Fig. 1

Transcellular and paracellular pathways of epithelial barrier prevents intestinal bacterial influx. Gut barrier is composed of epithelial cells with brush border (BB) as the transcellular barrier, and joined at their apical side by tight junctions (TJs) as the paracellular barrier. The BBs and TJs are physical ultrastructural barriers to prevent influx of commensal bacteria in healthy conditions. Upon epithelial barrier damages such as BB fanning and TJ opening, commensals and pathobionts may gain access to the lamina propria. Photoimages at the left side are (a) scanning electron micrographs of the en face view and (b) transmission electron micrographs of the longitudinal view of the highly organized brush borders in physiological conditions. Photoimages at the right side are (c) scanning electron micrographs of the en face view and (d) transmission electron micrographs of the longitudinal view of the disarrayed brush borders in pathological conditions. (a, c) Bar = 5 μm; (b, d) Bar = 0.5 μm