Periodontal Ligament (PDL) Flashcards
What is the periodontal ligament?
It is the dense fibrous connective tissue that occupies the periodontal space between the cementum of the root and the alveolar bone of the tooth socket
What is the average width of the periodontal space?
0.25mm
What are the 4 main functions of the periodontal ligament?
- It’s responsible for resisting displacing forces and for protecting the dental tissues form damage caused by excessive occlusal loads
- Responsible for the mechanisms whereby a tooth attains and maintains its functional position
- Its cells form, maintain and repair alveolar bone and cementum
- Its mechanoreceptors are involved in the neurological control of mastication
Which cell makes up most of the PDL
Fibroblasts
What is the PDL made up of?
Connective tissue (mainly fibroblasts) and an extracellular matrix
What is the extracellular matrix made up of?
Type I collagen Glycosaminoglycans Proteoglycans Glycoproteins Oxytalan Water
What are the 6 cells that make up the PDL?
- Fibroblasts
- Cementoblasts
- Osteoblasts
- Osteoclasts
- Epithelial cells (Rests of Malassez)
- Macrophages
What are the fibroblasts responsible for?
For the synthesis and degradation of collagen and for secretion of all components of the PDL ground substances
What type of cell and where are cementoblasts and osteoblasts found?
They are formative cells found on the surface of both the cementum and alveolar bone
Which cells are of mesenchymal origin?
Cementoblasts Osteoblasts Preosteoblasts Precementoblasts Epithelial cells
Which cells are involved in resorbing? And where are they found?
Osteoclasts and odontoclasts
They are found on the surface of bone and cementum
Which cells are of macrophage origin?
Osteoclasts
Odontoclasts
Macrophages
What are the rests of Malassez?
Epithelial cells
They are remnants of the epithelial root sheath
What are maacrophages?
Defense cells
They are blood borne cells that enter the PDL from blood vessels
They make up a small percentage of the cell population
What are the collagen fibres in the extracellular mastic gathered into?
They are gathered to form bundles called principal fibres
The region of attachment of the gingiva to the tooth is called what?
The gingival cuff
Where are elastin fibres found?
Elastin fibres are restricted to the walls of the blood vessels
Which fibres make up the periodontal fibres?
- Collagen:
Type I, III, IV, V, VI, VII, XII - Oxytalan, elastic and elaunin
- Reticulin
Which type of collagen makes up most of the PDL?
Type I (70%)
What other types of collagen (apart from type I) make up the PDL?
Type III - 20%
Types IV, V, VI, VII, XII In trace amounts
Where are elaunin fibres found?
They are found around the blood vessels
What might the elaunin fibres do?
They might provide mechanical protection for the vascular system
What are reticulin fibres now known to be?
Type III collagen
What do reticulin fibres form and what do they do?
They for crosslinks and a fine meshwork to aid tissue support
What found between the collagen fibres?
The ground substance
Where are the ground substances found and what do they do?
They are found between collagen fibres They are thought to have many important functions including: Water binding Control of collagen fibrillogenesis Fibre orientation Binding of growth factors
What is the basic unit of collagen fibres?
Triple helical tropocollagen molecules
What happens when triple helical tropocollagen molecules are secreted outside the cell?
They are cleaved and spontaneously aggregate in a staggered fashion, first as a 5 membered micro fibrils and then into classical fibrils
What is the characteristic banding of classical microfibrils?
Characteristic banding at 64nm
What are the fibrils grouped into after classical microfibrils
These fibrils are then arranged into larger structures called bundles ranging from 1 micrometer upwards
What are bundles then further arranged into?
These bundles are then arranged to give he overall tissue architecture called the principle fibre bundles
What is the diameter of small fibrils?
Approx 40nm
What are the 5 different orientation of the principle collagen fibres?
- Dentoalveolar crest fibres
- Horizontal fibres
- Oblique fibres
- Apical fibres
- Inter-radicular fibres
Where do the alveolar crest fibres run?
They run from the alveolar crest upwards to insert into the cementum below the crown
Where do the horizontal fibres run?
Run horizontally from bone to tooth
Which orientation of the principle collagen fibres is the largest?
Oblique fibres
Where do the apical fibres run?
They radiate from the root to the apex
Where do the Inter-radicular fibres?
In a multi rooted tooth the fibres radiate from the apex of the inter-radicular bone
Where do the oxytalan fibres run in the outer part of the PDL?
Runs obliquely down from the cementum to terminate in the vicinity of the periodontal capillaries
Where do the oxytalan fibres run within the periodontal ligament proper?
They tend to be more longitudinally orientated, crossing the oblique fibre bundles more or less perpendicularly
What is the suggested function of oxytalan fibres?
There is no clear idea but the suggested function of these fibres might be to aid fibroblast migration in the PDL
What does their ultrastructural characteristic of oxytalan suggest?
Suggests that they are immature elastin fibres called ‘pre-elastin’
What is the difference between the ‘pre-elastin’ and mature elastin?
Pre-elastin does not have a central amorphous core
What does the PDL demonstrate in response to axial loading?
The periodontal ligament demonstrates a BIPHASIC, ‘VISCO-ELASTIC’, response to axial loading
How does the PDL seem to resist intrusive loads
Some of the load applies to the tooth is probably dissipated through the oblique fibre system
Primarily the ligament appears to resist intrusive loads by compression and fluid flow
What does evidence suggest collagen fibres, vasculature and ground substance are involved in?
Tooth support
What fibres are involved in tooth support?
Collagen fibres,
Vasculature
Ground substance
The lamina propia of the gingiva is what?
A relatively dense and fibrous tissue that has insertions into both tooth and bone of the alveolar crests
What does the lamina propia of the gingiva do?
They act together with tissue fluid to support the free gingiva and hold the attached gingiva against the tooth
What is the gingival cuff?
The region of attachment of the gingiva to the tooth
What percentage of the extracellular matrix is made up of oxytalan fibres?
3%
What us the junctional epithelium
It is a specialised nonkeratinised stratified squamous epithelium which is unusual in having a basal lamina on both surfaces
They are part of the gingival cuff
Where is the basal lamina?
It is deposited on the enamel surface and the cells attach via the semi-desmosomes
Which organelles to fibroblasts have a lot of?
They are metabolically active with large amounts of rough endoplasmic reticulum
What processes surround the fibre bundles?
Cellular processes surround, or envelop, the fibre bundles
What are processes from adjacent cells joined by?
They are joined by intercellular contacts to form a cellular network
What are fibroblasts as well as metabolically active? How?
They are also metabolically coordinated as they demonstrate significant numbers of gap junctions between adjacent cells
This may help them to coordinate their matrix turnover activities.
What is an unusual feature of periodontal fibroblasts?
They frequently contain intracellular vacuoles containing fragments of fibrillar collagen
What type of event is the degradation of collagen fibrils in MOST connective tissue?
An extracellular event as the cells secrete collagenases into the extracellular environment
By what process does the degradation of collagen fibrils occur in the periodontal ligament?
The fibroblasts appear to degrade the fibrillar collagen by phagocytic process
Explain the phagocytic process of degrading collagen fibrils
Lysosomes fuse with the ‘phogosome’ containing the collagen fibril and form a ‘phago-lysosome’ where the fibril is degraded. .
Why might the phagocytic process of degrading collagen fibrils in the PDL be more advantageous
This may enable the cells to control the degradative process more precisely.
What do the epithelial cell rests of malassez look like?
They appear as islands of epithelial cells
What are the epithelial cell rests of Malassez?
Remnants of the epithelial root sheath of Hertwig which disintegrates following root development and have been considered to be relatively inactive
What can the epithelial cell rests of Malassez be triggered to do?
These cells can be triggered to proliferate and form peri-apical cysts
Where does the PDL get its blood supply from?
The PDL has a rich blood supply derived from the superior or inferior alveolar arteries
Where do the majority of capillaries within the ligaments have their origin?
Their origin is in the intra-bony spaces within the alveolus
Arterioles within the gingiva may also be involved
What do the capillaries and arterioles in the PDL form around the tooth?
They form a PLEXUS around the tooth which is primarily situated towards the socket wall, between the principal fibre bundles
Occupies 50% of the periodontal space
What is the PDL derived from?
Derived from the dental follicle
How is the PDL formed?
- Once the crown has fully formed the internal and external enamel epithelia proliferate downwards as a double layer sheet of flattened epithelial cells, epithelial root sheath of Hertwig that outlines the shape of the tooth
- Then the epithelial root sheath induces the peripheral cells of the dental papillae to differentiate into odontoblasts and begin producing dentine
- It’s in contact with the initial pre-dentine layer for only a short time before the continuity of its cell is lost
What lie close to the surface of newly formed unmineralised dentine?
Adjacent cells of the dental follicle come to lie close to the surface of the newly formed unmineralised dentine
What characteristics do dental follicle cells have prior to root formation?
They have the characteristics of undifferentiated mesenchymal cells
After root formation what happened to dental follicle cells?
They show an increase in cytoplasmic organelles especially those associated with protein synthesis and secretion
What do the cells of the dental follicle differentiate to form?
Cementoblasts
Fibroblasts
Osteoblasts
What do cells of the inner layer of the dental follicle differentiate to?
Cementoblasts that form the initial layer of cells on the surface of root dentine
What happens to dental follicle cells once cementogenesis has begun?
The remaining cells of the dental follicle become obliquely orientates along the root surface and show an increased content of intracellular organelles
They become fibroblasts
What do fibroblasts secrete into the PDL?
Collagen
What process triggered the formation of fibroblasts?
Cementogenesis
What do the unmineralised fibres of the PDL run into?
They run into the organic matrix of precementum secreted by cementoblasts
When are extrinsic fibres incorporated into the precementum?
Give an example of an extrinsic fibre
When further mineralisation occurs
An eg is Sharpeys fibres
Where are Sharpeys fibres found in abundance?
The aceullular cementum
What’s the difference between Sharpeys fibres found in the cementum and bone?
Sharpey’s fibres insertions into bone are larger but less numerous than those into cementum.
