22. Periodontal Ligament Flashcards
Tooth root formation is coordinated with development of …
Why?
- periodontium
- PDL attaches tooth to alveolar bone
3 differentiations of dental follicle cells
- cementoblasts for cementum
- fibroblasts for periodontal ligament
- osteoblasts for alveolar bone
Which genetic factors regulate differential formation of periodontal tissues?
- insufficiently investigated
- FGFs (cell proliferation, migration) and BMPs (cell differentiation, bone formation)
- use of growth factors e.g FGF2, BMP2, 7
- stem cells in PDL as source for regenerative therapies in periodontal disease
List functions of periodontal ligament
- tooth attachment
- withstand forces of mastication
- sensory receptor
- remodelling
- nutritive
How is the periodontal ligament for tooth attachment?
- PDL fibres insert into cementum and alveolar bone
- forms fibrous joint with very little/no movement (gomphosis/synarthrosis)
How is the periodontal ligament for withstanding force of mastication?
shock absorber
How is the periodontal ligament for sensory reception?
- sensations of pain and tension/compression
- repositioning of teeth to achieve occlusion
How is the periodontal ligament for remodelling function?
- high turnover of extracellular matrix and collagen fibres
- source of progenitor/stem cells
How is the periodontal ligament for nutrition?
- highly vascularised tissue
- connected to dental arteries, bone marrow and gingiva
Timing of PDL development and differentiation varies …
- among species
- between tooth types
- between primary and permanent teeth
What is the first stage of PDL development?
- initiation
- ligament space between cementum and bone consists of unorganised connective tissue (mainly fibroblasts and ECM)
- short fibre bundles are formed near the cementum and bone surface and extend only a short distance into ligament space
What happens in PDL formation after initiation stage?
- fine brush-like fibres emerge from cementum and only a few fibres project from alveolar bone into ligament space
- fibroblasts produce more collagen fibrils that assemble as fibre bundles and gradually extend from bone to cementum to establish a continuous attachment
- bone side is thick fibre bundles, cementum side is thin fibre bundles, fine intermediate meshwork
There are … and … PDL fibres at alveolar bone side whereas they’re … and … at cementum side
- thick and widely spaced
- thin and closely spaced
Once fibres have formed between cementum and bone in PDL development, what happens?
- alveolar crest fibres are formed first at cemento-enamel junction
- as root forms, fibre formation then proceeds apically
- orientation is initially oblique, then horizontal, then oblique again
- PDL is continuously modified by eruptive tooth movements and occlusal forces
- thick fibre bundles only form when teeth occlude and begin to function
- role of PDL in tooth eruption likely but still controversial
5 principle fibre groups of PDL
- alveolar crest group
- horizontal group
- oblique group
- apical group
- interradicular group
Explain alveolar crest group
- below CEJ - rim of alveolus
- resist extrusive forces
Explain horizontal PDL fibre group
- below alveolar crest group
- at right angle to tooth axis
- resist horizontal forces/’tipping’
Explain oblique PDL fibre group
- most abundant
- resist intrusive compressive forces in mastication
Explain apical PDL fibre group
- radiates around tooth apex
- forms base of socket
- resist extrusive forces
Explain interradicular PDL fibre group
- only multi-rooted teeth
- connects to crest of interradicular septum
- resist extrusive forces
Each collagen fibre bundle resembles …
What happens with individual fibrils? Overall fibre?
- a spliced rope
- continuously remodelled
- maintains core architecture and function (possible for it to adapt to varying mechanical/masticatory forces)
Types of PDL fibres
- collagen
- elastic
Explain elastic fibres in PDL fibres
- oxytalan fibres contain fibrillin but no elastin
- run perpendicular to collagen fibres in cervical region, associated with neurovascular bundles
- form 3D meshwork surrounding root
- possible role of regulating vascular flow
Sharpey’s fibres are … PDL fibres in … and …
- mineralised
- alveolar bone and cementum
Main function and types of fibroblasts in PDL
- produce collagen fibrils to form fibre bundles and ground substance in ECM
- have perivascular and endosteal ones - source of progenitor cells to maintain PDL
Jobs of fibroblasts in PDL
- secrete ground substance, collagen fibrils and growth factors/cytokines
- rich in organelles (ER, golgi) for protein production and secretion
- high levels of collagen produced (8x skin, 2x gingiva)
- form cell-cell contacts with adherens and gap junctions
- well developed cytoskeleton (actin network) - shape change and migration
- align along direction of fibre bundles
- contractile (mediated by integrins that bind to ECM) - mechano-transduction
- function tooth movements like mesial/vertical tooth drift
- remodelling (synthesis and degradtion of ECM and collagen)
PDL fibroblasts appear morphologically similar but …
- heterogenous cell population
What is PDL collagen half life?
3-23 days (variable - highest turnover at tooth apex)
- leads to scurvy
What kind of cell-cell junctions are in fibroblasts in PDL?
- adherens
- gap
How are fibroblasts in PDL contractile?
- mediated by integrins that bind to ECM
- mechano-transduction
Activity of fibroblasts in PDL is induced by what?
mecahnical or masticatory forces
How do fibroblasts in PDL have a role in remodelling?
- synthesis and degradation of ECM and collagen
- matrix metalloproteases (MMPs)
- a therapeutic target in periodontal disease
Composition of the PDL
- 60% ground substance
- rest is mainly collagen fibres, blood vessels and nerves
- fibres - 90% collagen, 10% oxytalan (elastic)
- collagens (type 1 - 80%, type 3 - 15%, type 4,5,6,7,12)
Type 3 collagen in PDL is what?
- in reticular fibres
- in meshwork
Type 12 collagen is what in PDL?
- fibril associated
- links other collagens
- only present after eruption
- expressed on pressure side following mechanical loading
Does collagen composition of PDL change with age?
no
Composition of ground substance
- complex
- of glycosaminoglycans (hyaluronic acid, dermatan sulfate, chondroitin and heaprin sulfate)
- proteoglycans
- and glycoproteins
Function of ground substance
- ion and water binding
- 70% water so acts as a shock absorber
- orientation of collagen fibres
How does ECM support ground substance?
- ground substance orientates collagen fibres
- ECM controls hydration of tissue and increases strength of collagen fibres
How does composition of ground substance vary?
- to developmental state
- hyaluronic acid conc decreases in development of PDL from dental follicle
- proteoglycan conc increases during tooth eruption
Role of fibronectin in ground substance
- mediates attachment of cells to collagen fibrils
- influences cell migration and differentiation
- clinically used to promote wound healing
ECM binds what 2 things?
- growth factors
- cytokines
List cell types in the PDL
- fibroblasts
- osteoblasts/clasts
- cementoblasts/clasts
- Rests of Malassez
- mesenchymal stem cells
- immune cells
- blood vessels
- nerve fibres
Osteoblasts and clasts are associated with … in PDL and are for …
- alveolar bone
- bone remodelling
Cementoblasts and clasts are associated with … in PDL and are for …
- cementum
- cementum remodelling
What are the Rests of Malassez?
- remnants of HERS
- possible source of epithelial stem cells
Role of mesenchymal stem cells in PDL
- source of all mesenchymal cell types
- e.g fibroblasts, osteoblasts, cementoblasts
What types of immune cells are in PDL?
- macrophages
- mast cells
- eosinophils
What blood vessels are present in PDL?
- endothelium
What nerve fibres are present in PDL?
axons of neurons
PDL is highly/poorly vascularised
Explain
- highly
- 10-30%
- differs between species, tooth types, PDL site, erupting teeth
- high turnover rate of PDL components needs constant nutrient supply
What is the vascular supply of PDL?
- branches of superior and inferior alveolar arteries
- branches of lingual and palatine arteries
Explain how superior and inferior alveolar arteries vascularise the PDL
- branches enter at pulp apex
- interalveolar vessels pass through alveolar process (‘perforating arteries’) - more abundant in posterior and mandibular teeth
- form ‘interstitial areas’ within PDL
- enables PDL function after endodontic treatment
- tooth extractions - formation of blood clot and invasion of cells involved in wound healing
Where do branches of lingual and palatine arteries enter oral cavity?
through gingiva
Where are neurovascular bundles in PDL?
- pass through perforations in alveolar bone
- form interstitial areas in PDL
What are the interstitial areas in PDL?
- located close to alveolar bone
- contain neurovascular bundles (with nerves and blood vessels)
- blood vessels form capillary plexus near root surface and postcapillary plexus from which venules pass into alveolar bone
In PDL, blood vessels in … direction and form …
Venous drainage occurs at the … and … follow the venules
- apical-occlusal
- arterio-venous anastomoses
- root apex
- lymphatic vessels
Circular plexus in PDL is … and crevicular is …
- surrounding the root surface
- surrounds the tooth in region beneatjh gingival crevice
Explain fenestrated capillaries in PDL
- large numbers of pores in endothelial cells
- special feature of PDL vasculature not seen usually in other connective tissue
- generates increased diffusion capacity e.g for larger molecules
- consistent with high metabolic rate of PDL especially during tooth eruption
Innervation of PDL
- well innervated and pattern of nerve fibres follows that of vasculature
- from apex to gingival margin and through lateral perforations of alveolar bone
- perforating nerve fibres divide into apical and gingival branch
How does PDL innervation show regional variation?
- more nerve endings in tooth apex
- upper incisors have denser innervation than molars
- can be related to masticatory response - initial contact with food could specify level of force required to process food
Types of nerve fibres in PDL
- myelinated fibres - sensory
- myelinated and unmyelinated fibres for sensory and autonomic
Roles of sensory nerve fibres
- nociception
- proprioreception - pressure
- food sensing, position of tongue and neck muscles, salivary reflexes
Role of autonomic fibres
- regulation of blood flow (constriction and dilation of blood vessels)
4 types of nerve endings
- free ending, treelike type
- Ruffini’s corpuscles
- coiled type
- encapsulated spindle type
Explain free ending treelike type
- evenly distributed across PDL
- unmyelinated fibres (enveloped by one Schwann cell/inset)
- extend up to cementoblast layer to sense pain and pressure
Explain Ruffini’s corpuscle
- found in PDL at root apex
- myelinated fibres with dendritic endings
- associate with collagen fibres - inset
- sense pressure
Where are coiled type nerve endings found?
- found in middle region of PDL
- unknown function
Where are encapsulated spindle type nerve endings found?
- found in PDL at root apex - infrequent
- surrounded by fibrous capsule
What is the general PDL thickness? Where is it thinnest?
0.15-0.38 mm
middle third of the root
PDL thickness at
- 11-16 years
- 32-52 years
- 53-67 years
- 0.21mm
- 0.18mm
- 0.15mm
PDL thickness decreases with …
increasing age
How does mastication link to PDL thickness?
- mastication induces periodontal remodelling
- results in increased PDL width by around 50% inc. thicker fibre bundles and increased alveolar bone size
- PDL thicker in areas of tension compared to areas of compression
- decreased function results in reduction/loss of periodontal tissues
Capacity of PDL remodelling forms basis for …
orthodontic treatment
- excessive force can cause localised necrosis of PDL by cutting off the blood supply
- inhibition of orthodontic tooth movement
Damaged PDL can be repaired. What if this goes wrong?
- incorrect repair causes localised resorption and tooth ankylosis
- fusion of tooth to bone - diagnosed using X-ray or percussion test - tapping of tooth
Accidentally lost teeth can be replanted. But if portions of ligament are permanently damaged, …
external root resorption and tooth ankylosis could occur
PDL is the target of therapies of periodontal disease. How so?
- prevention of undisirable wound healing
- growth factors, cytokines, stem cells to stimulate PDL regeneration
