Cementum / PeriodontiumDEVELOPMENT OF CEMENTUM

Cementum formation is called cementogenesis. Cementoblasts are the cells responsible for cementogenesis. CementogenesisIts occur right after the enamel was completely formed;The outer and inner enamel epithelia together form the epithelial root sheath of Hertwig which is responsible for determining the shape of the root;The growth of sheath does not occur downward into jaws but that proliferation in cells of sheath causes an upward movement of developing tooth.

These two epithelial layers which are separated by stratum intermedium and stellate reticulum become continous in the area of the future enamel-cementum junction;Hertwigs epithelial root sheathThe Hertwig's epithelial root sheath (HERS) is a proliferation of epithelial cells located at the cervical loop of the enamel organ in a developing tooth. Hertwig's epithelial root sheath initiates the formation of dentin in the root of a tooth by causing the differentiation of odontoblasts from the dental papilla. The root sheath eventually disintegrates, but residual pieces that do not completely disappear are seen as epithelial cell rests of Malassez (ERM).

Epithelial root sheath (diaphragm);Both epithelial layers merged apically into one;At the bottom form 4-5 cells - guides that do not share;

Cervically begins disintegration of epithelial sheat and begins penetration into mesenchymal cells.

Cementogenesis The differentiation of odontoblasts from ectomesenchymal cells;The fragmentation of Hertwig`s epithelial root sheath;The ensuing differentiation of cementoblasts from Hertwig`s sheath cells or follicle cells.

Cementum is made by cementoblasts

Derived from undifferentiated mesenchyme through intermediate precemntoblast stage in cuboidal cells form;They are arranged on outer surface of hyaline layer that covers the dentine.

Cementoblasts are responsible for deposition of organic matrix of cementum which consist proteoglycan and intrinsic collagen fibers.Also they are responsible for subsequent mineralization of organic matrix (cementoid or precementum).

A. HERS did not show any signs of fenestration at this stage;However, single cells (arrowheads) were positioned between ameloblasts (amel) and the beginning of HERS. (B). Tooth mineralization was significantly advanced . The bilayered unit of HERS was still distinguishable at the apex(hers) but had lost its continuity withthe ameloblast layer (amel). Instead,bundles of mesenchymal cells (mes,between arrowheads) as well asfibrous structures (fib) were occupyingthe developing root surface. Isolated epithelial cells (ep) were localized between mesenchymal tissues and HERS. Note the clear separation between ameloblast cell layer (amel, upper row of arrowheads) and mesenchymal cells (mes) covering the developing root surface.The fragmentation of Hertwig`s epithelial root sheat:

A1 and A2. Note how tubular cells from adjacent to the ameloblast cell layer (arrowheads) were interrupting the continuity of the ameloblast cell layer (amel). B1 and B2. The enamel layer (en) and the ameloblast cell layer (amel) clearly demarked the cervical margin of the developing toothcrown. A membrane (arrowheads) separated the ameloblast cell layer (amel) from the mesenchymal cells (mes) occupying the developing root surface. Therewere only a few epithelial cells (ep) in immediate proximity to the root dentin surface (de)..

Cementogenesis

Cementogenesis occurs when Hertwig`s root sheath disintegrates; This disintegration of the sheath allows the undifferentiated cells of the dental sac to come into contact with the newly formed surface of root dentin; This contact of the dental sac cells with the dentin surface induces these cells to become immature cementoblasts.Periodontal ligament and alveolar bone development

As the crown and root develop, the surrounding supporting tissues of the tooth are also developing. The mesenchyme from the dental sac begins to form the periodontal ligament (PDL) adjacent to the newly formed cementum. This process involves forming collagen fibers that are immediately organized into the fiber bundles of the PDL. The ends of these fibers insert into the outer portion of the cementum and the surrounding alveolar bone to support the tooth.The mesenchyme of the dental sac also begins to mineralize to form the tooth sockets or alveoli of the alveolar bone surrounding PDL.

Hertwig`s epithelial root sheat

The begining of cementogenesis

Differentiation of dental follicle cells

(1)Hertwig`s sheat;(2)Epithelial rests of Malassez;(3)The cells of the dental follicle; (4) Cementoblasts;(5)Priodontal ligaments;(6) Osteoblasts;(7) Alveolar bone;(8) Odontoblasts.

Cementogenesis

Mineralization occurs :

When the thin layer of cementoid has formed, mineral salt comes from tissue fluid;Calcium and phosphate ions will deposited as hydroxyapatite crystal along axes of collagen fibers.As cementum continues to increase in thinkness, more follicular (extrinsic) fibers become included in cementum, known as Sharpeys fibers right after periodontal ligament become established.

During the root is developing, those process occurred on apical direction :

Proliferation of cells of epithelial root sheath of Hertwigs;Dentine formation;Fragmentation of older part of sheath;Differentiation of new cementoblasts;Formation of cementum.

CementumA. Haversian bone ;B. Bundle bone (A & B make up alveolar bone proper);C. Reversal line;D. Osteoid ;E. Endosteum, composed of osteoblasts ;F. Cementoid (the organic matrix of cementum) ;G. Pericementum, composed of cementoblasts.

(A) the position of pulp (pulp), dental follicle (df), mineralized dentin (md), periodontal ligament (lig), Hertwigs epithelial root sheath (hers) epithelial diaphragm (epd), predentin (pd), dentin (dent), and several blood vessels (bv),

(B) Note the position of epithelial and mesenchymal tissues on the predentin surface at the apical tip of the root. At the apical margin of the root dentin the periodontal ligament (lig) was in direct contact with the non-mineralized predentin (pd).

Hertwigs root sheath (hers) was separated from the root surface by a periodontal ligament cell layer measuring at least 10 cell layers in thickness. The nude predentin surface (pd) was not covered by mineralized dentin or cementum.

At this stage, mesenchymal cells of the ligament (lig) had direct access to the root surface. Due to their spatial separation, HERS cells had lost their opportunity to deposit cementum on the root dentin prior to their departure from the root surface.Before tooth erupts :-After tooth erupts :

Extrinsic fiber are incorporated in cementum and lie parallel to the root structure.The fiber become oblique.It is known as the precursor of periondontal ligament fibers.

Extrinsic fiberCementum which is formed first:Does not contain any closed cells, but cementoblast are included in later cementum;These enclosed cementoblast are known as cementocytes and found in lacunae;

Multi-rooted teeth the formation of root diaphragm

Multi-rooted teeth:Like anterior teeth, multi-rooted premolars and molars originate as a single root on the base of the crown;This portion on these posterior teeth is called the root trunk;The root of a posterior tooth divides from the root trunk into the correct number of root branches for its type; During the formation of the enamel organ on a multi rooted tooth, elongation of its cervical loop occurs in such a way that long, tongue like horizontal epithelial extensions or flaps develop within.

In teeth with more than one rootThe initial single primary apical foramen, formed by the epithelial diaphragm of root sheath Hertwigs, become divided into two or more secondary apical foramina by tongues of epithelial tissue from diaphragm.

These fuse in future furcation area of the roots. The number of secondary apical foramina is determined by the presence of groups of blood vessels which enter the dental papilla.

Two types of cementum form: Cellular and Acellular

Acellular cementum forms first. The cementoblasts differentiate from follicular cells, which can only reach the surface of the tooth's root once Hertwig's Epithelial Root Sheath (HERS) has begun to deteriorate; The cementoblasts secrete fine collagen fibrils along the root surface at right angles before migrating away from the tooth; As the cementoblasts move, more collagen is deposited to lengthen and thicken the bundles of fibers; Noncollagenous proteins, such as bone sialoprotein and osteocalcin, are also secreted; Acellular cementum contains a secreted matrix of proteins and fibers; As mineralization takes place, the cementoblasts move away from the cementum, and the fibers left along the surface eventually join the forming periodontal ligaments

Cellular cementumDevelops after most of the tooth formation is complete and after the tooth occludes (in contact) with a tooth in the opposite arch.This type of cementum forms around the fiber bundles of the periodontal ligaments; The cementoblasts forming cellular cementum become trapped in the cementum they produce;The origin of the formative cementoblasts is believed to be different for cellular cementum and acellular cementum; One of the major current hypotheses is that cells producing cellular cementum migrate from the adjacent area of bone, while cells producing acellular cementum arise from the dental follicle;Nonetheless, it is known that cellular cementum is usually not found in teeth with one root; In premolars and molars, cellular cementum is found only in the part of the root closest to the apex and in interradicular areas between multiple roots.

The fate of Hertwig`s epithelial toot root sheath

Cemento enamel junction

In 30% the cementum and enamel meet as a butt joint, forming a distinct cementoenamel junction at the cervical margin edge to edge;In 60% the cementum overlaps the enamel;In 10% have a gap between the cementum and enamel it lead to sensitivity at this site.Cementum may be classified in the following ways:By location: Radicular cementum: The cementum that is found on the root surface.Coronal cementum: The cementum that forms on the enamel covering the crown.

By cellularity: Cellular cementum: Cementum containing cementocytes in lacunae within the cementum matrix.Acellular cementum: Cementum without any cells in its matrix.

By the presence of collagen fibrils in the matrix: Fibrillar cementum: Cementum with a matrix that contains well-defined fibrils of type I collagen. Afibrillar cementum: Cementum that has a matrix devoid of detectable type I collagen fibrils. Instead, the matrix tends to have a fine, granular consistency.

By the origin of the matrix fibers (applies only to fibrillar forms of collagen): Extrinsic fiber cementum: Cementum that contains primarily extrinsic fibers, i.e. Sharpey's fibers that are continuous with the principal fibers of the periodontal ligament. Since the fibers were originally produced by periodontal ligament fibroblasts, they are considered "extrinsic" to the cementum. These fibers are orientated more or less perpendicularly to the cementum surface and play a major role in tooth anchorage. Intrinsic fiber cementum: Cementum that contains primarily intrinsic fibers, i.e. fibers produced by cementoblasts and that are orientated more or less parallel to the cementum surface. This form of cementum is located predominantly at sites undergoing repair, following surface resorption. It plays no role in tooth anchorage. Mixed fiber cementum: Cementum that contains a mixture of extrinsic and intrinsic fiber cementum.

The description of the classes of cementumCementum Varieties

AAC - Acellular afibrillar (prymary) cementum1. Acellular, afibrillar cementumThis cementum is mostly composed of mineralized matrix, without detectable collagen fibrils or cementocytes. It is produced exclusively by cementoblasts. It is typically found as coronal cementum on human teeth over enamel and dentin in proximity to the cementoenamel junction

Cervical portion of cementum

2. Acellular, extrinsic fiber cementum

This type of cementum has a matrix of well-defined, type I collagen fibrils; The fibrils are part of the, densely packed Sharpey's fibers, that are continuous with the principal fibers of the periodontal ligament; Because of their dense packing, the individual Sharpey's fibers that form the bulk of the matrix may no longer be identifiable as individual fibers within the cementum layer; This cementum, which is acellular, is located in the cervical two-thirds of the root of human teeth; It plays a major role in tooth anchorage.

Acellular Extrinsic Fiber Cementum (Primary cementum)It is mainly found on cervical and middle root portions, covering 40% to 70% of the root surface;It serves the exclusive function of anchoring the root to periodontal ligament; The acellular extrinsic fiber cementum matrix consists of a dense fringe of short collagenous fibers that are implanted into the dentinal matrix (glycosaminoglycans) and are oriented about perpendicularly to the root surface;When they become elongated and eventually continuous with the principal periodontal ligament fibers they are called Sharpeys fibers.

Intermingling of collagen fiber bundles with those at the unmineralized dentin.The final connection between the collagen fiber bundels of acellular cementum and dentin surgace are shown.They are perpendicular to the dentin and penetrates between the cementoblasts.Cross section of the cementum

3. Cellular, intrinsic fiber cementum

This cementum contains cementocytes in a matrix composed almost exclusively of intrinsic fiber cementum; It is located at sites of cementum repair.; It plays no part in tooth anchorage.; However, it may be covered over by extrinsic or mixed fiber cementum, both of which are able to provide new anchorage;Once the tooth is in occlusion, a more rapidly formed and less mineralized variety of cementum, cellular intrinsic fiber cementum, is deposited;Differentiating cementoblast extend cell processes and deposit the collagen fibrils;As cementum deposition progresses, cementoblasts become entrapped in the extracellular matrix they secrete;These entraped cells, with reduced secretory activity, are called cementocytes, and the space they occupy is the osteocytic lacuna.

Cementocytes

Nourishment of the cells is believed to occur essentially by diffusion, and cementocytes in deeper layers may not be vital.

Collagen fibrils in the CIFCThey are deposited haphazardly during the rapid phase;Hawever, subsequently the bulk of fibrils organize as bundles oriented parallel to the root surface.Acellular and Cellular cementum

4. Cellular, mixed fiber cementum

It is found on the apical third of the root and in furcations (i.e. between roots); In these locations, the rate of cementum formation is usually more rapid than in the cervical region; The mineralized, extrinsic collagen fibers (Sharpey's fibers) run a more irregular course than in acellular, extrinsic fiber cementum; Intrinsic fibers are found interspersed among the extrinsic fibers of the cementum matrix, so that individual Sharpeys fibers are more readily identifiable than in extrinsic fiber cementum;Cementoblasts are trapped in hollow chambers (or lacunae) where they become cementocytes.

Cellular Mixed Fibrils Cementum

Cellular Mixed Fibrils CementumWhen the PDL becom organized, cellular cementum continues to be deposited around the ligament fiber bundles, wich become incorporated into the cementum and partially mineralized, thereby creating cellular mixed fiber cementum;It has:Inclusion of cementocytes;It has laminated structure;The presence of cementoid on its surface;Distinguishing between the fine-fibered, densely packed intrinsic fibers running parallel to the root surface and incorporated extrinsic fibers running at right angles to the root.

Intermediate CementumIC is a thin. noncellular, amorphous layer of hard tissue approximately 10m thick;It is deposited by the inner layer of the epithelial cellls of the root sheath; Deposition occurs immediately before the epithelial root cells disintegrate as a sheet and migrate away from the root into the periodontal tissue;It is the first layer of hard tissue deposited, and it seals the tubules of dentin;IC is composed of enamel protein rather than collagenLongitudinal section of cementum

Cross section of the toot roots incremental lines of cementum

Different layers in the cementum

CimenticlesA cymenticle is a calcified ovoid or round nudule found in the periodontal ligaments;Single or in a group they are near the surface of the cementum;Their origin may be a nidus of epithelial cells that are composed of calcium phosphate and collagen in the same amounts as cementum.

Composition of the CementumSince cementum is not a uniform, mineralized connectivetissue, differences in the proportional composition of the chemical constituents exist between the cementum varieties.Thus, the percentages of its chemical components may varyfrom sample to sample, particularly in different species.Cementum has a similar composition to bone - to about equal parts per volume,cementum is composed of water, organic matrix and mineral.About 50% of the dry mass is inorganic, and consists ofhydroxyapatite crystals. The remaining organic matrix contains largely collagens, glycoproteins and proteoglycans.Mineral compositionCementum is generally less mineralized than root dentin;Acellular extrinsic fiber cementum appears more highly mineralized than cellular intrinsic fiber cementum and cellular mixed stratified cementum;The mineral component is the same as in other calcified tissues, that is, hydroxyapatite - Ca10(PO4)6(OH)2, with small amounts of amorphous calcium phosphates present;As in other hard tissues, the hydroxyapatite of cementum is not pure, but contains other elements (ions) incorporated into mineral phase during mineralization; Thus, cementum contains 0.5-0.9%; Have a high fluoride content compared to other mineralized tissues (up to 0.9% as weight); Cementum also contains 0.1-0.3% sulfur, in particular Cu+2, Zn+2 and Na+.Organic compositionThe organic matrix of cementum is composed primarily of collagens type I and III collagen;Cementum contains two major non-collagenous proteins:bone sialoprotein (BSP);and osteopontin (OPN). Both are phosphorilated and sulfated glycoproteins.;These proteins,which are prominently expressed in acellular extrinsic fiber cementum and acellular afibrillar cementum, bind tightly to the collagenous matrices and hydroxyapatite, and they possess cell attachment properties.Comparison of the composition of the enamel, dentin, cementum, and bone % Enamel Dentin Cementum Bone

Mineral 95 70 61 45

Organic composition 1 20 27 30

Water 4 10 12 25Cement RepairCement repair is a protective function of cementoblasts after resorption of rooth dentin and cementum;These cells are programmed to maintain a smooth surface of the root;Loss of cementum is accompanied by loss of attachment fibers;When this occurs, repair cementum may be deposited by cementoblasts in the defect.The functions of the cementumPeriodontiumPeriodontiumThe periodontium is defined as those supporting the tooth and it consists of four principal components namely:GingivaPeriodontal ligament (PDL)CementumAlveolar bone; These tissues form a specialized fibrous joint and are thought to be of ectomesenchymal origin.

Periodontal ligamentsThe periodontal ligament is a fibrous connective tissue between the alveolar bone and the cementum covering the root;This ligament covers the root and connects with the tissue of the gingiva;The periodontal ligaments occupies the periodontal space and is composed of:Fibers;Cells;And intercellular substance.

Periodontal ligamentCells of dental follicle differentiate into collagen-forming cells of the ligament and of cementoblasts, which lay cementum on the tooth roots;Some cells of the ligament invade the tooth root sheath as it breaks apart;Other cells of the ligament area form delicate fibers, which appear along the forming roots near the cervical region of the crown.

Periodontal ligamentsThe thickness of the PDL varies from tooth to tooth and its position in relation to root;The thinnest portion of the PDL is near the middle of the root;The width of PDL shows a progressive decrease with age.

The PDL has a heterogeneous population of mesenchymal cells that when induced differentiate into fibroblasts, cementoblasts and osteoblasts;The fibroblasts also have the capacity to form PDL, cementum and alveolar bone any time in health;The factors secreted by PDL fibroblasts inhibit mineralization and prevent ankylosis of the tooth.

Development of PDLDuring the initial stage of root formation the follicular cells show increased proliferation;The innermost cells of the follicle near the forming root differentiate into cementoblasts and the outer most cells differentiate into osteblasts;The centrally located cells differentiate into fibroblasts.

Formation of the periodontal fibers

Fibroblasts produce the periodontal ligament fibers;They get embedded in the developing cementum and alveolar bone;At first, all the developing fibers of the periodontal ligament run obliquely in a coronal direction from tooth to bone;This changes as the tooth erupts;The part of PDL fibers present in the cementum and alveolar bone are called Sharpey`s fibers.

Formation of the periodontal spaceUnder the influence of the Hertwigs root sheath mesenchymal cells group are differentiated in fibroblasts;They are arranged under cementoblasts;Produce thick fibers primary arranged in parallel to the root;Fibers are converge towards the cement.

Formation of the first group of fibers - towards a cementum:

Approximation of the fibers

Sharpei`s fibers embedded in cementumSharpei`s fibers hanging in the periodontal spaceAfter attachment of the fibers to the dentin, they are coated with a cementitious matrix;Remain switched on in cement as external or extrinsic fibers;They are called Sharpei`s fibers;Their ends remain freely suspended in the periodontal space.

Sharpey's fibersThe Sharpey's fibers are the mineralized continuation of the thick fiber bundles (marked with an *) that originate in the periodontal ligament and help anchor the tooth to the bone;In this section the mineralized bone, that includes the Sharpey's fibers, appears light blue as compared to the purple color of the non-mineralized portions of the fibers.

Sharpey's fibers in Note the short Sharpey's fibers (SP) imbedded in the new osteoid seam (OS) secreted by osteoblasts (OB) in the periodontal ligament; OC, osteocytes; RL, reversal line.

Formation of the alveolar bone

Formation of alveolar bone and inserting the fiber in it

Intermediate fiber groups - almost perpendicular to the other

Intermediate fiber groups - almost perpendicular to the other

Formation of the third group of fibers

Intermediate plexus

Connection of the periodontal fibers

Increasing the fibers

This is a diagrammatic illustration of the remodeling process in the dental follicle that leads to the formation of the mature periodontal ligament

The earliest remodeling takes place along the tooth (T) and the alveolar bone (AB) side of the dental follicle (Figs. A and B). Small bundles of collagen align themselves perpendicularly to the tooth and bone surface. They gradually grow thicker and longer, with the fibers on the bone side becoming wider than those on the tooth side. On the tooth side, the fiber bundles become incorporated into the developing cementum layer and on the bone side into new bone.

The first stage in the formation of periodontal ligaments

The root surface at onset of cementogenesis and genesis of periodontal ligaments.Legends:D, root dentinDF, dental follicle (immature periodontal ligament)EF, extrinsic fibersF, fibroblasts

Second Stage

third stage

Fourth stage

Intermediate plexusIncompletely remodeled ligament or sections through neurovascular channels have been erroneously described in the dental literature as a so-called "intermediate plexus", a zone believed to allow for mechanical "slippage" between cemental and osseous fibers during rapid eruption. It is recognized today that rapid tooth movement takes place because of continuous remodeling of the structural elements of the ligament at the molecular level, and not by mechanical accomodation of the fibrous elements.

Parallel formation of cement and periodontium

Construction of the root and periodontium during tooth eruption

Crown of the tooth is ready, but the root is not;Dental follicle around the root has become a thin capsule that will remodeling periodontal ligaments (PDL);Some of pluripotent cells of the follicle producing cementoblasts, osteoblasts and fibroblasts, remain as undifferentiated cells;

Usually they are perivasculary, remodeling ligaments of the enamel-cement bond, pull it up apically.

Histology of PDLPDL like any other connective tissue is composed of cells and extracellular components;CellsFibroblasts;Cementoblasts;Osteoblasts;Osteoclasts;Undifferentiated mesenchymal cells;Epithelial cell rests of Malassez.

Extracellular componentsCollagenous fibers;Elastic fibers;Ground substance;Nerves;Blood vessels;

The main fibers are the principal fibers and additional fibers are oxytalan fibers.

Morphology of PDL

97Principal fibersThe fibre bundles that exit the cementum and alveolar bone from the periodontal ligament are called principal fibers;According to the location of those fibers they can be dentoalveolar fibers or gingival fibers.

Principal fibers of the periodontal ligamentThe periodontal fibers of the periodontal ligament are primarily composed of bundles of type I collagen fibrils. They have been classified into several groups on the basis of their anatomic location. The following constitute the principal fiber groups of the periodontal ligament: 1. Alveolar crest fibers;2. Horizontal fibers;3. Oblique fibers;4. Periapical fibers;5. Interradicular fibers.Dentoalveolar fibre groupAlveolar crest group near the cervical region;Horizontal fibre group near the midroot;Oblique fibre group immediately above apical group;Apical fibre group near the apical area of the root;Interradicular group between two roots.

Cementum-alveolar fibers

Gingival ligament

Alveolar crest fibers

Alveolar crest fibers - extend obliquely from the cementum just beneath the junctional epithelium to the alveolar crest.These fibers prevent the extrusion of the tooth and resist lateral tooth movements.

Transseptal fibers

Transseptal fibers (H): extend interproximally over the alveolar bone crest and are embedded in the cementum of adjacent teeth; They form an interdental ligament;These fibers keep all the teeth aligned; These fibers may be considered as belonging to the gingiva because they DO NOT have osseous attachment.

Transseptal fibers

Horizontal fibers

Horizontal fibers(J): attach to the cementum apical to the alveolar crest fibers and run perpendicularly from the root of the tooth to the alveolar bone.

Oblique fibers

Oblique fibers (K): are the most numerous fibers in the periodontal ligament, running from cementum in an oblique direction to insert into bone coronally.

Oblique fibers

Apical fibers

Radiating from cementum around the apex of the root to the bone, forming base of the socket

Interradicular fibers

Interradicular fibers are only found between the roots of multi-rooted teeth, such as molars. They also attach from the cementum and insert to the nearby alveolar bone.

Scheme of the periodontal ligament

The periodontal ligament is composed of collagen fibers that originate as fibers of different diameters at the mineralized surfaces where the fibers are continuous with their mineralized portion (Sharpey's fibers);

These fibers are of a wider diameter on the bone side than the cementum side.

Whether they originate from bone or cementum, they unravel into smaller fibers, which join up with those of adjacent fibers to produce a meshwork of interconnected fibers oriented between bone and cementum; Thus, the periodontal fibers do not stretch cable-like from cementum to bone, but form a meshwork of interconnected fibers.

Connective tissue in periodontiumConstruction of the periodontal ligament :Collagen moleculeCollagen type I

Collagen fibers form a network:

Histological cross-section through a periodontal ligament on the distal surface of a single-rooted tooth.The periodontal ligament fibers (F) originate from relatively thin fibers (*) inserted into the cementum.After they unravel and become intermeshed with adjacent fibers, they form thicker fiber bundles that insert into bundle bone (BB), so named because it contains numerous Sharpey's fibers (SF).Because of mesial drift, the tooth is slowly displaced mesially (to the left).This requires continuous remodeling of the ligament and deposition of new bone on the distal alveolar surface in order to maintain the width of the ligament constant. Neurovascular channels (NV) that house blood vessels, lymphatics and nerves in a loose connective tissue sheath course throughout the periodontal ligament. The ligament also contains cell rests of Malassez (M).Histological cross-section through a periodontal ligament on the mesial surface of a single-rooted tooth.

Mesial drift of the tooth results in cyclical episodes of alveolar bone (AB) resorption, followed by resting periods and short cycles of bone deposition. A distinct reversal line (RL) indicates the location where the last cycle of bone resorption was followed by bone deposition. The latter resulted from the production by osteoblasts (OB) of a thin seam of new bone in which very short Sharpey's fibers (SF) are imbedded. Note the thin fibers of the periodontal ligament (PDL) inserting into the cementum (C) laye

Cells, vessels and nerves of the periodontal ligamentThe periodontal ligament contains a unique assortment of cells that are capable of generating and maintaining three distinct tissues, namely the ligament itself as well as the mineralized tissues on either side of it, i.e. the cementum and the alveolar bone. The major cell types of the periodontal ligament include the following:Fibroblasts, macrophages and undifferentiated ectomesenchymal cells.Cementoblasts and cementoclasts.Osteoblasts and osteoclasts.Cell rests of Malassez.Vascular and neural elements.

Fibroblasts (FB). These cells are responsible not only for the synthesis of collagen and its assembly into collagen fibers (CF), but also for the removal of collagen fibers during the continuous remodeling that takes place in the ligament. Collagen fibrils are removed, in part, through intracellular degradation by fibroblasts in digestive vacuoles. ICC, intracellular collagen in digestive vacuole.

Dysfunctional periodontium

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E. Clinical considerations

1. The thickness of the periodontal ligament varies from 0.1 to 0.4 mm with a mean of around 0.2 mm. 2. The ligament is thicker in functioning than in non-functioning teeth, and in areas of tension than in areas of compression (see table below):3. The ligament cells are capable of remodeling the ligament and adjacent bone when functional forces are altered or the ligament is damaged.4. The periodontal ligament plays a key role in protecting the tooth from being resorbed by the normal remodelling process that affects the adjacent alveolar bone.

5. Excessive forces can cause localized necrosis (cell death) of the ligament by cutting off the normal blood supply to the cells. This situation immediately results in stoppage of remodeling at the affected site. Therefore, orthodontic tooth movement is no longer possible. Repair occurs via emigration of cells from adjoining vital periodontal ligament. In the event the ligament continuity is not restored, localized resorption and ankylosis may occur.6. Accidentally exfoliated teeth can be replanted. Complications include external root resorption and ankylosis if portions of the ligament are permanently damaged. These can be minimized by avoiding excessive handling of the torn ligament prior to replantation.7. Appropriate therapy can halt progressive destruction of the periodontal ligament by periodontal disease and can result in repair of periodontal defects.8. The periodontal ligament is unique among the periodontal tissues, in that it contains precursor cells for the production of the entire attachment apparatus of the tooth, i.e. cementum, periodontal ligament and bone. By using biologically compatible barriers, the therapist is able to promote the ingrowth of these cells into damaged sites where a new periodontal attachment is needed. This therapeutic principle is known as guided tissue regeneration.9. Current research on growth factors and cytokines is aimed at promoting the ingrowth of specific cell types into a wound, while keeping out undesirable cell types.

The PDL is always in a state of remodellingFunctions of PDLFormative;Supportive;Protective;Sensory;Nutritive.