The junctional epithelium surrounds the
tooth. It is composed of stratified squamous non-keratinized epithelium. It is
attached on one surface by the tooth and on another one to the gingival
connective tissue. It is composed of the internal basal lamina or suprabasal
layer, which extends to the tooth surface, and of the external basal lamina or
basal layer, which faces the connective tissue. The junctional epithelium forms
when the reduced enamel epithelium still lines most of the crown surface and it
is eventually remodeled into the junctional epithelium. It extends from the
base of the gingival sulcus to approximately 2 mm coronal to the alveolar bone
crest. The attachment of the junctional epithelium to the tooth is mediated
through the epithelial attachment apparatus. This consists of hemidesmosomes at
the plasma membrane of the cells directly attached to the tooth (DAT) and the
basal lamina on the tooth surface. The density of the intercellular junctions
is less than that observed in the mouth, which makes it more susceptible to
mechanical disruptions.
The
functions of the junctional epithelium are first to form an epithelial barrier
against plaque and bacteria. Secondly, to allow access of the gingival
crevicular fluid, inflammatory cells and components of the host’s defenses to
the gingival margin. Thirdly, the cells exhibit a rapid turnover, which
contributes to the rapid repair of damaged tissue and equilibrium of parasites.
During disease, the junctional epithelium allows the emigration of
polymorphonuclear cells and the migration of microorganisms from bacterial
plaque and associated toxins to enter the tissue. This process results in acute
inflammation and epithelial ulceration, which allows the damaging agents to
penetrate deeper into the periodontium. In addition, the gingival crevicular
fluid that passes through the junctional epithelium provides the nutrients
necessary for the directly attached to the tooth cells to grow. During health,
the amount of gingival crevicular fluid is minute, but during inflammation the
amount present increases and it becomes something resembling an inflammatory
exudate.
The
junctional epithelium is the first line of defense against microbial invasion
in tissue. Even though it provides a barrier many substances such as lipopolysaccharides
pass through, but have only limited access since both the internal and external
basal layers act as barriers. Another aspect of the defense mechanism is the
rapid turnover, which provides an effective removal of bacterial clinging to
the epithelial cells. Moreover, the junctional epithelium has enzyme rich
lysosomes such as matrilysin, cathepsin and alpha defensing. In addition, the
junctional epithelium cell surface receptors respond to extracellular molecular
changes by producing intracellular adhesion molecules (ICAM), and chemotactic
substances such as C5a, leukotriene B-4 and IL-8.
Continued
exposure of the junctional epithelium to bacterial challenges may lead to
subgingival plaque formation, transformation of the gingival sulcus into a
periodontal pocket, and an increase in the inflammatory focus in the connective
tissue. The consequences stated are reasons to encourage the understanding of
this structure of the oral environment and to encourage the maintenance of oral
health.