Root Development and Eruption Flashcards
When does root eruption start?
When does it finish?
After crown formation - at the bud stage.
Finishes 3 years after the tooth has erupted.
How many different cell populations are there at the early bell stage?
What are cervical loops?
4 different ones
Cervical loops are where there are two epithelia which come together (outer epithelium surrounding the bell and inner (ameleoblasts)). These loops proliferate and form Hertwig’s epithelial root sheath.
What are the 3 roles of HERS (Hertwig’s epithelial root sheath)?
- Maps out root shape
- Initiates odontoblast differentiation
- Break-down initiates cementum and PDL formation
Is HERS permanent or temporary?
Why?
Temporary
Break down initiates the formation of the periodontal ligament which enables cementum development.
Explain how HERS maps out root shape
what do mesenchymal cells next to HERS differentiate to?
Dentine of the crown is shown and next to this is HERS formed of two layers of epithelium which have proliferated and start moving downwards.
Mesenchymal cells differentiate to rootoblasts.
Explain how HERS initiates odontoblast differentiation
HERS grows down and as this happens it secretes molecules which act as signals on surrounding cells (especially mesenchymal cells of the dental papilla which will be included in the future dental pulp).
The signals induce the differentiation of the mesenchymal cells next to HERS to form odontoblasts.
Explain what happens once predentine is laid down
Predentine is laid down and mineralised to dentine.
After this, HERS is broken down and this then goes to form the periodontal ligament and cementum.
Cells come in contact with the dentine and either secrete collagen to form the periodontal ligament or become cementoblasts.
HERS grows down until the full length of the root is obtained.
What is the difference between acellular cementum and cellular cementum?
Acellular = coronal 2/3 of the root
Cellular = apical 1/3 of root
Give detail on HERS break down initiating cementum and PDL formation?
There are cluster of epithelial cells which remain from HERS. These remain around the root and can be reactivated and form cysts in later life.
Cell signalling is then what goes on to initiate PDL.
How does formation of the root apex occur?
HERS grow at a right angle to form an epithelial diaphragm. This does not completely close (leaves an open tip) called the apical foreman so the blood vessels can get in and vascularise the tooth.
How is HERS structures in a two root tooth?
Three root?
HERS forms around the two roots. It changes orientation and forms two tongues of epithelium which come together by moving at right angles and then fusing together.
Epithelium changes orientation and forms towards the centre of the tooth to separate the root into 3 parts.
Where does the tooth move from during eruption?
From its development position in the bony crypt to tis functional position in the occlusal plane.
How does the periodontal ligament form?
Fibroblasts secrete collagen which organises into bundles of fibres.
Some bundles change orientation.
Primary dentition:
What is the tooth covered by?
What do these cells fuse with?
All epithelial layers surrounding the cell types (4 cell types from ETD lecture).
These cells form the reduced enamel epithelium.
These cells then fuse with the oral epithelium and the cells degenerate.
When the reduced enamel epithelium and the —- —- fuse and the inner cells degenerate means the tooth will then erupt into the —- —-.
Cells which form the enamel will then be lost. When there is tooth decay later, there are no cells to replace the lost —- material as the cells laying down the enamel are now lost when the tooth —-.
Formation of the dental —- junction when the tooth erupts formed by two types of epithelium.
oral epithelium
oral cavity
mineralised
erupts
gingival
Explain deciduous eruption
Tooth is encased and the epithelial bell was connected to the epithelium (less bone). This allows space for the tooth to reach the oral cavity and this space is called the glomerular cord.
What type of cells degrade the deciduous roots?
Cells similar to osteoblasts
What are the main products of each of these origins?:
- Enamel organ
- Dental papilla
- Dental follicle
- Cervical loop (HERS), outer enamel epithelium, stellate reticulum, stratum intermedium, inner enamel epithelium (ameleoblasts - enamel)
- Odontoblasts (dentin). mesenchymal cells (pulp_
- Cementoblasts (Cementum), fibroblasts (periodontal ligament) and osteoblasts (alveolar bone)
Where are the two tensions coming from in tooth eruption?
Pull - tension from the PDL
Push - Root growth, alveolar bone remodelling
Is root growth a cause of eruption? Explain
It coincides with tooth eruption but it is not causative.
Metal replica of a tooth with no roots but with a dental follicle. Still erupted into the oral cavity.
Dentin dysplasia 1 - very short roots but still erupted into the cavity.
What are 3 theories about what causes tooth eruption?
1 - traction by the periodontal ligament
2 - tension from the periodontal ligament (but still have eruption without PL - may just accelerate eruption)
3 - alveolar bone remodelling (as roots grow, more and more bone is deposited around it)
The dental follicle controls alveolar bone —- during tooth eruption.
Main role of it?
What happens if you remove the coronal 1/2 of follicle?
Removing the basal 1/2?
Metabolism
Main role - root lengthening
Coronal = no bone resorption Basal = no bone formation
No tooth eruption will happen in both cases as no root formation.
DENTAL FOLLICLE IS IMPORTANT IN ALVEOLAR BONE STRUCTURE.
Which part of the dental follicle is osteoclastic?
Which part is osteogenic?
Osteoclastic = upper(coronal) Osteogenic = lower(basal)
The coronal dental follicle secretes chemokines. What do these promote?
What does the basal part secrete and what does this do?
The differentiation of osteoclasts from monocyte precursors.
BMP2 and this promotes the differentiation of osteoblasts.
Give some anomalies of root development
- Rootless teeth when NF1c is transcribed (this is expressed in the odontoblasts). HERS present but no odontoblast differentiation so no dentine released.
- Dentin dysplasia 1 = short roots as dentine formation is disrupted.
- OFCD = abnormally long roots as the apex does not close so the root will continues to grow. Signals tell the mesenchyme to be more stem like so they do not differentiate.
- Taurodontism = Only one root forms which splits right at the end because. Failure of HERS to invaginate at the proper horizontal level.
Leads to an enlarged pulp chamber, apical displacement of the pulp floor, no constriction at the cemento-enamel junction - Variable root number in mutants
QUIZ:
Is HERS formed of epithelial or mesenchymal cells?
Which cells differentiate into root odontoblasts?
In taurodont teeth, are the root canal longer or shorter?
Which important tooth de development process is controlled by the ectodysplasin pathway, in addition to the determination of the number of tooth roots?
Epithelial
Mesenchyme of the papilla, those adjacent to HERS
Shorter
Number of cusps
QUIZ:
What is the key process involved in tooth eruption?
What embryonic tissues form the dentogingival junction?
Does bleeding occur during tooth eruption?
Cite a syndrome in which teeth with long roots form
Bone metabolism and remodelling
Oral epithleium, inner and outer reduced enamel epithelium, stellate reticulum.
No
OFCD
QUIZ:
How long after tooth eruption is root formation complete?
What is the main histological feature of developing tooth root?
3 years
Open apex
