Embryology of Eruption and Exfoliation

Tooth Eruption and Exfoliation

• The eruption process is continuous throughout life, beginning in early embryonic development
• The process involves three phases
• The first tooth becomes visible in the oral cavity around 6 months of age, but the process begins much earlier
• The process is not completely understood, but various theories attempt to explain it
• The process is multi-factorial, with ameloblasts, cells of the enamel organ, osteoclasts, odontoclasts, and oral epithelium playing a role

Learning Outcomes

• Describe the three phases of tooth eruption
• Describe how the dento-gingival junction originates
• Describe the process of tooth exfoliation
• Outline the theories of tooth eruption
• Link the processes to the developmental timeline of teeth and eruption/exfoliation ages for each tooth

Selecting the Best Answer (Questions)

• Eruption begins in early childhood (around 6 months of age).
• Eruption begins before birth and continues throughout life
• Eruption is completed once all teeth are present in the mouth
• Eruption is completed once all the primary teeth erupt, and permanent dentition erupts after the primary dentition
• The correct answer is "Begins before birth and continues throughout life"

Definitions

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

Embryological Links to Eruption and Exfoliation

• Amelogenesis begins during the bell stage.
• Ameloblasts, cells of the enamel organ, osteoclasts, odontoclasts, and oral epithelium play roles in eruption.

Clinical Links to Eruption and Exfoliation

• The developmental timeline during embryology—the eruption timeline—is essential for identifying missing or supernumerary teeth, or crowding to assist with orthodontic referral.

Workbook Activity

• Complete Section 1, Question 1-2 to review existing knowledge of embryology relevant to tooth eruption.

Three Phases of Eruption Process

• Pre-eruptive phase: The developing tooth moves within the alveolar bone until crown formation is complete. This movement starts during the bell stage, remodeling the bony crypt to create more space.
• Active eruption phase: Tooth movement through the alveolar bone (intraosseous), then the soft tissue (supraosseous) to the oral cavity. This begins at about the same time as root formation and continues until the tooth reaches occlusion.
• Post-eruptive phase: Tooth movements after active erupiton to maintain occlusion, compensate for occlusal and proximal tooth wear, and allow for continued growth. This continual movement occurs throughout life.

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.
• Muscular forces of the tongue, cheeks, and lips guide the tooth into its position.
• A sustained force of 4–5 g is required.

Single Best Answer (Question)

• The eruption process begins during the bell stage of tooth development.

Exfoliation (Shedding) of Primary Teeth

• Permanent successor teeth develop, increasing in size, initiating the eruptive phase.
• This triggers the exfoliation process for primary teeth. Odontoclasts slowly resorb the roots of primary teeth.
• Masticatory forces also contribute to exfoliation.
• The process is usually consistent, making variations in the pattern a clinical indicator of abnormalities.

So How Does the Tooth Actually Erupt?

• There are various theories attempting to explain the tooth eruption process.
• Research is ongoing.

Root Formation Theory

• The tooth crown is elevated by the thrust of root development.
• This theory is refuted because eruption occurs throughout life and root development doesn't necessarily drive the eruption process.

Bone Remodelling

• Whether bone resorption and deposition cause tooth eruption or is an effect is not clear.
• Not the only mechanism; the dental follicle plays a role.

Dental Follicle Theory

• Signals between the dental follicle and reduced enamel epithelium induce bone remodeling.
• This theory may explain the consistent timing of eruption linked to the ameloblast lifecycle.

Periodontal Ligament Theory

• Periodontal ligament formation by fibroblasts contributes to tooth movement during eruption.
• This theory is refuted similar to the root formation theory.

Molecular Determinants of Tooth Eruption

• Various molecules are involved in the complex tooth eruption process, each playing a distinguishable role.
• Researchers are also exploring more recent theories, including bite forces on tissues and neuromuscular forces.

Summary

• Tooth eruption and exfoliation are multifactorial, complex processes with distinct phases: pre-eruptive, active, and post-eruptive phases, connected to embryological development.
• A range of theories (root formation, bone remodeling, dental follicle, periodontal ligament, molecular determinants) may explain these processes, with ongoing research.