20. Tooth Root Development and Cementum Flashcards
Tooth root formation is coordinated with …
development of periodontium
Explain reciprocal tissue interactions in cervical loop during tooth crown formation
- inner enamel epithelium of cervical loop is separated from dental papilla cells by cell-free zone
- IEE cells become elongated (preameloblasts) and secrete signalling molecules to induce odontoblast differentiation from dental papilla cells
- odontoblasts align and produce predentin
- signals from odontoblasts in predentine induce differentiation of preameloblasts into ameloblasts that start producing preenamel
How do reciprocal interactions during tooth crown formation compare to tooth root formation?
- very similar
- just no ameloblasts in roots
Stages of root formation
- formation of Hertwig’s epithelial root sheath (HERS)
- induction of odontoblasts and root dentine formation
- root completion
Explain formation of Hertwig’s epithelial root sheath (first stage of root formation)
- once crown formation complete
- epithelial cells of IEE and OEE initially proliferate downwards from cervical loop of enamel organ to form double layer of epithelial cells, HERS
- HERS extends around and closes the pulp and defines shape of future root
Difference in cervical loop in root formation
- no stratum intermediate and no stellate reticulum
- similar reciprocal interactions between HERS and dental papilla are thought to occur
- but genetic factors are not well studied
Proteins in epithelium
- PTHrP
- enamel proteins (transient)
Proteins in mesenchymal
- TGF-beta
- BMP
- RUNX2
What do proteins in mesenchyme cause?
- cementoblast differentiation
- periodontal regeneration
Explain ‘induction of odontoblasts and root dentine formation’ (second stage of root formation)
- IEE of HERS induces odontoblast differentiation
- no ameloblast differentiation occurring like crown
- odontoblasts secrete predentine that mineralises to form root dentine
- curved end of HERS is termed epithelial diaphragm and outlines primary apical foramen
- growth of dentine layer causes HERS to be stretched and epithelial cells degenerate in this area
- continued growth of HERS, odontoblast induction and dentine formation until root is finished
At eruption stage, tooth root is only … of final length
What does this mean?
- 65%
- wide, open root apex
Explain root completion (3rd stage of root formation)
- 1.5 years in primary teeth
- 3 years in permanent teeth
- very narrow apical foramen formed - blood vessels and nerves run
How are multiple rooted teeth formed?
- HERS encloses pulp and appears as a curtain hanging down
- primary apical formation divides by fusion of epithelial folds (ingrowths) from HERS (formed in avascular areas)
- secondary apical foramina
- formation of three rooted tooth
Explain root elongation after root formation
- HERS produces new epithelial cell proliferation
- but it doesn’t grow downwards into underlying connective tissue
- HERS position remains stationary
- consequently when root dentine is formed the tooth root is growing/moving upwards
Role of root elongation in tooth eruption
- growth replacement theory (disproved)
- rootless teeth can erupt
- periodontal ligament promotes eruption via rearrangement of collagen fibres and contraction
Layers of root dentine
- cementum
- hyaline layer
- Tome’s granular layer
- root dentine
Cementum is a/cellular, has fibrils/doesn’t and is solid/granular
- acellular
- doesn’t
- granular
Hyaline layer of root dentine is analogous to …
mantle dentine of crown
Features of hyaline layer in root dentine
- non-tubular
- structureless layer of dentine
- 20 micrometres
- first dentine formed
- possibly bonds dentine to cementum
2 explanations of Tome’s granular layer
- extensive branching and backward looping of odontoblast processes
- incomplete fusion of calcospherites (like interglobular dentine)
Root dentine is considered similar to coronal dentine but …
- collagen fibres deposited parallel and at a distance from basal lamina of HERS
- root dentine less mineralised and mineralises faster
- less phosphophoryn (dentine phosphoprotein - binds calcium and regulates dentine mineralisation)
Role of phosphoryn
- dentine phosphoprotein
- binds calcium and regulates dentine mineralisation
Define ‘cementum’
- avascular connective tissue covering roots
Cementum is produced by …
cementum-forming cells (cementoblasts)
Function of cementum
- attachment of root dentine to periodontal ligament
- to alveolar bone via Sharpey’s collagen fibres
Composition of cementum by weight
- 40 to 50% hydroxyapatite (similar to bone but harder and more mineralised)
- type 1 (90%), 3 and 7 collagens
- alkaline phosphatase, bone sialoprotein, dentine matrix protein 1, osteoclacin, osteonectin, osteopontin
- similar proteins to dentine and bone
Classifications of cementum
- on absence or presence of cells is acellular (primary) or cellular (secondary)
- on origin of collagen fibres in cementum - extrinsic or intrinsic
Stages of cementogenesis
- odontoblast induction and dentine formation
- stretching and disintegration of HERS
- differentiation of dental follicle cells into osteoblasts, fibroblasts and cementoblasts
2 theories of cementoblast differentiation
- undifferentiated dental follicle cells can migrate through gaps in disintergrating HERS and receive iinductive signals from newly formed predentine and/or HERS cells
OR - HERS cells undergo epithelial-mesenchymal transition (EMT) to become cementoblasts (or a partial contribution)
How does early formation of cementum occur?
- cementoblasts align along newly formed hyaline dentine (predentine)
- extension of cell processes into predentine and deposition of collagen fibrils that intermingle with unmineralised predentine matrix (collagen fibres)
- extension of collagen fibres and stitching of PDL fibres to fibrous fringe
- mineralisation of predentine spreads through cementum and traps fibres
- strong connection of cementum to dentine
- further embedding of fibres into cementum through PDL
Differences between acellular and cellular cementum
- no cells vs lacunae and canaliculi (directed towards PDL) containing cementocytes and processes
- border with dentine not clearly demarcated vs clearly demarcated
- rate of development slow vs fast
- incremental lines close together vs wider
- precementum layer virtually absent vs present
Incremental growth lines in cementum are called …
Lines of Salter
Cementum enamel junction differs in people. Percentages?
- 60% cementum overlaps enamel
- 30% cementum meets enamel
- 10% they don’t meet
Distribution of acellular cementum
- covers root adjacent to dentine
- primary attachment
Distribution of cellular cementum
- found in apical and interradicular areas
- adaptation and repair
- thickness of cellular cementum increases with age
Role of acellular vs cellular cementum
- primary attachment
- adaptation and repair
Give 2 developmental anomalies in cementum
- free cementicle (completely split off)
- sessile cementicle (protrusion from cementum)
Cementicles are usually … and caused by …
– acellular
- unknown, protentially microtrauma
What are cementicles?
What happens to them?
- detachment of a group of cementoblasts
- free cementicles may become incorporated as sessile or embedded cementicles as thickness of cementum layer increases
What are epithelial rests of Malessez?
- HERS become stretched and degenerates by apoptosis
- leaves behind a group of epithelial cells in periodontal ligament (near root surface)
- involved in periodontal repair?
- source of epithelial stem cells?
Explain enamel pearls
- epithelial rests can form enamel pearls (mostly on root surface in bifurcation areas of maxillary molars)
- can cause plaque accumulation and periodontal disease
Explain enamel pearl formation
- localised attachment of epithelilal cell rests to predentine (caused by absence of cementum deposition)
- molecular signs from predentine could initiate ameloblast differentiation
- when HERS/root development is initiated, stratum intermedium/stellate reticulum cells of cervical loop can be trapped in subset of epithelial cell rests
- combine both these theories is the idea
Define ‘concrescence’
- union of two teeth after eruption
- resulting from fusion of cementum surfaces
Causes and clinical considerations regarding consrescence
- trauma and tooth crowding
- radiograph and surgery .
Define ‘dilacerated roots’
curved or bent roots
Causes and clinical considerations of dilacerated roots
- developmental trauma (for example playground incidents in children)
- radiograph and careful extraction
Cause and clinical consideration of multiple roots/root canals
- abnormal folding of HERS
- radiograph and variation of extraction technique
- careful cleaning of extra root canal during endodontic treatment
What needs to be remembered about the complex root canal system?
- endodontic treatment can be difficult
- failure to notice and clean accessory root canals is a common cause of persistant infection after root canal treatment
Mechanism of lateral root canal formation
- continuity of HERS is interrupted too early
- during root dentin formation e.g a blood capillary running between dental papilla and dental follicle
- this interface affects local formation of odontoblasts resulting in canal in dentine connecting dental pulp and PDL of mature tooth
- frequently found at root apex
Define ‘hypercementosis’
- abnormally increased production of cementum
Causes of hypercementosis
- age change
- idiopathic
- Paget’s disease
- occlusal trauma (bruxism)
Clinical considerations of hypercementosis
- difficult to extract
- increased distance from CE junction to root apex for endodontic treatment
