Transcellular and paracellular monocyte traversal across human choroid plexus papilloma cells HIBCPP. HIBCPP were stained for zonula occludens (ZO)-1 (red), actin (yellow) and cell nuclei (blue); additionally, monocytes were stained for a monocyte marker (CD14- fluorescein isothiocyanate (FITC), green). (A-D) Three-dimensional immunofluorescence images of HIBCPP and transmigrating monocytes reconstructed from 0.3 μm Apotome optical sections, using Zeiss software Inside 4D. The images show different transmigration routes. Para- and transcelluarly migrating monocytes are shown after TNFα stimulation (A, C) and under control conditions (B, D). The figure shows a respresentative example of three independent experiments that all gave similar results. Scale bar as indicated.