c.  Tooth eruption and formation of the junctional epithelium

As the erupting tooth approaches the overlying epithelium, the external cells of the reduced enamel epithelium proliferate, causing the epithelial covering of the enamel to thicken.  Proliferation of the externel cells of the reduced enamel epithelium begins around the cusp tips and slowly progresses toward the cervix of the tooth.  The proliferating cells eventually displace any remaining reduced ameloblasts, thus replacing the relatively inert reduced enamel epithelium with an epithelial collar of cells with a high turnover rate.  This collar of cells with a high turnover rate is the early junctional epithelium.

Eventually, the entire reduced enamel epithelium will become replaced by junctional epithelium, a process that begins at the cusp tips and proceeds in an apical direction.


Fig.   44: Tooth about to erupt through the oral epithelium (OE).  Reduced enamel epithelium (REE) still lines much of the crown surface.  The junctional epithelium (JE), located coronally, is formed by the progressive proliferation of the outer layers of the reduced enamel epithelium beginning near the cusp tips. D, dentin; ES, enamel space.




Fig. 45: Erupting tooth.  Cellular proliferation of the outer cells of the reduced enamel epithelium gives rise to junctional epithelium (JE), an epithelium with a rapid turnover rate. The proliferative cell layer is located mainly in the basal cell layer adjacent to the gingival connective tissue (CT).  The new cells migrate coronally and toward the tooth. In the process,some cells may displace remaining ameloblasts and become attached to the enamel surface.  Eventually, all cells are exfoliated through the gingival sulcus.  The junctional epithelium gradually replaces the reduced enamel epithelium (REE) from the coronal end of the crown apically.  D, dentin; ES, enamel space; OE, oral epithelium.

Fig. 46:  Magnified view of a section of Fig. 45.  The line "x" marks the separation between the junctional epithelium (JE) and the remaining reduced enamel epithelium (REE).  Eventually, the entire reduced enamel epithelium will be remodelled into junctional epithelium, a process that may take several months, if not years.  In the process the primary epithelial attachment (REE to enamel) is replaced by a secondary epithelial attachment (JE to tooth surface). CT, gingival connective tissue; D, dentin; ES, enamel space.


Fig. 47: Transmission electron micrograph of mature junctional epithelium (JE) on an enamel surface. The proliferative cell layer of the junctional epithelium is located primarily along the connective tissue (CT) side to which it is connected by the external basal lamina (EBL).  The basal cells in this layer tend to be cuboidal.  As they migrate simultaneously toward the sulcus and toward the tooth, the cells tend to become more squamous (flattened). The cells on the tooth side of the epithelium are attached to the enamel by hemidesmosomes and the internal basal lamina (IBL). ES, enamel space.

   
The epithelial cells in the junctional epithelium are connected to one another primarily by desmosomes.  However, the density of the intercellular junctions in the junctional epithelium is about one-third that observed in the oral epithelium. This structural difference accounts, in part, for its greater susceptibility to mechanical disruption.

Fig. 48: Higher magnification of the junction between the tooth and the junctional epithelium, as shown in Fig. 47.  The epithelial attachment is mediated by hemidesmosomes (HD) and an internal basal lamina (IBL). Immunocytochemical studies in rats indicate that the internal basal lamina contains laminin, but is missing the type IV collagen normally found in the lamina densa of the external basal lamina.  ES, enamel space.

Fig. 49: Higher magnification of the junction between the gingival connective tissue (CT) and the junctional epithelium, as shown in Fig. 47.  The junction is mediated by hemidesmosomes (HD) and an external basal lamina (EBL).

The dental cuticle is an amorphous layer that appears electron-dense after staining with heavy metal salts such as uranyl acetate and lead citrate. It is typically located between the junctional epithelium and the tooth surface. It varies in thickness up to about 0.5 micrometers (Fig. 50 C).  It probably represents an accumulation of basal lamina material, although its origin as an inflammatory exudate cannot be excluded.

Fig. 50 (From Schroeder, H.E. and Listgarten, M.A., 1977): This composite photomicrograph consists of 3 different views of the interfacial region between junctional epithelium and the enamel surface. is a section through a non-decalcified tissue block in which the dense packing of hydroxyapatite crystallites in the enamel (E) illustrate the true density of the enamel layer as compared to the adjacent soft tissues.  The presence of the hydroxyapatite results in poor tissue staining of the junctional epithelium (JE), internal basal lamina (IBL) and hemidesmosomes (HD). B is a section from a comparable location of a decalcified tissue block. The clear enamel space (ES) represents the previous location of the hydroxyapatite crystallites.   Note the improved staining of the soft tissues, with the internal basal lamina (IBL) in direct contact with the enamel space.  C is a section from a comparable region that includes a dental cuticle (DC) interposed between the enamel space (ES) and the internal basal lamina (IBL).

Tooth eruption is the result of both active eruption and passive eruption. Active eruption refers to the bodily movement of the developing tooth through the overlying jawbone and oral mucosa into the oral cavity. Active eruption normally stops when the erupting tooth comes in contact with its antagonist in the opposite jaw.  Passive eruption refers to the uncovering of the anatomic crown because of apical recession of the surrounding tissues, rather than bodily movement.   Uncovering of the anatomic crown, which results in a longer clinical crown, is due to both active and passive eruption.

Fig. 51 (From Smith, R.G.,1982): Diagramatic representation of the eruption of a maxillary canine from the time of its emergence into the oral cavity until 60 weeks later.  D represents the unerupted portion of the anatomic crown, C the clinical crown, and E the distance from the cusp tip to the occlusal plane. The distance (D + C) represents the length of the anatomic crown. As can be seen from this diagram, the height of the clinical crown increases with time, in part as the result of active eruption which lasts until around week 30, but also as a result of passive eruption which is taking place for the entire 60 weeks.

Passive eruption has been classified into 4 stages by Gottlieb and Orban (1933):

Stage 1: The junctional epithelium is located entirely over the enamel.
Stage 2:   The junctional epithelium is located in part over the enamel and in part over the cementum.
Stage 3:   The entire junctional epithelium is located over cementum, with its coronal end at the
                     cemento-enamel junction.
Stage 4:   The entire junctional epithelium is located apical to the cemento-enamel junction.

University of Pennsylvania and Temple University © 1999. All rights reserved. Created: May 8, 1999  Revised: URL: Max A. Listgarten: Comments to author: max@listgarten.com (Technical Support: Center for Dental Informatics: Heiko Spallek)