ciBECs were cultured onto cell-culture inserts as described above. ciBECs were washed once with PBS and fixed in 2% glutaraldehyde (Microscopy Sciences) and 4% paraformaldehyde (Sigma-Aldrich) overnight at 4°C. All sections were treated with OsO4 (1% for 1 min and 0.5% for 20 min at 4°C) in PBS, dehydrated in ethanol and propylene oxide, and embedded in Luveak 812 (Nacalai Tesque). Ultrathin sections were cut with an ultramicrotome (Leica) and observed with a transmission electron microscope (H-7650; Hitachi).
