A. We prepared conditioned media from OPCs (OPC-CM), and then added them to cerebral endothelial RBE.4 cells. B. OPC-CM increased in vitro BBB tightness (i.e. decreased permeability of endothelial monolayer), and the TGF-β-receptor signaling inhibitor SB431542 (10 µM) cancelled the effects of OPC-CM. Mean ± SD of n = 3. *p<0.05 vs control (OPC-CM: −, SB431542: −), #P<0.05 vs OPC-CM only (OPC-CM: +, SB431542: −). C–D. Correspondingly, OPC-CM increased the expressions of tight junction proteins, and once again, SB431542 (10 µM) cancelled the effects of OPC-CM. E. Immunostaining confirmed that our cultured rat OPCs produced TGF-β1. PDGF-R-α staining was used to confirm the purity of our OPC cultures. F. LDH assay showed that the TGF-β receptor inhibitor SB431542 (10 µM) did not affect endothelial viability. Mean ± SD of n = 3.