1. Development of the dentogingival junction
Dr Jamal Naim
PhD in Orthodontics
Development of the dentogingival junction

2. Early stages
During the bell stage of tooth development, the developing tooth consists of an ectodermal component, the enamel organ, derived from the oral ectoderm, that surrounds an ectomesenchymal component, the dental papilla (DP).  

3. Early stages
Within the bell-shaped ectodermal structure several layers are visible. The innermost layer, or inner dental epithelium (IEE) is in intimate contact with the dental papilla.  Next to it is the stratum intermedium (SI) consisting of 2-4 layers of undifferentiated epithelial cells.

4. Early stages
The bulk of the cells are part of the stellate reticulum (SR), a network of star-shaped epithelial cells.
The outermost layer consists of a single layer of cells, the outer dental epithelium (OEE).  

5. Early stages
The cells of the dental papilla in contact with the inner enamel epithelium differentiate into odontoblasts (OB) that proceed to form predentin (PD) and dentin (D).  The inner enamel epithelium cells, in contact with the first-formed dentin, differentiate into preameloblasts and then into ameloblasts that produce enamel on top of the already formed dentin. 

6. Reduced enamel epithelium
Shortly after the onset of amelogenesis, the stellate reticulum (SR) shrinks considerably so that the outer enamel epithelium (OEE) comes into close contact with the stratum intermedium cells.
The collapse of the enamel organ results in the formation of the reduced enamel epithelium.
The ectomesenchymal tissue that surrounds the enamel organ forms the dental follicle (DF).

7. Reduced enamel epithelium
Once the enamel is completely mineralized, the ameloblasts shrink from columnar to cuboidal or flattened cells, but remain a part of the reduced enamel epithelium that forms a more or less continuous lining over the completed enamel.

8. Reduced enamel epithelium
The reduced enamel epithelium consists of two major cellular layers:
the reduced ameloblasts that are in contact with the enamel, but are no longer able to undergo cell division, and
the external cells of the reduced enamel epithelium that consist mostly of stratum intermedium cells, and cellular remnants of the stellate reticulum and outer enamel epithelium.

9. Reduced enamel epithelium
The external cells of the reduced enamel epithelium consist of undifferentiated epithelial cells able to divide and multiply. 
It is the external cells of the reduced enamel epithelium that will eventually give rise to the junctional epithelium.

10. Reduced enamel epithelium
epithelial plug
Unerupted tooth after completion of amelogenesis. The crown is located within a bony housing. The enamel is covered by a thin reduced enamel epithelium (REE).

11. Reduced enamel epithelium
The reduced enamel epithelium (REE) adjacent to the enamel space (ES) ends at the cementoenamel junction, near the bottom of the CT, follicular connective tissue

12. 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 external cells of the reduced enamel epithelium begins around the cusp tips and slowly progresses toward the cervix of the tooth. 

13. formation of the junctional epithelium
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.

14. formation of the junctional epithelium
The junctional epithelium gradually replaces the reduced enamel epithelium (REE) from the coronal end of the crown apically.
The line "x" marks the separation between the junctional epithelium (JE) and the remaining reduced enamel epithelium (REE). 

15. formation of the junctional epithelium
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.

16. Tooth eruption
Tooth eruption is the result of both active 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.

17. Tooth eruption
The outer layer of the reduced enamel e layer of oral epithelium will proliferate into the degenerated C.T. to form a mass of cells over the erupting tooth (epithelial plug).

18. Tooth eruption
Cell death in the middle of the epithelial plug leads to the formation of epithelial lined canal through which tooth will erupt without hemorrhage.

19. Tooth eruption
Once the tip of the crown appears in the oral cavity enamel epithelium will be called primary attached epithelium, and the shallow groove present between the tooth and the gingiva is called gingival sulcus.

20. Tooth eruption
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.