Root Development & Eruption

• is permanent teeth and primary teeth eruption the same?

  eruption of permanent teeth involves:

  - the resorption of the roots of the deciduous teeth which takes place by odontoclasts.

  - the shedding of the crown of the deciduous tooth so the permanent tooth can erupt within the oral cavity
• post- eruptive phase

  
  

  when does the post-eruptive phase stop?

  The post-eruptive phase continues as long as the tooth remains in the oral cavity.
• post- eruptive phase

  
  

  what causes the mesial shift?

  by a combination of forces acting on the teeth:

  - the anteriorly directed force generated by the teeth when they're in occlusion

  - the pressure exerted by trans-septal fibers of the PDL to keep neighboring teeth in contact

  - the pressures exerted by the surrounding soft tissue like the tongue and cheeks.
• post- eruptive phase

  
  

  what is interproximal wear compensated by?

  Interproximal wear, which is the wearing of the contact surface between the teeth, is compensated by a process called mesial or approximal shift.
• post- eruptive phase

  
  

  what is occlusal wear compensated by?

  occlusal wear, which is the loss of tooth structure on occlusal surface, is compensated by cemental deposition around the apex.
• post- eruptive phase

  
  

  what happens are the jaw height keeps increasing?


  New bone is deposited along the socket floor to maintain the height of the erupted tooth in the arch.
• post- eruptive phase

  
  

  what is the post- eruptive phase?

  it is the phase of checking to see if the teeth remain in the optimal position and correcting any abnormalities due to jaw growth and occlusal or interproximal wear.
• eruptive stage

  
  

  what does eruption of the tooth leave behind?

  It leaves behind some space at the bottom of the bony crypt.

  -> This space gets filled up by fine bony trabeculae, or bone with tiny gaps in its structure. The bony trabecule/bony ladder, provides support to the apical tissues.

  At the end of the eruptive phase, this bony ladder is resorbed to give way for the developing root tip.

  
  

  Ultimately, bundles of PDL fibers are laid down at the apex, extending from the root tip to the bony floor
• eruptive stage

  
  

  how do PDL fibres stabilise the tooth and help in the eruption process?

  Researchers have found that PDL fibers continuously attach and detach from the tooth as it erupts. It is thought that these dynamically changing PDL fibers play a role in stabilizing the tooth and help in the eruption process.
• eruptive stage

  
  

  what cells speed up eruption?

  In addition to fibroblasts, PDL fibers contain a special type of fibroblast called the myofibroblast.

  -> Myofibroblasts are cells with contractile properties, and are thought to speed along the eruption.
• eruptive stage

  
  

  where does PDL fibres first appear?

  The first bundle of PDL fibers (alveolar crest fibres) appear at the cervical area of the tooth root and radiate coronally to the alveolar process.

  As the tooth erupts, more PDL fibers appear along the length of the root and existing ones become more prominent.
• eruptive stage

  
  

  what happens as the tooth erupts?

  - Epithelial root sheath begins to proliferate, and starts to form the root.

  
  

  - Bundles of collagen fibers start appearing in the surrounding dental follicle. The collagen bundles get invaded by fibroblasts that lay down fibrous tissue, and thickens the bundles to form the periodontal ligament, or PDL.
• eruptive stage

  
  

  what happens to the ameloblasts that cover the tooth crown during tooth eruption?

  they are lost

  
  

  -> so enamel cannot be repaired if damaged
• eruptive stage

  
  

  why is eruption relatively painless and bloodless?

  Due to the lack of blood vessels and nerves.
• eruptive stage

  
  

  how is the eruptive pathway formed?

  The reduced enamel epithelium covering the dental follicle secretes colony stimulating factor 1 ( CSF-1) and transforming growth factor beta-1 (TGF-beta-1) into the surrounding tissue.

  ------> The CSF-1 and TGF-beta-1 attract tons of monocytes that then differentiate into osteoclasts which resorb the part of the bony crypt overlying the tooth. Without the bony crypt, the dental follicular cells become continuous with the cells of the lamina propria of the overlying oral mucosa.

  
  

  
  

  The reduced enamel epithelium then goes on to secrete some enzymes that break down overlying the connective tissue, nerves and blood vessels, forming an eruption pathway for the tooth.
• eruptive stage

  
  

  how is the gubernacular cord linked with the eruptive pathway?

  The gubernacular cord is a structure composed of conjunctive tissue which link the tooth follicle to the overlying gingiva.

  -> it directs the course of the tooth eruption.
• eruptive stage

  
  

  what is the eruptive pathway?

  it is a pathway cleared through the bone and connective tissue which allows the tooth to emerge.
• eruptive stage

  
  

  what happens once the tooth has entered the oral cavity?


  The tooth gradually rises into the oral cavity till it reaches the plane of functional occlusion.
• eruptive stage

  
  

  What is the fused epithelium lining the sides of the tooth after the tooth enters the oral cavity called?

  the functional, or attachment epithelium.
• eruptive stage

  
  

  describe what happens during the eruptive stage

  - The tooth ascends through the bony crypt and the connective tissue and comes into contact with the oral epithelium.

  - The reduced enamel epithelium over the tooth crown then proliferates and fuses with the overlying oral epithelium to form a single membrane.

  - This epithelium is pierced by the tooth to finally enter the oral cavity.
• eruptive stage

  
  

  what does the eruptive stage begin with?

  begins simultaneously with root formation.
• pre-eruptive stage

  
  

  what happens to the permanent molars as they do not have a primary predecessor?


  you'd think that without a primary predecessor, the permanent molars would be developing unhindered.

  But their location right near the ends of the jaw is a very cramped up space.

  And so, the permanent molar tooth germs in the maxilla grow with their crowns tilted distally, and those in the mandible have their crowns tilted mesially.

  Only when the jaw grows to a sufficient size can the permanent molars move into a more vertical position.
• pre-eruptive stage

  
  

  describe the positions of the predecessor and succesional teeth during the pre-eruptive stage


  At the beginning of the pre-eruptive phase:

  the successional tooth germs lie lingual to their primary predecessors, with their occlusal surfaces on the same plane.

  
  

  By the end of the pre-eruptive phase:

  the permanent anterior teeth ends up lingual and apical to the primary anterior teeth. And the permanent premolars come to rest underneath the divergent roots of the primary molars.

  
  

  ------> this change in position is more due to the movement of the primary tooth and growth of the jaws, rather than the movement of the permanent tooth itself.
• pre-eruptive stage

  
  

  describe eccentric growth


  a part of the tooth grows relatively faster than the rest of the tooth.

  -> Eccentric growth helps the crown of the tooth reorient itself as the jawbone keeps growing.
• pre-eruptive stage

  
  

  describe bodily movements

  there is a remodeling of the bony crypts through resorption of bony crypt wall in the direction that the tooth is moving.

  This is followed by depositing new bone on the opposite crypt wall, hence creating a path for the tooth germ.
• pre-eruptive stage

  
  

  what are the two methods of movement of the tooth germs in the pre-eruptive stage?

  1- bodily movements

  2- eccentric growth.
• pre-eruptive stage

  
  

  what happens to the tooth germs as the jawbone increases in height?

  the tooth germs move closer to the surface; that is, the mandibular tooth germs move upwards, and the maxillary tooth germs move downwards.
• pre-eruptive stage

  
  

  what happens to the tooth germs as the jawbone widens?

  the tooth germs move buccally, or outward
• pre-eruptive stage

  
  

  what happens to the tooth germs as the jawbone lengthens?

  the anterior tooth germs move mesially, or forward, and the molar tooth germs to move distally, or backward.
• pre-eruptive stage

  
  

  what happens during the pre-eruptive stage?


  the teeth are positioned within the jaw for eruptive movement

  
  

  -> the tooth germs are growing within the jawbones.

  As the bones grow in length, width, and height, more space becomes available for the tooth germs to spread out and grow without affecting each other.
• which part of the dental follicle is involved in bone formation?

  the basal part (it is osteogenic)

  
• which part of the dental follicle is involved in bone resorption?

  the coronal part (it is osteoclastic)

  
• through experimental methods, is alveolar bone metabolism required for tooth eruption?

  yes

  - the alveolar bone needs to be removed for the tooth to erupt
• through experimental methods, is PDL formation and root formation required for tooth eruption?

  no.

  - if there are no roots or PDL, eruption still occurs so they are not the main driving forces of tooth eruption
• what is the driving force behind the eruption of teeth?

  it is still unknown, but it is thought to be due to a combination of a few factors:

  - signals from the dental follicle for alveolar bone metabolism (removal) (PUSHES TOOTH OUT OF SOCKET )

  - root formation (PUSHES TOOTH OUT OF SOCKET )

  - tension form formation of PDL (PULLS TOOTH OUT OF SOCKET )
• what are the 3 phases of tooth eruption?

  1- pre-eruptive

  2- eruptive

  3- post-eruptive
• when do permanent teeth start erupting?

  around 6 years of age
• when do deciduous teeth start erupting?

  around 6 months of age
• What is tooth eruption?

  The movement of the tooth from its developmental position in the bony crypt to its functional position in the occlusal plane
• anomalies of root development

  
  

  what causes the formation of an abnormal number of roots?

  due to the disturbances of the Hertwig's epithelial root sheath forming the root.