Embryology of Tooth Eruption and Exfoliation medium

Embryology of Tooth Eruption and Exfoliation

• Tooth eruption is a continuous process throughout life, beginning in early embryological development.
• It involves three phases: pre-eruptive, eruptive, and post-eruptive.
• Eruption begins during the bell stage of tooth development.
• Pre-eruptive movement of the developing tooth occurs, completing crown formation within the alveolar bone.
• Active eruption involves movement of the tooth through the alveolar bone (intraosseous) and then the soft tissue (supraosseous) to the oral cavity.
• This phase begins approximately around the time of the root formation and continues until occlusion.
• Post-eruptive movement continues throughout life to maintain occlusion and compensate for occlusal/proximal tooth wear and growth.
• This movement can occur when an opposing tooth is removed.
• The rate of eruptive movement through bone is slow (1-10μm/day), while movement through soft tissue is faster (75μm/day) until occlusion is reached.
• Dental follicle, bone remodelling, and periodontal ligament theories exist but scientists aren't sure how tooth eruption precisely works.
• Muscular forces of the tongue, cheek, and lips guide the tooth into position.
• Sustained force of 4-5g is required.
• Various theories attempt to explain the precise molecular mechanisms for this process, which are still under investigation.

Learning Outcomes

• Describe the three phases of tooth eruption.
• Describe how the dentogingival junction originates.
• Describe the process of tooth exfoliation.
• Outline the theories of tooth eruption.
• Link the processes to the developmental timeline of teeth and to eruption/exfoliation ages for each tooth.
• Clinically distinguish normal and abnormal tooth eruption/exfoliation.
• Identify supernumerary/missing teeth.

Definitions

• Eruption: Physiological movement of teeth from their developmental position in the alveolar bone through soft tissues to their functional position in the oral cavity.
• Exfoliation: Physiological resorption of primary teeth until they are lost.

Embryological Links to Eruption and Exfoliation

• Processes including initiation, morphogenesis, cell differentiation, and matrix secretion are involved in tooth eruption and exfoliation.
• Cells of the enamel organ, osteoclasts, odontoclasts, and oral epithelium play a role.

Clinical Links to Eruption and Exfoliation

• Developmental timeline during embryology continues into life and is essential to identify teeth presence/position.
• Crucial for identifying missing/supernumerary teeth and crowding.

Workbook Activity

• Complete Section 1, Questions 1-2 to recap embryological knowledge relevant for tooth eruption.

Three Phases of the Eruption Process

• Pre-eruptive: tooth movement within the alveolar bone until crown formation is complete.
• Eruptive: movement through alveolar bone (intraosseous) and soft tissue (supraosseous) to the oral cavity.
• Post-eruptive: movement after active eruption to maintain and compensate for occlusal and proximal tooth wear/growth.

The Role of the Reduced Enamel Epithelium

• Once amelogenesis is complete, the ameloblasts shrink, combining with outer enamel epithelium, intermedium, and residual stellate reticulum.
• This forms the reduced enamel epithelium, protecting the developing tooth crown.
• It also merges with oral epithelium to form the dentogingival junction.

The Eruption Pathway

• The bone overlying the crown is resorbed by osteoclasts (and odontoclasts), allowing a pathway for movement through initiating complex signaling in the cells.
• The reduced enamel epithelium protects the tooth crown from osteoclasts/odontoclasts.
• Reduced enamel epithelium and oral epithelium fuse to form an eruption pathway.

Formation of the Dentogingival Junction and Sulcus

• As the tooth breaks through the oral epithelium, the reduced enamel/oral epithelium combine, creating the dentogingival junction that seals the external oral cavity.
• It forms a gingival sulcus—a shallow trough.
• This junction is significant for periodontal disease and the long junctional epithelium of the gingiva.

Rate of Eruptive Movement

• Movement through bone is slow (1-10 μm/day).
• Movement through soft tissue is faster (75 μm/day) until occlusion is reached.

Single-Best-Answer Questions/Answers

• Various questions and answers are provided regarding tooth eruption, including the most correct answer per question, like who develops the reduced enamel epithelium.

Exfoliation (Shedding) of Primary Teeth

• As permanent successors develop in position (lingualling), they increase in size which starts the eruptive phase.
• Odontoclasts slowly resorb the roots of the primary teeth until the crown largely remains intact.
• Masticatory forces contribute to the entire process (exfoliation).
• The pattern of exfoliation provides information about abnormalities.

So How Does the Tooth Actually Erupt?

• The exact mechanisms aren't entirely understood, and ongoing research attempts to uncover better answers.

Root Formation Theory

• The idea that tooth crowns elevate due to the force of root development has been refuted since eruption occurs throughout life and root development does not occur until later phases.

Bone Re-modelling

• It is unclear whether bone resorption/deposition directly triggers eruption or is a resulting effect.
• The dental follicle plays a role in modulating bone remodelling.

Dental Follicle Theory

• Dental follicle and reduced enamel epithelium signaling is linked to bone remodelling consistency
• Consistent with ameloblast lifecycles.

Periodontal Ligament Theory

• Periodontal ligament fibroblasts contribute to tooth movement.
• Though, this theory is refuted similar to the root formation theory.

Molecular Determinants of Tooth Eruption

• Various molecules play diverse roles in the complex process of tooth eruption.
• Recent theories also investigate the influence of bite forces on soft tissues and neuromuscular forces on movement.

Summary

• Tooth eruption and exfoliation are complex, encompassing three phases.
• The active phase is linked to embryological development (detailed in these notes).
• Several theories aim to explain tooth eruption but remain under investigation.