Embryology of Tooth Eruption and Exfoliation

Embryology of tooth eruption and exfoliation

• Tooth eruption is a continuous process that begins during early embryonic development
• It involves three phases
• Crown development is complete during the bell stage, alongside root development and is visible in the oral cavity around 6 months of age, but the process begins earlier
• Tooth eruption is a multi-factorial process, and various theories explain the mechanisms

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

Select the single best answer (Question 1)

• The eruption process of teeth begins in early childhood around 6 months of age, but the process begins much earlier

Select the single best answer (Question 2)

• The eruption process of teeth begins before birth and continues throughout life

Eruption of teeth

• The eruption of teeth is a continuous process throughout life that begins during early embryological development
• The process happens in three phases

Definitions

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

Embryological links to eruption and exfoliation

• Initiation, morphogenesis, differentiation, and matrix secretion
• Ameloblasts, osteoclasts, odontoclasts, and oral epithelium play a role
• Underlying knowledge explains the various theories on the eruption mechanisms of teeth

Clinical links to eruption and exfoliation

• Developmental timelines during embryology, continue throughout life
• Essential to identify the presence and position of:
  • Missing teeth
  • Supernumerary teeth
  • Crowding of teeth
  • Orthodontic referrals

Workbook activity

• Complete section 1, question 1-2, to recap existing knowledge of embryology relevant for tooth eruption

Three phases of the eruption process

• Pre-eruptive phase: The movement of the developing tooth within the alveolar bone until crown formation is complete
  • Movement starts during the bell stage
  • Reshaping the bony crypt to allow more space for development
• Eruptive phase: The movement of teeth through the alveolar bone and soft tissue to the oral cavity
  • Occurs around the time of root formation
  • Continues until the tooth reaches occlusion
• Post-eruptive phase: The movement of teeth after active eruption to maintain and compensate for occlusion, wear, and growth
  • Occurs throughout life, as opposed teeth are removed

1. The pre-eruptive phase

• The movement of a developing tooth within the alveolar bone until crown formation is complete
• Occurs during the bell stage
• Bone remodelling allows for more space in the bony crypt for the developing tooth

2. Active eruption phase

• The movement of teeth through the alveolar bone (intraosseous) and soft tissue (supraosseous) to the oral cavity
• Similar time as root formation
• Continues until the tooth reaches occlusion; root formation continues after this phase

3. Post-eruptive phase

• The movement of teeth after active eruptions to maintain and compensate for occlusion, wear, and growth
• Throughout life, such as when an opposing tooth is removed

Single best answer (Question 3)

• The eruption process of teeth starts during the bell stage of development

Active eruption phase in more detail

• Diagrams showing the active eruptive phase:
  • Completion of crown formation
  • Fusion with the oral epithelium
  • Eruption into the oral cavity

The role of the Reduced Enamel Epithelium

• Once amelogenesis is complete, the ameloblasts shrink, and combine with outer enamel epithelium
• Protect developing tooth crowns
• Forms dento-gingival junction (an essential seal)
• Fuses with oral epithelium creating an eruption pathway

The eruption pathway

• The bone overlying the developing crown is resorbed by osteoclasts.
• Creates a pathway for the development of eruption

The eruption pathway (cont.)

• The reduced enamel epithelium fuses with the oral epithelium, forming an eruption path.
• There are no blood vessels and nerves present at that time.
• Stimulation and trauma to the ectomesenchyme enable the fusion.
• Fusion prevents exposure of the ectomesenchyme and hemorrhage

Formation of the dento-gingival junction and sulcus

• As the tooth breaks through the oral epithelium, reduced enamel epithelium and oral epithelium combines
• Creates the dento-gingival junction, sealing the oral cavity off
• Forms a shallow trough that will create the gingival sulcus

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, like tongue, cheek, and lips, guide proper positioning of the tooth
• Sustained force of 4-5g is required

Single best answer

• The reduced enamel epithelium fuses with the oral epithelium to form the periodontal attachment with the tooth

Exfoliation (shedding) of primary teeth

• As the permanent successor teeth grow, signaling the exfoliation of primary teeth.
• Odontoclasts slowly resorb the roots of primary teeth.
• Masticatory forces contribute to exfoliation by applying pressure.
• Variation in the pattern is a key clinical indicator of abnormalities

So how does the tooth actually erupt?

• The exact mechanisms of tooth eruption are not fully known
• Various theories attempt to explain the process

Root formation theory

• (Refuted) The tooth's crown elevation is because of root development

Bone re-modelling

• It is uncertain whether bone resorption causes teeth eruption or if it's an effect
• This is one of the mechanisms modulated by the dental follicle

Dental follicle theory

• Signals between the dental follicle and the reduced enamel epithelium are related to bone remodelling.
• May explain consistency in eruption times, linked to the ameloblast lifecycle.

Periodontal ligament theory

• The power of the formation of the periodontal ligament by fibroblasts contributes to tooth movement during eruption.
• This theory is refuted similarly to the root formation theory

Molecular determinants of tooth eruption

• Numerous molecules are involved in the process, each with a distinct role.
• Recent theories include bite forces on the soft tissues and neuromuscular forces

Summary

• Eruption and exfoliation are complex, multi-factorial processes
• Three phases of the process are discussed in detail, along with how these phases are linked to embryonic development.
• The different theories of eruption mechanisms are outlined.