Characterization of junctional structures in the gingival epithelium as barriers against bacterial invasion

Abstract Background and objective Adherens junctions (AJs) and tight junctions (TJs) are known to play a crucial role in maintaining the physical barrier function of the epithelium. Here, we aimed to characterize the distribution of AJs and TJs throughout the gingival epithelium and to obtain insights into the physiological importance of these junctional structures. Methods Sections of mouse gingival tissue were examined using transmission electron microscopy (TEM) and bio‐high voltage electron microscopy tomography. The gingival sections were stained for E‐cadherin and JAM‐A as markers of AJs and TJs, respectively, and examined using confocal microscopy and lattice structured illumination microscopy. Bacteria within the gingival epithelium were examined using in situ hybridization. Results Junctional structures, including desmosomes, AJs, and TJs, were observed throughout the gingival epithelium. The expression levels of E‐cadherin were particularly low in the granular/keratinized layers of the oral epithelium (OE), while extremely low JAM‐A levels were detected in the granular/keratinized layers of the sulcular epithelium (SE). The three‐dimensional rendering of the junctional structures revealed that both AJs and TJs in the gingival epithelium formed discontinuous short bands or patches. Interestingly, strong bacterial signals were observed at the granular/keratinized layers of both SE and OE, but a few bacteria were detected within the junctional epithelium (JE) and the basal/spinous layers of the SE and OE. Conclusions AJs and TJs form a discontinuous barrier throughout paracellular passage in the gingival epithelium; nevertheless, they seem to play an important role in defending against invading bacteria.