Periodontal ligament

•Dense fibrous tissue,derived from dental follicle.

•Continuous with the gingiva coronally and with the pulp apically.

•It's width is .25 mm (variations between different teeth, and within the same individual), it decreases with age and increases with function.

•PDL is hourglass in shape, being narrower in permenant teeth .

PDL functions

•It's the tissue of attachment between teeth and alveolar bone.

•It helps teeth to attain then maintain it's functional position.

•It's cells form,maintain and repair cementum and alveolar bone .

•It's mechanoreceptors are involved in the neurological control of mastication.

The periodontal ligament consists of:

1.Stroma of fibers.

2.Ground substances containing:

cells. blood vessels. nerves.

1. Fibres

Collagen 90% ,main one. Oxytalan. Reticulen. Elastin, in some species

Collagen

Type Ⅰcollagen

Is the main one, 80%. Consists of two identical α1 chains and one α2

chain. Low in hydroxylysine and glycosylated

hydroxylysine .

Collagen type Ⅲ

Three identical α1chains. High in hydrxyoproline , low in hydroxylysine. Contains cysteine. Present in sites with rapid turnover. Linked covalently to type Ⅰcollagen . Found in the periphery of sharpey’s fiber attachments

to alveolar bone and around nerves and blood vessels.

Collagen typeⅤ

Present in small amounts. Coats cell surfaces and other types of

collagen. Increases in periodontal inflammation.

Collagen type Ⅵ

Absent from the middle zone of erupting molars and tooth related portion of the ligament.

Promote ploriferation of fibroblasts. Part of Oxytalan fibre system.

collagen type ⅩⅡ

Non fibrous collagen. is present only when PDL is fully erupted and

functional . present at pressure sites following orthodontic

loading.

Collagen type ⅩⅣ

Can be expressed in the PDL

Collagen is gathered to form bundles, 5 μm in diameter called principle fibers.

Collagen fibrils are formed of individual tropocollagen molecules.

In close relation with fibroblasts since they are responsibIe for their synthesis and degredation.

Periodontal collagen does not alter with age or function: difference from other fibrous C.T

Principle fibersDentoalveolar crest fibers.Horizontal fibers.Oblique fibers(form suspensoy

ligament).Apical fibers.Interradicular fibers.

Fibers of the periodontal ligament cross the whole length of the periodontal space in form of sheets but branch and intercalate to form a plexus which is seen in longitudinal sections only.

Area of remodelling of the intermediate area of the plexus Of collagen during eruption is called "ZONE OF SHEAR".

Fibers do not necessarily run the PDL in a straight line, it is said to be wavy.

A specific type of waving is called " crimping" . A crimp is gradually pulled out when the ligament is subjected to mechanical tension.

Sharpey's fibers

collagen fibers embedded into cementum and bone called "sharpey's fibers".

The mineralization in alveolar bone occurs at right angle to the fibers which indicates that in function the fibers are subjected to tensional forces.

More numerous,but smaller at the cementum. There's heavy fibers insertion at the alveolar crest ,

elsewhere fibers appear to end around the blood vessels of the ligament.

Cont'd Highest turn over rate than all other connective

tissue collagen, being highest at the root apex Related to the functional demands (increases in

occlusal stresses and tooth movement). No difference between erupting and fully erupted

teeth. Turn over does not reflect total protein turn over as

protein turn over is dependant on extracellular processing .

Oxytalan Immature elastin (pre elastin, no central

core). They have Elastin and type Ⅳcollagen. 3% of extracellular fiber composition. .5-2.5 μm in diameter, oval in shape. Attached to cementum and rarely to the

bone.

function of oxytalan: Tooth support. Help in fibroblast migration in the PDL.

• Run longitudinally in the PDL space• Cross the oblique fibers at right angles.• In the outer part of the PDL they terminate around blood

vessels and nerves.

•Elastin fibers are restricted to the walls of blood vessels.

•Reticulin fibers are related to the basement membrane in the PDL.

2. Ground substances glycoaminoglycans. proteoglycans. glycoprotein.

All are secreted by fibroblasts

Functions : Ions and water binding and exchange. Control collagen synthesis and fiber

orientation. Binding of growth factors. Nature of ground substances explains why

the PDL never mineralizes as it acts as an inhibitor to this process.

Ⅰ.GlycoaminoglycansMuch of the GAGS are located near the surface of collagen.

Types: 1.Hyaluronan : main one, influence tissue

permeability, affects cell mobility.

2.Dermatan. 3.Chondroitin. 4.Heparin sulphate.

Ⅱ. proteoglycans Are compounds containing anionic

polysaccharide attached to a protein core.

Proteodermatan sulphate Chondroitin sulphate/ dermatan sulphate

hybridsDecorin: aid in collagen synthesis and

strength.Biglycan: hydration of the extracellular matrix

Ⅲ. GlycoproteinsA. Fibronectin. Uniformly distributed in PDL. Has fetal like characteristics ( Immature C.T).

functions:

- Promote attachment of cells to the substratum especially collagen.- Cell migration and orientation.

B.Tenascin Fetal like characteristics. Uniformly distributed in PDL. c. Vitranectin Associated with elastin fibers.

Tenascin and Vitranectin are important in tissue remodelling.

That there's a diffrence in the ground substances between PDL and adjacent alveolar bone is a clinical diagnostic tool when analysing gingival crevicular fluid to assess and predict patient susceptibility to periodontal disease.

Cells1. Fibroblasts

Predominant one, very active cell. Responsible for regeneration of tooth support

apparatus. Have adaptive responses to mechanical forces. Variety of shapes, usually fusiform, with many

cytoplasmic processes. Flattened , disc shaped. Rich in protein synthesis organells ( rough E.R,

mitochondria, ..) FB have migratory potential along collagen fibers in

apicoronal direction New population arise from blood vessels reaching PDL.

In vitro studies show that FB have the potential for being motile-contractile cells .

Motile being able to organize a fibrous network,and can generate signification force.

Contractile , cells have the properties of both smooth muscle and fibroblast, found in contracting wounds.

Fibroblast are fibroclastic too.Intracellular collagen profiles are responsible for collagen degredation

Fibroblasts secrete :1- Matrix metalloproteinase (degrades collagen).2- Tissue inhibitor metalloproteinase. 3- Prostaglandins (regulate collagenase production and phagocytosis).

Fibroblasts of the PDL label for vimentin intermediate cytoskletal filaments at all stages of development and in response to mechanical loading.

Cytokeratin 19 labelling occurs during active stage of tooth eruption.

Fibroblasts produce growth factors and cytokines: insulin like GF, ILs, PDGF, BMPs.

• Fibroblasts may produce factors that inhibit osteoclastic activity, by expressing RANKL.

2.cementoblast Protein synthesis. deposit cellular and a cellular cementum.

3. Osteoblasts

cuboidal. Have rough E.R , mitochondria , secretory vesicles . Microfilament are present on the secreting surface

of the cell. Cells contact each other by tight junction to form a

transport system in the bone.

4.osteoclasts/cementoclasts Arise from blood cells of monophage type. Has vesicles containing acid phosphate. Bone shows resorption concavities called

"Howship's lacuna" which contains osteoclasts.

5.Epithelial cells Rests of Malassez ( epithelial cells) : remnants of

Hertwing epithelial root sheath. In the first 2 decades they are found more apically ,

between 3rd and 7th decades they are found more cervically, they have slow turn over.

Function: could direct cementogenesis and might inhibit

cementogenesis in mature teeth .

epithelial cells appear in clusters

6.Defence cells1- Monocytes/Macrophages

4% of cell population in PDL. Near nerves and blood vessels. Monocytes have numerous microvilli at it's border. Monocytes contain enzymes that degrade C.T

extrecellular matrix. FB produce a protein to attract monocyte in response

to inflammation. Macrophages are derived from monocytes and

responsible for phagocytosis. Macrophages secrete molecules such as interferon,

prostaglandins, and factors that inhance the growth of FB and endothelial Cells.

2-mast cells Associated with blood vessels. Have large number of granules. It produces histamine , heparin, and factors associated

with anaphylaxis.

3-Eosinophils Only seen in normal PDL. Possess granules containing peroxisomes. Functions: phagocytosis.

Neutrophils and Lymphocytes are also found in case of inflammation.

Mast cell with numerous dense, membrane bounded vesicles

Cell kinetics During PDL development , cell populating the dental

follicle are ectomesencymal in origin. In mature PDL progenitor cells population of the PDL are

located adjacent to blood vessels , near the surface of the alveolar bone and in endosteal spaces.

Rate of mitotic activity is modest (.5-3%) being highest in the cental part of the PDL.

Rate of mitosis is higher in wound healing and during orthodontic movement.

The FBs of the PDL requires continued expression of EGERs , they inhibit cell diffrentiation into mineralized tissue forming cells.

Apoptosis is not a common features in the PDL, but increases with age.

Blood vessels

PDL is rich in blood supply and derived from superior and inferior alveolar arteries.

Arteries from gingiva (lingual and palatine arteries) are also involved.

Blood supply is mainly from perforating arteries passing through alveolar bone rather than arteries entering the pulp apex.

volume of PDL accupied by blood vessels is between 10- 30%

Special features of the vasculature in the PDL

Crevicular plexus of capillaries loops. Large fenestration in capillaries , this is a specialised

feature in th PDL. Veins do not accompany arteries, they enter the

alveolar bone into intra alveolar venous network. Dense venous plexus are present around the apex of

the alveolus. Open junction between the endothelial Cells for fluid

and molecular transport.

Nerves PDL is well innervated . Two types : Sensory and autonomic. Sensory fibers are associated with nociception and

mechanoreception. Nerve fibers entering the PDL are derived from two

sources: fibers entering the pulp , or as final branches through alveolar bone.

Autonomic nerve fibers are associated with blood vessels.

Myelinated nerve fibers are sensory 5 μm , some may reach 15 μm.

Unmylinated nerve fibers are both sensory and autonomic (.5 μm).

Mechanoreceptors have low threshold , activated at .01 N. They exhibit directional sensitivity in that they respond

maximally to a force applied to the crown in one particular direction.

Aß fibers conduction velocity ( 50 m/s). They are either rapidly, intermediate or slowly adapting

fibers depending on the magnitude of the force. Also temperature dependant, peaking at body temp. 75% of mechanoreceptor have their cell body in the

trigeminal ganglion: touch, pressure, and proprioception.

proprioception helps in: Mastication refluxes.Control tongue position(modulate hypoglycaemia activity).Salivary refluxes. Modulate neck masculature.

Nerve ending representing Ruffini –like mechanoreceptor

PDL as a specialised tissuePDL have fetal like characteristics which is important

in:

Determining the characteristics of the PDL . In periodontal diseases: 1. Wound healing is different than other tissues .2. Repair /periodontal reattachment using connective

tissue grafts.

Tooth support mechanism It is frequently stated that the PDL behaves as a "suspensory

ligament“ implying Hook’s law (tooth recoils to it’s resting position after release of load).

Mobility studies show that teeth do not follow that law, instead they show property of hysteresis, meaning they exhibit responses which are time dependent (viscoelasticity).

When there's marked trauma to the a region of the PDL the remaining tissue can still perform a supporting function in the short term.

Collagen fibers, vasculature and ground substances are all involved in tooth support.

Clinical consideration In inflammatory periodontal disease, toxins

products from dental plaque and host defence mechanisms cause destruction of the PDL and alveolar bone: pockets, mobility, exposure of the roots.

We either repair to stop the progression of the disease.

generation of lost tissue:• The problem that the junctional epithelium proliferates rapidly, which prevents PDL Fibers to reattach to cementum. •So we use bioactive molecules such as growth factors, cytokines placed on suitable carriers to prevent gingival tissues from deeper part of the wound ’Guided tissue regeneration’

Tipping movement during orthodontic loading: Crown and root tips are in opposite direction

producing pressure and tension zone on either sides of the root.

Side of tension: (tooth drown from the bone)• periodontal space is wider, fibers are stretched, alveolar crest is pulled, cell number increases esp. FB, osteoid deposition, calcification starts in the deeper tissues, sharpy's fibers secretion with new bone formation, collagen fibers are incorporated in the new bone.

Side under pressure:

Periodontal space becomes narrower. Alveolar crest is slightly deformed . Vascular activity is low.

Changes can be categorized intoI. Direct resorption(when the pressure is light) II. Hyalinization (pressure is high) osteoclasts activity is

abscent, odema,obliteration of blood vessels , degenerative changes in FBs and necrosis.

Necrotic tissue removed by macrophages, and this may cause resorption of teeth roots esp. Maxillary central incisors.

Resorption of roots is decreased by providing constant force over the period of treatment.