Transepithelial channels for leukocytes in the junctional epithelium

Abstract Background and Objectives The junctional epithelium (JE) has been recognized as a defensive organ rich in polymorphonuclear leukocytes (PMNs). However, the migration of PMNs through the JE has not been clearly documented. For mucosal defense, PMNs migrate outwards over the epithelium to defend the intestinal or respiratory tract on the epithelial surface. With this background, the present study investigated whether there is any structural evidence showing the transepithelial migration of PMNs through the JE in gingival mucosa. Methods Three‐dimensional modeling of gingiva surrounding mouse molars at varying ages was performed by array tomography. Images of the serial sections for array tomography at the 800 nm thickness were obtained using back scattered electron (BSE) detector equipped in the field‐emission scanning electron microscopy (FESEM). Expressions of neutrophil marker or CD47 were immunohistochemically examined on the frozen sections. Results Array tomography using FESEM and 3‐dimensional modeling clearly showed that a system of epithelial channels developed between keratinocytes and generally ran along the long axis of the JE. Most PMNs were found inside the channels, rather than being scattered throughout the JE. The channels could be traced from the base of the JE to the bottom of the gingival sulcus, although some channels were fragmented and interrupted with short intercellular gaps. Conclusions These findings suggest that the JE may be an organ for transepithelial migration of PMNs to the bottom of the gingival sulcus through epithelial channels, as occurs in the epithelial lining of other organs such as the intestinal or respiratory tract.