Tight junctions (TJ) in human malignant choroid plexus papilloma cell line (HIBCPP) grown on transwell filter supports in the standard (A, C) and the inverted (B, D) culture system, respectively. Transmission electron microscopy (A, B) shows that in both culture systems the cells are connected by TJs (arrows), which are located close to the apical side as indicated by the presence of microvilli. Examination of HIBCPP by freeze fracture electron microscopy (C, D) revealed a broad band of closely meshed TJ strands. The diameter of mesh was in the magnitude of 0.2 to 0.4 μm. Reproduced with permission from the author .