Models for transepithelial migration across polarised epithelial cells by C. jejuni . Simplified schematic diagram depicting cell junctions and two considered routes of bacterial travel across a polarized epithelium. The apical surface of the epithelial monolayer faces the external environment to the gut and forms the first barrier for C. jejuni invasion. Cell junctions important for the structural stability of a polarized epithelium include the tight junctions, adherens junctions, and matrix receptors as indicated. Various routes for C. jejuni transmigration have been proposed. (A) The transcellular route is characterized by pathogens crossing the epithelial barrier through entering the cells at the apical surface and exiting the cells at the basal membrane. (B) The paracellular route is taken by the bacteria entering the epithelium between two neighboring cells, thus crossing cells through the tight and adherens junctions. Opening of the cell-to-cell junctions maybe a temporal process and potentially close again after C. jejuni have passed. Basal exiting C. jejuni express the adhesin CadF which can bind to the fibronectin→integrin complex utilized for invasion from the bottom of epithelial cells.