The term Junctional- epithelium (JE) denotes the tissue that is affixed to the tooth on one side and to the oral sulcular epithelium and connective tissue on the other side1,2. Junctional epithelium is the epithelial component of dento-gingival unit that is in contact with tooth surface. It is not keratinized on its free surface layer, so cannot act as a physical barrier. So, other structural and functional characteristics of JE must compensate for the absence of this barrier. The JE fulfills this difficult task with its special structural framework and the collaboration of its epithelial and non-epithelial cells that provide very potent antimicrobial mechanism.
Histological section of Gingiva
However these defence mechanisms do not preclude the development of extensive inflammatory lesions in the gingiva. The conversion of JE to pocket epithelium is regarded as the hallmark in the progression of gingivitis to periodontitis. That is why it is a dynamic structural entity as its structure and function differs significantly from oral gingival epithelium.
2) General and microscopic features of Junctional epithelium (JE) include:
- Thickness varies from 15-18 cells at the base of gingival sulcus to 1-2 cells at its most apical portion.
- Cells are arranged in basal and supra-basal layer only.
- Basal cells and adjacent 1-2 suprabasal layers of cells are cuboidal or slightly spindle shaped. All remaining cells of the suprabasal layer are flat, oriented parallel to the tooth surface and closely resemble each other.
- The inner most layer of suprabsal cells (facing the tooth surface) are called DAT cells (directly attached to tooth)3.
- Lysosomal bodies are found in large number in JE cells. Enzymes contained within these lysosomes participate in eradication of bacteria.4
- JE cells have numerous golgi apparatus, abundant rough endoplasmic reticulum with cisternae and polyribosomes.
- JE cells exhibit a unique set of cytokeratins including cytokeratin 5, 13, 14 and 19. Occasionally weak activity of cytokeratins 8,16 and 18 is also seen5. Compared to other epithelia, JE cells are interconnected by a few desmosomes and occasionally by gap junctions6-9 .These features account for the remarkable permeability of JE.
- A variety of mononuclear leucocytes occupy these interstitial spaces. neutrophills(PMN’s)are found in the central region of the JE and near the tooth surface.1o
- These mononuclear cells along with their products, products of junctional epithelial cells, blood and tissue fluid represents the first line of defense in the control of the perpetual microbial challenge. These molecules include α- and β-defensins, cathelicidin LL-37, interleukin (IL)-8, IL-1α and -1β, tumor necrosis factor-α, intercellular adhesion molecule-1, and lymphocyte function antigen-3.
- Antigen presenting cells (APC’s) and langerhans and other dendritic cells are present as well.11
- JE, particularly its basal cell layers is well innervated by sensory nerve fibers.12-15
- In absence of clinical signs of inflammation, approximately 30,000 PMN’s migrate per minute FROM JE of all teeth into the oral cavity.16
- Cells originate in basal layer and migrate in an oblique direction towards and along the tooth surface, where they are sloughed from the free surface.
- Junctional epithelial cells show no signs of synthesis of membrane coating granules,a finding that agrees with the fact that the JE is highly permeable to water soluble substances. The chief barrier to passage of substances larger than 100 KDa is provided by the external basal lamina.
3) Formation of Junctional Epithelium (JE):
To understand the formation of JE we must understand the epithelium- tooth interface first. The following concepts were given historically to explain the epithelium-tooth interface and subsequently formation of junctional epithelium was described.
Gottlieb’s experimental and clinical observations led to the concept that the soft tissue of gingiva is organically united to enamel surface17,18 . He termed the epithelium contacting the tooth surface along with the interface substance the “epithelial attachment”. According to this concept, final task of ameloblasts upon completion of enamel matrix formation is the production of primary enamel cuticle19 which is continuous with the enamel matrix and attaches the cells of REE (reduced enamel epithelium) to the calcified tooth structure.
At the onset of tooth eruption the cells of REE unite with proliferating oral epithelium. As eruption proceeds the epithelial cells adjacent to the enamel surface produce a cornified layer of material Gottlieb referred to as secondary enamel cuticle and subsequently become separated from the tooth surface leaving a V-shaped groove, the gingival crevice.
Orban and his colleagues gradually modified Gottlieb’s concept in 1944. Orban20 incorporated the views of Mayer21-22,beck23-24 and weski25by stating that the separation of the epithelial attachment cells from the tooth surface involved preparatory degenerative changes in the epithelium26. This statement was a sharp departure from the Gottlieb’s concept of production of a cornified cutical layer.
Inspite of objections concept of epithelial attachment dominated throughout until 1952, when Waerhaugh presented the concept of “epithelial cuff”27. This concept was based on insertion of thin blades between the surface of the tooth and gingiva. The blades could be passed apically to the connective tissue attachment at the cement-enamel junction without resistance. Based on these findings and other microscopic findings he concluded that the gingival tissue and tooth are closely adapted but not organically united.
Schrouder and listgartan’s concept:
Resolution of the controversy regarding the nature of epithelial- tooth interface was not possible until the introduction of transmission electron microscope. Subsequently in extensive studies, Schrouder and listgartan illustrated the details of the structural relationship of epithelial tooth interface28They gave following terminologies-
Primary epithelial attachment is the term that has been used to describe the relationship of epithelium to unerupted tooth. This is formed during the maturation of enamel but prior to tooth eruption. The reduced amealoblasts elaborate a basal lamina referred to as the epithelial attachment lamina. This structure lies in direct contact with the enamel surface and the epithelial cells are attached to it by hemi-desmosomes; no evidence indicates the presence of dental cuticle at this stage. As eruption proceeds, mitosis occurs in the basal layer of the oral epithelium and the outer layers of REE, but the amealoblasts no longer divide.
The reduced amealoblasts and the other cells of REE are transformed in to junctional epithelial cells and the primary enamel epithelial attachment then becomes the secondary epithelial attachment. The amealoblasts do not degenerate or cornified as was previously supposed; they undergo dramatic nuclear and cytoplasmic reorganisation, including the development of cytoplasmic filaments, golgi apparatus and other features that make them indistinguishable from junctional epithelial cells. The secondary epithelial attachment in its simplest form is made up of the epithelial attachment, lamina and hemi-desmosomes.
At the site near to CEJ the complete enamel may become denuded of its epithelial covering and subsequently may lie in direct contact with the connective tissue. When this development occurs, the connective tissue product, afibrillar cementum may be elaborated and deposited on surface of enamel.29
4) Epithelial attachment at molecular level:
JE faces both the gingival connective tissue and tooth surface. Basement membrane is interposed between the basal cells of JE and gingival connective tissue. Basal lamina forms part of the interfacial matrix between the tooth facing junctional epithelial cells and tooth surface (DAT cells). At the apical end of JE, the basal lamina and basement membrane are continuous.
Basement membrane is specialised extracellular matrix that is interposed between connective tissue and the epithelium. It is thought to play the role in compartmentalisation (physical barrier function), filtration (selective permeability barrier function), cell polarisation, migration, adhesion and differentiation. It usually consists of lamina lucida (also known as lamina rara) towards the epithelium, lamina densa towards the connective tissue and a lamina fibroraticularis (also known as sub-basal lamina). The lamina fibro-reticularis is a discontinuous layer consisting of reticular and anchoring fibrils and face the connective tissue site from which it is supposed to originate. The width of basal lamina is reported to be in the vicinity of 800 A˚ -1200 A˚.
Constituents of basement membrane are collagen type IV and VII, leminin, heparin sulphate proteoglycan, fibronectin, nidogen (entactin), and proteoglycan perlecan. Basement membrane of junctional epithelium resembles other basement membranes but basal lamina has distinctly different structural and molecular characterstics. It lacks most of the common basement membrane components such as collagen type IV and VII, most laminin isoforms, perlecans and a lamina fibroreticularis30-32. Thus basal lamina of JE has its own characterstics aans cannot be regarded as the basement membrane in true sense.
5) Location and functions of molecular factors associated with JE:
Cells have surface or cell membrane molecules that play a role in cell matrix and cell-cell interactions. JE cells express numerous cell adhesion molecules (CAM’s), such as integrins and cadherins33.
Knowledge about structures and molecules involved in the maintenance of cell- cell contact is particularly important in view of the pathological changes that the junctional epithelium undergoes during its conversion to a pocket lining. Following are the general facts about the molecular factors associated with junctional epithelium.
- Integrins are cell surface receptors that mediate interactions between cell and extracellular matrix, and also contribute in cell to cell adhesion.34-35
- The cadherins are responsible for tight contacts between cells.36-37
- E-cadharin, an epithelium specific cell adhesion molecule, plays a crucial role in maintaining the structural integrity.
- Intercellular adhesion molecule-1 (ICAM-1 or CD-54) and lymphocytic function antigen- 3 (LFA-3) are additional cell adhesion molecules.
- Cells in contact with the internal basal lamina express the α6β4 integrins, a laminin receptor.
6) Role of ICAM in immuno-regulation:
ICAM’s are immunoglobulins like transmembrane glycoproteins that mediate cell-cell interactions in inflammatory reactions. They function as ligands for β2 integrin molecules present on leucocytes and participate in the control of leucocyte migration into inflammatory sites. Expression of ICAM-1 and lymphocyte function antigen-3(LFA-3) has been demonstrated in the junctional epithelial cells.38-41
The establishment of a gradient of ICAM-1 expression with in JE is thought to be an important mechanism for guiding PMN’s towards the bottom of the sulcus, where they could counteract the bacterial challenge. In this respect high expression of IL-8, a chemotactic cytokine, in the coronal most cells of the JE may be additional mechanism of routing PMN’s towards the bacterial challenge.42-43
7) Mucosa and junctional epithelium around implants:
The mucosa that surrounds an oral implant (made of commercially pure titanium) is different from the gingiva, and the bone tissue that enables osseointegration contains hardly any periodontal structures. Studies done on animal models have demonstrated the structural characteristics of the gingival (at teeth) and the mucosa that encompass implants44-49. Their observations were; the healthy soft, keratinized tissues facing teeth and implants frequently have a pink color and a firm consistency. The two tissues have several microscopic features in common.
The gingiva as well as the keratinized, peri-implant mucosa is lined by a well-keratinized oral epithelium that is continuous with a junctional epithelium that is about 2 mm long. The junctional epithelium at a tooth site terminates at the cementoenamel junction, apical of which an acellular, extrinsic fiber cementum establishes an important component of the supra-alveolar attachment apparatus. The distance between the cementoenamel junction and the bone crest is about 1 mm and is characterized by the collagen fibers that project from the cementum into the connective tissue and the bone. In the implant site, the apical portion of the junctional epithelium is consistently separated from the alveolar bone by a zone of noninflamed, collagen-rich but cell-poor connective tissue. This zone, which is about 1-1.5 mm high, is continuous with the junctional epithelium and establishes together with the epithelium an implant-mucosa attachment that is about 3-4 mm high. In the collagen-rich zone, the fibers invest in the marginal bone and run a course more or less parallel to the implant surface.
At the biochemical level as well, there are no differences between the peri-implant and periodontal soft tissue, even if some higher amounts of collagen type V and VI were noticed50. The vascular supply of the peri-implant gingival or alveolar mucosa is more limited than that around teeth. Indeed, because of the lack of a periodontal ligament, this vascular supply is often reduced51 because the principal propreoception in natural dentition comes from the periodontal ligament; its absence around the implants reduces the tactile sensitivity52 and reflex function.53
8) Regeneration of junctional epithelium:
Injury to junctional epithelium may occur due to intentional or accidental trauma. Usually it occurs due to accidental trauma during brushing, flossing or eating. Intentional trauma occurs during periodontal surgeries where the junctional epithelium is completely lost. Many studies have been done to investigate the renewal of junctional epithelium. These include studies done on renewal of junctional epithelium on tooth and implant surface after mechanical detachment by probing, studies done on mechanical trauma during flossing and studies on regeneration of junctional epithelium after gingivectomy procedure which completely removes junctional epithelium.
A study was done on marmosets in which probing was used to mechanically detach junctional epithelium. A new and complete attachment indistinguishable from that in controls was established 5 days after complete separation of the junctional epithelium from the tooth surface54. Another study done on dental implants revealed that healing around implants takes almost the same time for re-establisment of junctional epithelium as that of tooth55. Both of the above studies showed that probing injury leads to a completely reversible injury to junctional epithelium.
One study investigated the healing of junctional epithelium after mechanical trauma by flossing. The results of the study revealed that detachment of cells persisted for 24 hrs after flossing ceased. New attachment of junctional epithelial cells started 3 days after flossing ceased. After 2 weeks, the cell populations on the experimental and control surfaces were again indistinguishable from each other.56
Many studies have been done to investigate the formation of new junctional epithelium after periodontal surgeries as they completely remove the junctional epithelium57-60. In a human study, that evaluated healing following an inverse bevel flap in man demonstrated newly differentiated attachment apparatus with normal hemidesmosomal attachment is possible following surgery. This new attachment apparatus was seen on cementum as well as dentin58. Another study performed gingivectomies on two young cynomolgus monkeys and examined block sections of tooth/gingiva at 12 days and 3, 4, and 7 weeks post surgery. The electron microscopic examination revealed that the JE was completely re-established at 12 days. Hemidesmosomes appeared to form prior to the basal lamina. The basal lamina initially formed in close proximity to the hemidesmosomes at both the tooth and connective tissue interface. At 4 to 7 weeks, the basal lamina appeared complete61. Studies have shown that regeneration of junctional epithelium after gingivectomy procedure usually occurs within 20 days.61-63
9) Turnover of the attachment epithelium:
One study investigated renewal time in the gingival epithelium of marmosets using injections of tritiated thymidine and autoradiography. Epithelial cells in the attached gingiva exhibited a renewal rate of 10.4 days, whereas the corresponding rate for the epithelial cuff was 5.8 days64. In another study autoradiography was used to study the rate of migration of attachment epithelium. The authors observed that the rate was comparable to the rate of tooth eruption, suggesting that the location of the attachment is relatively stable.65
Junctional epithelium is important because of its anatomical location. It is the site of host-bacterial interaction in initiation of periodontal diseases. There is a constant presence of bacteria and their products in gingival sulcus which makes junctional epithelium an important structural component of periodontal defense mechanism. As discussed in the present article, junctional epithelium plays an important role in immunological response of the host against bacterial challenge. When the bacterial challenge overwhelms the host response, periodontal diseases are initiated. The conversion of the junctional epithelium to pocket epithelium is regarded as a hallmark in the development of periodontitis. Our future research is directed to find out the therapeutic strategies that halt the disease progression at this important tooth-tissue interface.
The concept of Biological width: The biological width is defined as the dimension of the soft tissue, which is attached to the portion of the tooth coronal to the crest of the alveolar bone. It is important from restorative point of view because its violations lead to complications like gingival inflammation, alveolar bone loss and improper fit of the restorative component.
Diagram showing Biological width (Gargiulo et al)
Gargiulo et al (1961)66 in their study described the dimensions and relations of dento-gingival junction in humans. The average histological width of connective tissue attachment was 1.07 mm. The mean average length of epithelial attachment was 0.97 mm with a range of 0.71-1.35mm. The average combined histological width of connective tissue attachment and junctional epithelium was 2.04 mm, which is referred to as the Biological width.
Vacek et al (1994)67 evaluated 171 cadaver tooth surfaces. They observed mean measurements of 1.34 mm for sulcus depth, 1.14 for epithelial attachment, and 0.77 mm for connective tissue attachment.