Junctional epithelium

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The junctional epithelium (JE) is the tissue that forms the seal where the gums attach to a tooth, at the base of the gingival sulcus (the space between the gum and the tooth). It lies just below the sulcular epithelium and creates the epithelial attachment (EA) to the tooth surface.

In health, the JE is a nonkeratinized, thin layer that attaches to enamel, cementum, or dentin. It is about 1 mm wide from top to bottom, making up roughly half of the biologic width. The JE starts near the crown of the tooth when it first becomes functional after eruption, and a primary EA forms as the tooth develops its first attachment.

The JE comes from the reduced enamel epithelium during tooth development. Before eruption, a basal lamina forms on the tooth surface and becomes part of the EA. As the tooth erupts, the coronal portion of the surrounding tissue peels away, leaving the cervical portion attached to the tooth. This fused tissue near the cementoenamel junction is the early JE and is later replaced by the definitive JE as the root forms.

Structurally, the JE has wide spaces between cells and fewer desmosomes, which helps white blood cells move into the gingival sulcus to fight bacteria. However, this also means JE is more permeable to microbes. The JE is thinner than the sulcular epithelium, ranging from about 15–30 cell layers thick at the sulcus floor to only 3–4 cells thick at the deepest part near the tooth.

Superficial JE cells form the actual epithelial attachment to the tooth through hemidesmosomes and an internal basal lamina. This internal basal lamina connects with the collar of connective tissue beneath. In health, the JE acts as a strong seal between soft gum tissue and the hard tooth surface.

JE cells are highly active but immature, continually dividing at the base to push newer cells coronally as older cells are shed into the sulcus. Unlike keratinized gum tissue, JE does not mature into a keratinized surface, and it lacks a protective keratin layer. Its barrier against microbes comes from its cellular structure and defense mechanisms, including immune cells and lysosomes that help destroy bacteria.

If the JE is damaged, its surface becomes irregular, and pocket epithelium can form inside periodontal pockets. Inflammation can lead to ulceration, gingival overgrowth, changes in tissue structure, increased shedding of cells, more bacterial invasion, and even loss of bone and connective tissue attachment if disease progresses.