The junctional epithelium is found at the base of the gingival sulcus and, in a healthy sulcus, defines this base. The junctional epithelium is made up of mostly flattened epithelial cells that are layered on a basal membrane; only one layer is attached to this basal membrane, and the others adhere to one another. This allows the basal membrane to remain intact and undamaged while outer layers of epithelium are sloughed off through the consistent friction experienced by the inner lining of the mouth.
The junctional epithelium is apical, meaning closer to the root, or apex, of the tooth than the sulcular epithelium. The sulcular epithelium protects the inside of the gingival sulcus and merges with the epithelium of the rest of the inside of the mouth at the free gingival margin. The junctional epithelium is attached to the tooth by the epithelial attachment and is about 1 mm in width. The junctional epithelium attaches to the tooth’s surface on either the enamel, the cementum, or the dentin and is derived from the epithelium of the tooth’s enamel during the development of the teeth. The cells of the junctional epithelium have wider intercellular spaces to allow white blood cells to freely reach the bottom of the gingival sulcus and help prevent disease that may form there. The junctional epithelium is thinner than the other epithelial layers in the mouth and undergoes constant cell division; it does not mature and become keratinized, or thickened and hardened by protein cells, like the other gingival tissues do. This means that there is no thickened, physical barrier protecting the junctional epithelium from microbial attack, and that functional and structural characteristics must instead protect the tissue. This protection occurs through structural arrangement and cellular antimicrobial mechanisms, like the presence of white blood cells and other cells and organelles that contain enzymes that counteract the bacteria in dental plaque. These structural and functional mechanisms can protect the junctional epithelium to an extent, but they may not prevent the development of inflammatory lesions that can lead to loss of bone and the deterioration of the connective tissue that holds the teeth to the gums.
The junctional epithelium has higher permeability which allows the white blood cells to enter and protect the tissue, However, their higher permeability may also allow microorganisms that are present in dental plaque to enter the tissues of the junctional epithelium and create an environment that is conducive to infection. If it is damaged, the junctional epithelium can become irregular in texture on its surface, developing pockets in the epithelium within periodontal pockets and loosening the epithelial attachment to the teeth -- one of the primary characteristics of periodontal disease. This weakened tissue can lead to increased exfoliation of these cells, as well as increased migration of white blood cells and internalization of bacteria. When bacteria are left unaddressed in the gingival sulcus, they can cause the tissues of the sulcus to loosen and the tissues of the junctional epithelium to deteriorate, which can cause the teeth to loosen and fall out.