyanate (FITC)-dextran of different masses (4, 10, 20, 40, and 70 kDa), which cross the BBB non-specifically by paracellular diffusion. ciBECs showed significantly decreased permeability of FN (39.3% ± 3.8%) and 4-kDa FITC-dextran (26.1% ± 5.6%) compared with ECs, hCMEC/D3, and HUVEC (Figures 5E and S6). Heavier FITC-dextran also tended to have less permeability across ciBECs compared with ECs (Figure 5E). Taken together, hiPSC-derived ciBECs have a strong barrier function, including high transendothelial resistance and low rates of paracellular leakage.
