Cementum as a Dyanamic Tissue

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Periodontology 2000. Vol. 13, 1997, 41-75Printed in Denm ark All rights reserved C o p y r i g h t 0 M u n k s p a a r d 1 9 9 7PERIODONTOLOGY 2000ISSN 0906-6713

Dental cementum: the dynamictissue covering of the rootD I E T E R . BOSSHARDT KNUTA . SELVIG

In humans and other mammals, the teeth are notattached rigidly to the alveolar bone. A soft connec-tive tissue, referred to as the periodontal ligament, isinterposed between the tooth root and the sur-rounding alveolar bone. In the mature periodontalligament, principal fiber bundles span between theroot and the bone surface. At the soft-hard tissueborders of the periodontal ligament, the principalperiodontal ligament fibers are embedded in boneon one side and radicular cementum on the otherside. The embedded terminations of these collagenfibers are referred to as Sharpeys fibers. Due to itsintermediary position between the radicular dentinand the periodontal ligament, cementum is a com-ponent of the tooth itself, but belongs functionallyto the dental attachment apparatus, that is, the peri-odontium. The latter tissue unit comprises the peri-odontal ligament, gingiva, alveolar bone and ce-mentum. One of the main functions of cementum isto anchor the principal collagen fibers of the peri-odontal ligament to the root surface. Besides its in-dispensable role in tooth attachment to the sur-rounding alveolar bone, root cementum has import-ant adaptive and reparative functions. The dynamicand highly responsive features of cementum are cru-cial for maintaining occlusal relationship and for theintegrity of the root surface and its function in toothsupport.

The morphogenesis of cementum has been com-prehensively described in various mammalian spe-cies (38). In addition, the physical properties (170)and the basic chemical composition (125) of ce-mentum are known. Cementum is a nonuniform,mineralized connective tissue. Several distinctly dif-ferent cementum varieties are found on humanteeth. They differ with respect to location, structure,function, rate of formation, chemical compositionand degree of mineralization. In fully formed andfunctioning teeth, cementum is firmly attached tothe radicular dentin and covers the entire surface ofthe root. In addition, small localized areas of enamel

close to the cementoenamel junction are frequentlycovered with a particular type of cementum. Ce-mentum increases in thickness towards the apex andmay extend partially into the apical foramen.

Although the morphogenesis and the establishedstructure of the various cementum varieties havebeen described by many researchers, knowledge ofcementum physiology still lags behind what isknown about the other dental and periodontaltissues. The interest in cementum, however, hasnever been given up by researchers, and the ultimategoal of true periodontal regeneration after treatmentfor periodontitis has revived vigorously the interestin this unique mineralized tissue. Although recentstudies have contributed to an understanding of thepossible involvement of some of the molecular fac-tors in cementum regeneration (1191, cementogen-esis, on a cell biological basis, continues to be poorlyunderstood. Virtually nothing is known about thedifferentiation mechanisms of cementoprogenitorcells and the cell dynamics during normal develop-ment, repair and regeneration.

Radicular cementum is unique in that it is avascu-lar, does not undergo continuous remodeling likebone, but continues to grow in thickness throughoutlife. New cementoblasts must, therefore, be continu-ously recruited from committed cementoprogenitorcells in order to replace cementoblasts that havereached the end of their life span. During initial peri-odontal wound healing, new cementoblasts have tobe generated as well from ancestral cementoprogen-itor cells, although much faster than for normaltissue maintenance. It is likely that new cemento-blasts for both maintenance (homeostasis) and re-pair and regeneration take their origin in the sameroot-related portion of the intact periodontal liga-ment. In both circumstances, however, their preciseorigin and the molecular factors regulating new cellrecruitment and differentiation are not known (38,129). The clue to answer these questions lies in thestill growing root and the non-pathologically altered

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Human cementumroot surface. Basic knowledge of cementum develop-ment during normalcy is therefore of utmost import-ance.

The rodent has provided the most popular modelfor the study of tooth development in general. Al-though the tremendously fast growth rates of ratsand mice may render these animals an excitingmodel for studying periodontal ligament develop-ment and tooth eruption, the rodent molar does notprovide a good parallel for the human situation withregard to cementogenesis (38). Over recent years, anincreasing quantity of data has accumulated thatallows human cementogenesis to be described ex-clusively. The aim of this chapter is to give a compre-hensive insight into the structure, function, physicalproperties and chemical composition of human ce-mentum during undisturbed development and re-pair as well as under some pathological conditions.

DevelopmentThe formation of cementum can be subdivided intoa prefunctional and functional developmental stage.The prefunctional portion of cementum is formedduring root development. Since the formation of hu-man tooth roots occurs over an extended period oftime ranging between 3.75 and 7.75 years for perma-nent teeth, the prefunctional development of ce-mentum is an extremely long-lasting process. Duringthis period of time, the primary distribution of themain cementum varieties is determined for eachroot. The functional development of cementum, onthe other hand, commences when the tooth is aboutto reach the occlusal level, is associated with theattachment of the root to the surrounding bone andcontinues throughout life. It is mainly during thefunctional development that adaptive and reparativeprocesses are carried out by the biological respon-siveness of cementum, which, in turn, influences thealterations in the distribution and appearance of thecementum varieties on the root surface with time.

Root formationFor an understanding of cementogenesis, the cellu-lar events occurring during root formation are of ut-most importance. Root formation commences whenthe enamel organ has reached its final size and theinner and outer cell layers of the enamel epithelium,which delineate the enamel organ, proliferate fromthe cervical loop to form Hertwigs epithelial rootsheath (59). Continuous cell mitotic activity at theapical termination of Hertwigs root sheath leads toa coronoapical growth of this double cell layer. Itsmost apical portion, that is, the diaphragm, separ-ates the dental papilla from the dental follicle (Fig.la,b) . The inner and outer cell layer of Hertwigs rootsheath is surrounded by a basement membrane (Fig.lc,d). Similarly to the reciprocal epithelial-mes-enchymal interactions occurring during crown for-mation (141, 158, 159, 209, 2121, cells originatingfrom the peripheral dental papilla differentiate alongthe internal basement membrane of the diaphragminto odontoblasts (Fig. Id) (8, 215). Once the firstmatrix of radicular mantle dentin is formed by thematuring odontoblasts and before the mineraliza-tion of the dentin matrix reaches the inner epithelialcells, Hertwigs root sheath becomes discontinuous.Epithelial cell remnants of Hertwigs root sheath per-sist in the still developing and, later in time, in theaging periodontal ligament at an approximate dis-tance of 30-60 pm remote from the root surface,where they are referred to as the epithelial rests ofMalassez (123). Although seen in longitudinal sec-tions as isolated cell clusters surrounded by a base-ment membrane, which separates them from thesurrounding connective tissue, they apparently forma continuous network ensheathing the root at a cer-tain distance (21, 41, 53, 143, 167, 182, 183, 184).Al-though the number of epithelial rests of Malassezdecreases with age (86, 151, 154, 199, 2201, cell mi-totic activity has also been observed (90, 107, 220).Their existence in the periodontal ligament through-out life implies that they represent more than a

Fig. 1. Light (a) and transmission electron (b-d) micro-graphs showing the most apical portion of two still grow-ing human premolar roots developed to 50%(a) and 75%(b-d) of their final length. a, b. Hertwigs root sheath(HRS) consists of an inner (IE) and outer epithelial celllayer (OE). Cell mitotic activity at the apical loop (arrowin a points at a dividing cell) results in an apical growthof Hertwigs root sheath, thereby separating the cells ofthe dental follicle proper (DFP) from the pre-odontoblasts

(pOB) of the dental papilla. Note the striking morpholog-ical diversity of the epithelial cells within Hertwigs rootsheath in b. Source: Bosshardt &Schroeder (32) with per-mission from the publisher. c, d. A basement membrane(BM, arrowheads) separates the epithelial cells of Hert-wigs root sheath from the surrounding mesenchyme.OB: odontoblasts;PD: predentin. Original magnification:a: x 800;b: x 2200; c, d x 7700.

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Fig. 2. Transmission electron micrographs illustrating thedevelopment of the dentinocemental junction (DCJ) in thecoronal half of a human premolar root developed to 50%of its final length. a. Following the disintegration of Hert-wigs epithelial root sheath, cytoplasmic processes (CP)originating from pre-cementoblasts (pCB) penetrate thenot yet mineralized matrix of the radicular mantle dentin,that is, the external predentin (PD), immediately coronalto the advancing root edge (ARE). The cell labeled withIE is separated from the newly deposited predentin by abasement membrane (BM) and represents the coronal ter-mination of the intact inner epithelial cell layer of Hert-

wigs root sheath. b. When the precementoblasts differen-tiate along the external surface of the predentin into ce-mentoblasts (CB), they implant the initial collagen fibrilsof the cementum matrix among those of the predentin. Inthis way, the intimate interdigitation of the collagen fibrilsat the dentinocemental junction is accomplished. Notethat there is no intermediate layer interfacing the two dif-ferent fibril populations. c. The external mineralizationfront (MF, arrowheads) in dentin (D) is about to reach thefibrillar dentinocemental junction, when the dentinalmatrix is almost completely covered by the cementummatrix. d. When the cementum matrix is established on

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merely vestigial structure. Their function, however,continues to be unknown.

Cementoblast originThe differentiation of cementoblasts from cement-oprogenitor cells and the formation of the dentin-ocemental junction are temporally and spatiallyclosely related to dentin formation. The initiation ofcementogenesis is, therefore, restricted to a narrowband encircling the forming root at its most apicalportion. This circular band extends only 200-300 pmcoronally from the advancing root edge and shifts inthe apical direction while the root elongates (30, 32).

Based on transplantation and 3H-thymidinestudies performed in mice (71, 96, 145, 201, 202, 204,234) and supported by ultrastructural indications fordirected cell migration towards the root surface inrat molars (46), it is widely accepted today that thecementoprogenitor cells arise from the dental follicleproper, which is of ectomesenchymal origin (that is,a derivative of the cranial neural crest). However, aspointed out by Thomas & Kollar (214), labeled ce-mentoblasts could also be of epithelial origin, sincecells of the enamel organ, which give rise to Hert-wigs root sheath, also incorporate 3H-thymidineprior to transplantation. Recent ultrastructural andimmunohistochemical studies support, indeed, thehypothesis that the cementoblasts originate fromepithelial cells of Hertwigs root sheath when theyundergo an epithelial-mesenchymal transformation(33, 38, 121, 214, 216). Such phenotypic transform-ations have been well documented during embry-onic development (93) and include, for instance,neural crest (110), sclerotome (191) and cardiaccushion mesenchyme (126). Another example ofphenotypic transformation has recently been shownduring palatogenesis when the ectodermal cells ofthe palatal medial edge epithelium transform intomesenchymal cells (66, 87). This example is of par-ticular interest in analogy to a possible epithelial-mesenchymal transformation of Hertwigs rootsheath cells, since the epithelial cells of the palatalmidline seam belong to the oral epithelium and theunderlying mesenchymal cells are believed to beneural crest in origin.

the root surface, the mineralization front has reached andpartially passed the fibrillar dentinocemental junction.OB: odontoblasts. Original magnification: a-d: x 6250.

Differentiation of cementoblastsFor the time being, the nature and origin of the mol-ecules that trigger both a possible cell migration to-wards the root surface and cementoblast differen-tiation are not known. However, several possibilitieshave been suggested and, notably, all of them havebeen derived from experiments in rats and mice. Achemical substance produced early in rat molar den-tinogenesis has been suggested to act as a chemoat-tractant for the cells of the dental follicle proper (46).Since it has been postulated that the disruption ofHertwigs root sheath appears to be a consequenceof this directed cell migration (461, such a chemoat-tractant must have effect through the intact Hert-wigs root sheath, which is composed of two celllayers surrounded by a basement membrane oneach side. Although the dentin matrix is known toinduce in vitro cell migration (1681, it seems very un-likely that a matrix component can be effectivethrough such a barrier.

Although repeatedly suggested (96, 113, 143, 1651,interactions between the dental follicle proper andHertwigs root sheath, which would eventually leadto cementoblast differentiation, have never beenshown. In analogy to the reciprocal epithelial-mes-enchymal interactions between the inner epithelialcells of Hertwigs root sheath and the differentiatingodontoblasts, timed modulations in basement mem-brane composition could possibly act as inductivesignals for cementoblast differentiation. Results fromrecombination experiments using murine molars in-dicate indeed that a mineralized tissue adhering tothe developing dentinal root surface depends on thepresence of basement membrane components (120).These experiments could, however, not clarifywhether the tissue formed on the root surface waseither bone or cementum. In addition, it remains tobe determined whether components of the base-ment membrane induce the cells from the dentalfollicle proper to differentiate into cementoblasts.

Other extracellular matrix proteins that have beensuggested to play a role in cementoblast differen-tiation are noncollagenous proteins also found inbone. High expression of the two major noncolla-genous proteins, bone sialoprotein (122, 193) andosteopontin (192, 194), has been detected on thesurface of the forming molar roots in mice, and ithas been proposed that bone sialoprotein might beinvolved in precementoblast chemoattraction, ad-hesion to the root surface and cell differentiation(122). Since these results were derived from im-munohistochemical studies of thick sections, they

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Human cementumdo not allow a precise immunolocalization. Thesequential appearance of bone sialoprotein and os-teopontin during root development and their preciseroles remain to be determined. The forming rootcomprises the initial mineralization of both dentinand cementum, a process that has to be preciselyharmonized in time and space. The high expressionsof bone sialoprotein and osteopontin are likely to berelated to the mineralization process of mantle den-tin and cementum, including their interface. Thesituation at this interfacial site is more complicatedin the rodent molar, since an interfacial layer, whichappears to be rich in glycoproteins, is frequently in-terposed between dentin and the cementum properand may significantly contribute to the highimmunoreaction on the root surface (see: The devel-opment of the dentinocemental junction).As a mat-ter of fact, there exists no tissue intermediate be-tween cementum and dentin in human teeth, and itremains to be determined whether bone sialoproteinand/or osteopontin expression precede and there-fore induce cementoblast differentiation.

Another group of proteins, that are immunolog-ically related to enamel proteins have also been pro-posed to be involved in early cementogenesis (164,186, 187). They appear to be a normal feature o nthe root-analogue surface of rodent incisors and afrequent matrix constituent of the cervical root sur-face of rodent molars and have also been character-ized biochemically from extracts of human ce-mentum (188). The inner epithelial cells of Hertwigsroot sheath, which are a derivative of the inner cellsof the enamel organ, do, at least for a certain time,maintain the potential for the production and secre-tion of enamel or enamel-related proteins, as can beclearly seen in the extreme case of enamel drops andpearls occasionally covering the root surface. Al-though repeatedly suggested, it is still not clear

whether and how these proteins influence the initia-tion of cementogenesis.

The development of the dentinocemental junctionNo matter what the factors are that trigger the differ-entiation of cementoprogenitor cells into fully acti-vated cementum-producing cells, they differentiatealong the newly deposited and not yet mineralizedmatrix of the radicular mantle dentin into cemento-blasts. At the beginning of their maturation on theroot surface, they extend numerous tiny cytoplasmicprocesses into the loosely arranged and not yet min-eralized dentinal matrix (Fig. 2a). This enables thecementoblasts to position the initially secreted colla-gen fibrils of the cementum matrix among those ofthe dentinal matrix (Fig. 2b), and this crucial stepleads eventually to a n intimate interdigitation of thetwo different fibril populations (Fig. 2c) (30, 32). Themineralization of the outermost layer of the dentinmatrix, that is, the mantle dentin, appears to be de-layed and the mineralization front in dentin doesreach the future dentinocemental junction, not be-fore the implantation of the cementum matrix is es-tablished and the dentinal matrix is completelycovered with the collagen fibrils of cementum (Fig.2d), The term intermediate cementum appears re-peatedly in the literature and in textbooks. Althoughoriginally described for the apical portion of humanteeth (191, there exists no interfacial layer betweendentin and cementum in hum an teeth (38). In ro-dent molars and incisors, on the other hand, an in-termediate layer has frequently been observed, par-ticularly between acellular extrinsic fiber cementumand dentin (112, 144, 146, 172, 225, 226). This layerappears to be rich in glycoproteins but containssparsely distributed collagen fibrils (225, 226). Theorigin of this layer is still controversial. It is either

Fig. 3. Light micrographs showing the different cementumvarieties found on human teeth. a. The acellular afibrillarcementum (AAC) shown in this micrograph overlaps theenamel on the crown (ES; enamel space), apposes to thedentin (D) and merges into acellular extrinsic fiber ce-mentum (AEFC). b, c. The acellular extrinsic fiber ce-mentum is usually found on the cervical half of the root.The extrinsic matrix fibers remain short during the pre-functional development (b) but become continuous withthe principal fibers of the periodontal ligament (PL) whenthe tooth is in functional occlusion (c). d, e. Cellular in-trinsic fiber cementum (CIFC) during its initial attach-ment to the root dentin at the apical portion of a humanpremolar root developed to 75% of its final length (d)

and at a more advanced stage of formation (e).f. Later indevelopment, the cervically located acellular extrinsicfiber cementum splits into several layers which interfacewith the more apically located cellular intrinsic fiber ce-mentum layers to form the cellular mixed stratified ce-mentum (CMSC). Source: Schroeder (169). g. Cellular in-trinsic fiber cementum is also found as a reparative tissuefilling resorptive defects of the root. Note the reversal line(RL) demarcating the junction between old and newtissues. Source: Bosshardt (35)with permission from thepublisher. HRS: Hertwigs root sheath; O B odontoblasts;PD : predentin. Original magnification: a-d: ~ 4 4 0 ; :X 1 3 0 ; f: ~ 8 0 ;: X 2 3 0 .

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Fig. 4. Transmission electron micrographs of acellularafibrillar cementum {AAC) at the cementoenamel junc-tion. The area outlined in a corresponds to b. a. The acel-lular afibrillar cementum apposes to the enamel space(ES),which contains residual enamel proteins, and tapersin the coronal direction. b. The acellular afibrillar ce-

mentum is characterized by numerous incremental layerswith varying electron density and texture. c. In the apicaldirection, the acellular afibrillar cementum abuts againstthe dentin (D ) and the acellular extrinsic fiber cementum(AEFC). Original magnification: a: ~ 3 2 0 0 ; , c: x 13,000.

formed by cementoblasts (47, 167, 225) or by the in-ner epithelial root sheath cells (144, 146, 161, 188,194).

cal and middle root portions (Fig. 3a-c). On frontteeth, it may also cover part of the apical root por-tion, since the apical extension of acellular extrinsicfiber cementum on the root surface increases fromposterior to anterior teeth. Cellular intrinsic fiber ce-

The cementum varieties:their location, formation, structureand functionHuman teeth have three fundamentally differentvarieties of cementum. The location of these variet-ies shows tooth-type specific distribution patternsbut may also vary along and around the surface ofthe same tooth. The following general rule appliesto human teeth: acellular afibrillar cementum coversminor areas of the enamel, particularly at and alongthe cementoenamel junction (Fig. 3a). Acellular ex-trinsic fiber cementum is mainly found on the cervi-

mentum is initially deposited on root surface areaswhere no acellular extrinsic fiber cementum hasbeen laid down on the dentin (Fig. 3d). This mayoccur in the furcations and on the apical root por-tions (Fig. 3d,e). Cellular intrinsic fiber cementummay overgrow layers of acellular extrinsic fiber ce-mentum, and acellular extrinsic fiber cementum, inturn, can overlay cellular intrinsic fiber cementum.The so formed mingled cementum is called cellularmixed stratified cementum and is confined to theapical root portions and to the furcations (Fig. 3f).In addition, mainly cellular intrinsic fiber cementumparticipates in the repair process of previously re-sorbed roots (Fig. 3g).

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Acellular af5brillar cementumThe acellular afibrillar cementum consists of a min-eralized matrix, which appears similar to the inter-fibrillar matrix of acellular extrinsic fiber cementum,but contains neither collagen fibrils nor embeddedcells. The lack of collagen fibrils indicates that thiscementum variety has no function in tooth attach-ment. The acellular afibrillar cementum can be iden-tified by light and electron microscopy. Under thelight microscope, the acellular afibrillar cementumstands out by its basophilia and its more or less uni-form appearance (Fig. 3a). In the electron micro-scope, however, the structure of acellular afibrillarcementum is less homogeneous (Fig. 4a-c). A vari-able number of layers with varying electron densityand different texture, which can be either granularor reticular, give the acellular afibrillar cementum amultifarious appearance.

Acellular afibrillar cementum is deposited as iso-lated patches over minor areas of enamel and den-tin. Cementum islands represent isolated patches ofacellular afibrillar cementum deposited on the en-amel over small areas of the dental crown just cor-onal to the cementoenamel junction. Cementumspurs are found around the cementoenamel junc-tion, where they cover minor areas of the enameland the adjacent dentin of the root (Fig. 3a, 4a). Ce-mentum spurs may be covered by acellular extrinsicfiber cementum and /o r by junctional epithelium.The areas and location of acellular afibrillar ce-mentum vary from tooth to tooth and along thecementoenamel junction of the same tooth. This un-predictable distribution pattern indicates that acel-

yet mineralized matrix of the radicular mantle dentin(NMD). The latter is continuous with the pulpal predentinlayer (PD) and tapers in the coronal direction. Note thatHertwigs epithelial root sheath (HRS) is associated withthe nonmineralized matrix of the radicular mantle dentinfor an extremely short distance a t the advancing root edge(ARE); 2. Establishment of the acellular extrinsic fiber ce-mentum matrix on the root surface in form of a shortcollagenous fiber fringe (FF);3. When the fiber fringe hasattained its maximum numerical density, a cell-fiberfringe meshwork establishes on the root surface. 1-3. Theexternal mineralization front in dentin (mineralized den-tin, MD) is gradually approaching the base of the fiberfringe implantation on the root surface. 4. The externalmineralization front in dentin has reached the future den-tinocemental junction. E: enamel; ERM: epithelial cellrests of Malassez. Source: Bosshardt &Schroeder (30)withpermission from the publisher.

Fig. 5. Schematic diagram depicting the gradual develop-ment of acellular extrinsic fiber cementum during its pre-functional genesis along a human premolar root de-veloped to about 50% of its final length. 1.Attachment ofthe acellular extrinsic fiber cementum matrix to the not

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lular afibrillar cementum formation is likely to be adevelopmental curiosity that deviates from the normrather than an indispensable tissue.

The cells responsible for the formation of acellularafibrillar cementum have still not been determinedwith precision. Its formation commences at the endof enamel maturation and continues for an un-known period of time. It is believed that connectivetissue cells are responsible for the acellular afibrillarcementum formation when they come in contactwith the enamel surface (166, 167). To make thispossible, cells of the reduced enamel epitheliummust be lost or detached from the enamel. On theother hand, it cannot be completely ruled out thatacellular afibrillar cementum is an epithelia1 productinitially produced when the ameloblasts transforminto the reduced enamel epithelium and when thecells of the inner enamel epithelium are about togenerate the inner cells of Hertwigs root sheath. Invitro experiments showed that calcified layers, whichmorphologically resembled acellular afibrillar ce-mentum, formed around demineralized dentin slicesimmersed in serum-containing culture mediumsupplemented with alkaline phosphatase and an or-ganic source for phosphate such as the monophos-phate ester P-glycerophosphate (17).The notion thatsuch acellular afibrillar cementum-like layersformed also in the absence of cells suggests that thismatrix represents a co-precipitate of medium- orserum-derived components and mineral. However,the enzyme alkaline phosphatase is required formineralization to occur. In the in vivo situation, thisenzyme is particularly associated with periodontalligament cells in the vicinity to bone and cementum(88).

Acellular extrinsic fiber cementumThe acellular extrinsic fiber cementum is usuallyconfined to the coronal half of the root. Its formationcommences therefore shortly after crown formationis completed and always before cellular intrinsicfiber cementum starts to form on more apical rootportions. The gradual development of acellular ex-trinsic fiber cementum can be followed along theforming root (Fig. 5 , 6a,b) (30, 31, 33). The cemento-blasts producing acellular extrinsic fiber cementumcommence their cell differentiation in closest prox-imity to the advancing root edge. This may occuronly about 20 to 30 pm coronal to the first depositeddentinal matrix. These cells resemble fibroblasts, re-veal a well-developed rough endoplasmic reticulum,are interconnected by desmosome-like junctionsand commence to produce and attach the colla-genous cementum matrix as close as 50 pm coronalto the root edge (Fig. 6c). Further collagen depo-sition results in a complete covering of the not yetmineralized dentinal matrix along the next 100 pmof the root surface (Fig. 6d). About 200 to 300 pmcoronal to the advancing root edge, the initial acellu-lar extrinsic fiber cementum matrix is established onthe dentinal matrix (Fig. 6e). The acellular extrinsicfiber cementum matrix consists of a dense fringe ofshort collagenous fibers that are implanted into thedentinal matrix and oriented about perpendicularlyto the root surface (Fig. 6e, 7a). The outwardly pro-gressing mineralization front in dentin (Fig. 6d) doesnot reach the future dentinocemental junction untilthe collagenous interdigitation of the two fibrilpopulations is established (Fig. 6e). Since the min-eralization of the dentinal matrix commences about

Fig. 6. Light (a, b) and transmission electron (c-e) micro-graphs illustrating the initial attachment and gradual de-velopment of the acellular extrinsic fiber cementum mat-rix (that is, fiber fringe; FF) along the apical portion of ahuman premolar root developed to about 50% of its finallength. The area outlined in a corresponds to b. The posi-tions of the micrographs c and d are indicated in b bythe corresponding letters. a, b. Immediately coronal to thedisintegration of Hertwigs epithelial root sheath (O E indi-cates the outer and IE the inner epithelial cell layer), in-tensely stained cells distend the space between the rootsurface and the outer epithelial cell layer. More coronally,the cells do no more reveal the same intense staining anda fiber fringe (arrows) appears on the root surface. Theonset of dentin mineralization is seen 100 pm coronal tothe advancing root edge (ARE) and somewhat away fromthe external root surface (lowest arrowhead). In the cor-

onal direction, the external mineralization front (MF, ar-rowheads pointing to the left) in dentin (D) is gradualiyapproaching the root surface. c. The cementoblasts (CB)on the root surface commence to implant the fiber fringeamong the collagenous matrix fibrils of predentin (PD) atthe future dentinocemental junction (DCJ). Source: Boss-hardt (35)with permission from the publisher. d. Whenthe fiber fringe is covering almost the entire predentin,the external mineralization front in dentin is about toreach the fibrillar dentinocemental junction. Source:Bosshardt & Schroeder (30) with permission from thepublisher. e. The established fiber fringe continues fora short distance into the space of the immature peri-odontal ligament (PL). The mineralization front has com-pletely reached and partly passed the fibrillar dentinoce-mental junction. OB: odontoblasts. Original magnifi-cation: a: x 130; : ~ 4 4 0 ;, d x6700; e: X4400.

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Fig. 7. Light micrographs demonstrating the growth ofacellular extrinsic fiber cementum (AEFC) on human pre-molar roots. a. The acellular extrinsic fiber cementummatrix consists of short fringe fibers (FF) that emergefrom the root surface and appose to a fibrocellular mesh-work occupying the space of the immature periodontalligament (PL). A cementum layer is not yet visible. b. Min-eralization of the fiber fringe, which is indicated by round,basophilic dots within the fiber fringe base (arrows), pro-ceeds very slowly. This acellular extrinsic fiber cementum

100 pm coronal to the advancing root edge andsomewhat underneath the root surface (Fig. 6b1, themineralization of the mantle dentin seems appar-ently to be delayed. With the onset of cementummineralization, the acellular extrinsic fiber ce-mentum commences to grow in thickness (Fig. 7b,c).This growth is extremely slow but quite constant(Fig. 10a) (33, 179).The extrinsic fibers remain shortuntil the tooth is about to reach the occlusal level(Fig. 7c). On cervical root surfaces of human pre-molars, the prefunctional acellular extrinsic fiber ce-mentum development may last 5 years or more, thatis, until acellular extrinsic fiber cementum hasreached a thickness of about 15 pm (Fig. 7c) (31,33,38). How the short fiber fringe becomes elongatedand eventually continuous with the principal peri-

layer has developed over approximately 2 years. Note thatthe fiber fringe is still short and apposes to a fibrocellularmeshwork running broadly parallel to the root surface.c. This 15-pm-thick acellular extrinsic fiber cementumlayer has developed over approximately 5 years. Note thechange in the orientation of the periodontal ligamentfibers and the continuation of some of the fringe fiberswith the developing principal periodontal ligament fibers.D dentin. Original magnification: a-c: X440.

odontal ligament fibers is still an open question. Theacellular extrinsic fiber cementum continues to growas long as the adjacent periodontal ligament remainsundisturbed. The extraordinarily high numericaldensity of fibers inserting into acellular extrinsicfiber cementum (approximately 30,000/mm2; (167))is a reflection of the significant function of this ce-mentum variety for tooth anchorage to the sur-rounding bone. Due to posteruptive tooth move-ments, changes can occur in the direction of theSharpey's fibers. These changes are accentuated byindividual acellular extrinsic fiber cementum layersinterfaced by growth lines, also known as resting orincremental lines. Although acellular extrinsic fibercementum is a quite constantly growing tissue, theselines appear to represent the periodic deposition of

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Human cementumcementum layers in frequent association with anabrupt change in the direction of Sharpeys fibers.Moreover, as can be deduced from faster growthrates on distal (4.3 pm/year) than on mesial (1.4 pm/year) root surfaces (56), acellular extrinsic fiber ce-mentum has the potential to adapt to functionallydictated alterations such as mesial tooth drift.

Cellular and acellular intrinsic fiber cementumAlthough the intrinsic cementum alone has no im-mediate function in tooth attachment, its importantrole as an adaptive tissue (167) that brings and main-tains the tooth in its proper position should not beunderestimated. In addition, only cellular intrinsicfiber cementum can repair a resorptive defect of theroot in a reasonable time due to its capacity to growmuch faster than any other known cementum type.Functional stimuli, that is, the force generated bytooth contact and mastication, are widely held re-sponsible for the onset and appositional growth ofcellular intrinsic fiber cementum. This assumptionprobably originates from observations showing thatthe genesis of this cementum variety coincides withthe first occlusal tooth contact (51, 58, 95, 140) andthat functioning teeth appear to have thicker ce-mentum layers than teeth which are not in function(84, 99). Several observations, however, are not inkeeping with this concept. As already suggested byKronfeld (105, 106) and Kellner (1041, mastication isapparently not a prerequisite for cellular intrinsicfiber cementum genesis, since i) the furcations ofhuman teeth are covered with thick cementumlayers before they emerge into the oral cavity (1051,ii) impacted and erupted teeth without antagonistsappear to have thicker cementum layers than fullyerupted and functioning teeth (9, 103, 104, 106, 181)and iii) over-compression of the periodontal liga-ment causes root resorption. Thus, it seems that theinitiation of cellular intrinsic fiber cementum gen-esis does not depend on stimuli transmitted by mas-ticatory forces and that influence by pressure mayreduce the rate of matrix deposition.

As for acellular extrinsic fiber cementum, theinitiation of cellular intrinsic fiber cementum gen-esis on the forming root commences in closest prox-imity to the advancing root edge (32, 34). Prece-mentoblasts differentiate along the not yet mineral-ized dentinal matrix into large, basophilic cells (Fig.8a). They first project numerous cytoplasmic pro-cesses into the loose dentinal matrix and immedi-ately commence to implant the initial collagen fibrils

among those of the dentinal matrix (Fig. 8b). Ad-ditional cementoblasts, which are remote from thedentinal surface, deposit their cementum matrix atvarious locations around themselves (Fig. 8c). Thismultipolar and fast matrix deposition, which occursin the space between deviating epithelial cells ofHertwigs root sheath and the dentinal surface (Fig.8a), appears to be the reason for the incorporationof some of the cementoblasts (32, 67, 146). The cellsentrapped in the mineralized cementum are referredto as cementocytes and occupy lacunae, which areinterconnected through canaliculi (see: Physiologicalactivity of cementocytes) (Fig. 16, 17). The cemento-blasts attain their full synthetic activity approximate-ly 100 pm coronal to the advancing root edge (Fig.9a). They are large, basophilic cells with an euchro-matin-rich nucleus and an abundant endoplasmicreticulum. The rapid matrix deposition slows downsoon, and further collagen matrix is deposited in amore unipolar mode of secretion (Fig. 9b-e). In rarecases, the intrinsic cementum is formed in an ex-tremely unipolar mode of matrix deposition andcompletely lacks cementocytes (Fig. 8d, 9d,e). Thisparticular tissue consists of densely bundled colla-gen fibrils and is named acellular intrinsic fiber ce-mentum (29). The collagen fibrils produced duringthe fast, multipolar cellular intrinsic fiber cementuminitiation show a more random orientation thanthose of the subsequently deposited matrix. There-fore, the bulk of the intrinsic collagen fibrils formdiscrete bundles oriented mainly parallel to the rootsurface (Fig. 1 a). Polarized light microscopic in-vestigations in deciduous (1021, permanent (163)and impacted human teeth (99) suggest that theseintrinsic fibers are arranged in an orderly fashionaround the root (Fig. ll b) .

Unlike in rat molars (227, 228), initial cellular in-trinsic fiber cementum deposition on human toothroots is not necessarily associated with the simul-taneous formation of extrinsic fibers, that is, the fu-ture Sharpeys fibers. In humans, the extrinsic fibersare oriented about perpendicularly to the root sur-face and traverse the intrinsic cementum varietyeither sporadically or densely arrayed in parallel. Al-though the numerical density of these highly aggre-gated extrinsic fibers may be distinctly less than inpure acellular extrinsic fiber cementum (1671, theyare considered as the matrix of acellular extrinsicfiber cementum that intermingles or alternates withthe intrinsic fibers. This mixed cementum is referredto as cellular mixed stratified cementum. When theextrinsic fibers are continuous with the functionallyoriented principal fibers of the periodontal ligament,

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Human cementum

Fig. 9. Light microscopic radioautographs demonstratingthe prefimctional deposition of cellular intrinsic fiber ce-mentum (CIFC) and acellular intrinsic fiber cementum(AIFC) close to (a) and more remote from the advancingroot edge (ARE) (b-e). These human premolars with rootsdeveloped to 75-95% of their final length were in vitropulse-labeled with 3H-proline for 15 min followed by achase incubation for 45 min (a, d), 1 h 45 min (b), 4 h 45min (e), and 23 h 45 min (c). Silver grains are, first, local-

ized in clusters over the paranuclear cytoplasm of ce-mentoblasts (arrowheads in a, d), appear, later, over theperipheral cytoplasm of cementoblasts and the subjacentcementum matrix (b), and eventually cover the peripheralcementum matrix (c, e). Source: Bosshardt & Schroeder(34) with permission from the publisher. D: dentin; HRS:Hertwigs epithelial root sheath; OB: odontoblasts; PD:predentin; P L periodontal ligament. Original magnifi-cation: a: x440;k.700.

Fig. 8 . Light (a) and transmission electron (b-d) micro-graphs illustrating the genesis of cellular intrinsic fibercementum (CIFC) and acellular intrinsic fiber cementum(AIFC) at the advancing root edge (ARE) (a-c) and on theestablished cementum layer (d), espectively Human pre-molar roots developed to 75% of their final length. a. Atthe advancing root edge, the inner cells of Hertwigs epi-thelial root sheath (HRS) cover not more than 20 pm ofthe newly deposited predentin (PD), whereas the outercell layer of Hertwigs epithelial root sheath continues forabout 150 pm in the coronal direction. Cementoblasts(CB) distend the space between the root surface and thedeviating outer epithelial cell layer. The cellular intrinsicfiber cementum thickness increases rapidly in the coronaldirection. Arrowheads point at the mineralization front(MF) of both dentin (D ) and cellular intrinsic fiber ce-

mentum. The interrupted line corresponds to the dentin-ocemental junction. b. Large cementoblasts commence toproduce and attach the initial cellular intrinsic fiber ce-mentum matrix to the external predentin matrix about 50pm coronal to the advancing root edge. Note that thereis no intermediate layer between the two matrices at thedentinocemental junction (DCJ).c. The cellular intrinsicfiber cementum matrix is also produced away from thepredentin among the cementoblasts. d. Further matrix isdeposited by cementoblasts, which form a continuous celllayer when they are associated with acellular intrinsicfiber cementum. Sources: a-c: Bosshardt & Schroeder(32); d Bosshardt & Schroeder (34) with permission fromthe publisher. OB: odontoblasts. Original magnification:a: x440; b: X7700; c: X3200; d X4400.

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Fig. 10. Fluorescence lines in cementum, dentin (D) andalveolar bone (AB) f a mandibular second deciduous mo-lar of a Macaca fmcicularismonkey. The animal receivedsequential injections of calcein (green lines) and xylenolorange (orange lines) on average every 33 days. The twofluorochromes bind to sites of ongoing mineralization andproduce clear fluorescence lines. Note the difference inthe labeling pattern between acellular extrinsic fiber ce-

they can be regarded as Sharpeys fibers. As layers ofacellular extrinsic fiber cementum and cellular andacellular intrinsic fiber cementum develop unpre-dictably in time, space and thickness (167, 1691,par-ticular root surface areas covered with cellular mixedstratified cementum may temporarily remain unsup-ported by periodontal fibers.

Like pure acellular extrinsic fiber cementum onthe coronal half of the root, cellular mixed strati-fied cementum increases in thickness throughoutlife (236). However, the unpredictable dynamics ofthe tissue alternations renders growth rate deter-mination much more difficult for cellular mixedstratified cementum. Nevertheless, the initial cellu-

mentum (AEFC) and cellular intrinsic fiber cementum(CIFC). The labeling pattern and the difference in thegrowth rates between acellular extrinsic fiber cementumand cellular intrinsic fiber cementum are basically thesame in human teeth, albeit acellular extrinsic fiber ce-mentum and cellular intrinsic fiber cementum grow moreslowly. Source: Bosshardt et al. (28). PL: periodontal liga-ment. Original magnification: a, b: x250.

lar intrinsic fiber cementum growth has beenmeasured in deciduous teeth of a non-human pri-mate (28). It may be up to 30-fold faster than themore regular acellular extrinsic fiber cementumdeposition (Fig. lob) . The dynamic tissue alter-nations and the variations in growth rates are re-flected by a tissue layering of cellular mixed strati-fied cementum with layers of acellular extrinsicfiber cementum and incremental lines (that is,growth lines or resting lines) interfacing layers ofcellular and acellular intrinsic fiber cementum. Thepatch-wise deposition of cellular mixed stratifiedcementum results in great circumferential vari-ations in cementum thickness and reflects periods

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Human cementum

Fig. 11. Polarized light photomicrographs of cementum.a. In a section made parallel to the root surface, intrinsiccollagen fibers in cellular intrinsic fiber cementum appearas wide sheets of alternating directions. b. Cementum on

of accelerated deposition of cellular intrinsic fibercementum, which are probably due to functionaldemands in order to reposition the tooth when itis shifting in its bony socket during its post-erup-tive tooth movements.

MineralizationMineralization begins in the depth of precementum.Fine hydroxyapatite crystals are deposited, first be-tween and, secondly, within the collagen fibrils (Fig.12) by a process which, apparently, is identical to themineralization of bone tissue. Zander & Hurzeler(236) examined the thickness of cementum on hu-man teeth extracted from individuals of varying ages.From their data it can be calculated that the mean,linear rate of cementum deposition on single-rootedteeth is about 3 pm per year, but varying greatly withtooth type, root surface area, and type of cementumbeing formed. A similar rate has been found for acel-lular extrinsic fiber cementum in young human pre-molars (33) and in nonfunctioning, impacted teeth(9). Cementum forms at a much higher rate in de-ciduous teeth of the Macacafasciculuris monkey (0.1pm/day for acellular extrinsic fiber cementum and

an impacted third molar. Collagen fibers are almost ex-clusively of the intrinsic type arranged in an orderlyfashion parallel to the root surface (arrow). D: dentin.Original magnification x 130.

up to 3.1 pmlday for cellular intrinsic fiber ce-mentum (2811, and, presumably, in most other mam-mals.

The width of the precementum layer in the hu-man is about 3-5 pm (74, 173). The mineral crystalsreach mature size similar to mineral crystals in boneand dentin within 1 to 4 pm from the calcificationfront (173).Thus, it appears that the processes of es-tablishing the appropriate condition for crystalliza-tion as well as for the appositional growth of the in-dividual crystals in cementum normally are ex-tremely slow and extend over a period of severalmonths.

The distribution of mineral within the maturetissue shows a great deal of variability. Studies by mi-croradiography, using a technique that basically re-flects the distribution of calcium in the tissue, haveshown that cellular mixed stratified cementum gen-erally has a lower mineral content than acellular ex-trinsic fiber cementum. This difference can in partbe accounted for by the nonmineralized structurespresent in cellular intrinsic fiber cementum. Thesemay include cementocyte lacunae as well as largerinclusions of cellular elements. In addition, the Shar-peys fibers of cellular mixed stratified cementumgenerally retain an unmineralized core (Fig. 13) (61,

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173). The latter feature is in contrast to the intrinsicfibers and to the Sharpeys fibers in acellular extrin-sic fiber cementum which exhibit a more completedegree of mineralization. A similar difference in min-eralization pattern between Sharpeys fibers and in-trinsic fibers is present in the alveolar bone as well.Selvig (173) has pointed out that Sharpeys fibers arederived from periodontal fibers, which are not calci-fiable in their original location and that, therefore,these fibers will calcify after they have become em-bedded in bone and cementum only if they have ac-quired the concentration of inorganic ions and othercomponents required for calcification, and if poss-ible inhibiting substances have been removed. This

Fig. 12. a. Crystals at the calcification front of human ce-mentum are plate-like structures that appear as fine, elec-tron-dense threads when standing on edge. Crystals lyingflat in the plane of the section exhibit lower electron den-sity and are barely identifiable (arrows). The characteristiccross-banding pattern of the collagen fibrils inserting inthe cementum can be seen. The crystals are generallyoriented with their long axis parallel to the direction ofthe collagen fibrils, which course horizontally in the fig-ure. b. Thin section parallel to the root surface at the calci-fication front illustrates cross-sectioned collagen fibrils(circular images) and individual mineral crystals (fine,electron-dense structures). As cementum deposition ad-vances, crystals are deposited, first, within collagen fibrils(left) and, later, between the fibrils to form the completelycalcified tissue (right). c. Crystals in deep layer of ce-mentum. Great variations in length and width among thecrystals of low electron density are evident. Their thick-ness, as revealed by the crystals standing on edge, showsless variation. The organic matrix has been extracted, inorder to enhance electron contrast of the mineral com-ponent. Transmission electron micrographs. Originalmagnification: a, c: x 120,000; b: x 60,000.

exchange would be less complete in regions wherehard tissue formation progresses at a more rapidrate, such as during formation of alveolar bone orcellular intrinsic fiber cementum. More recent dataseem to indicate that these fibers have a coating oftype I11 collagen, which may prevent mineralizationof the type I collagen in the core (218).

Although additional cementum is laid downthroughout life, the mineral content of this tissue,once formed, does not seem to change significantlywith age (136, 170, 197). This is in contrast to rootdentin, which increases in mineral content and roottransparency with age by obliteration of the dentinaltubules.

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Fig. 13. a. Microradiograph illustrating uncalcified cores exhibits an irregularly shaped, uncalcified core, sur-of Sharpeys fibers (dark, radially oriented structures) and rounded by an electron-dense peripheral part. The fiberslacunae in cellular cementum. b. Thin section prepared in this section are 10pm or less in diameter. Transmissionparallel to the root surface of cellular cementum, illustrat- electron micrograph. DCJ: dentinocemental junction.ing Sharpeys fibers in cross section. Each Sharpeys fiber Original magnification: a: X200; b: x4000.

BiochemistrySince cementum is not a uniform, mineralized con-nective tissue, differences in the proportional com-position of the chemical constituents exist betweenthe various cementum varieties. Thus, the percen-tages of its chemical components may vary fromsample to sample, particularly so when they orig-inate from different species. Biochemical studieshave shown that cementum has a chemical com-position similar to bone. To about equal parts pervolume, cementum is composed of water, organicmatrix and mineral. About 50% of the dry mass isinorganic and consists of hydroxyapatite crystals.The remaining organic matrix contains largely col-lagens and to a lesser degree mainly glycoproteinsand proteoglycans.

Organic matrixCollagens.The organic matrix of cementum consistsprimarily of collagens. Like in bone and periodontal

ligament, the two typical fibril-forming collagenstype I and I11 are also found in cementum. Biochem-ical analyses of bovine (26, 27) and human (48) ce-mentum have revealed that approximately 90% ofthe organic matrix is type I collagen and a minorproportion of approximately 5%accounts for type I11collagen. It has been suggested that the collagentype I fibrils are coated by type I11 collagen (218).Onthe other hand, immunocytochemical and biochem-ical (14, 83,94 , 97, 101, 137) as well as in v i m studies(108),using different tissues, have shown that colla-gen type I is apparently co-localized with collagen/procollagen TVpe I11 in the same fibril rather thansurrounded by it. The collagens are composed ofthree polypeptide alpha chains coiled around eachother to form the classic triple helix configuration.The procollagen molecules are secreted and aggre-gate extracellularly to form cross-striated collagenfibrils with the typical 67 nm banding pattern. Thisstriking banding pattern stands out in electronmicrographs and is partly obscured when the colla-genous matrix is mineralized.

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Bosshardt & SeluigIn most tissues, the collagens play important

structural and morphogenic roles (93). In mineral-ized tissues, they provide also a scaffold for the min-eral crystals (49). Banded collagen fibrils are fre-quently observed in membrane-bounded compart-ments within the cementoblast cytoplasm (32).Theirrole is still a controversial subject (32, 109, 124).These compartments appear to be continuous withthe extracellular space and serve to position newlyproduced fibril segments to already existing fibrilbundles (22-25). However, in most studies, primarilyconcerning fibroblasts from the periodontal liga-ment and the gingiva, these membrane-boundedcollagen fibrils have been associated with enzymaticcollagen degradation (57, 69, 77, 78, 162, 180, 203,205,206,229),whereas only a few authors associatedthem with polymerization and secretion of collagen(43-45, 79).Noncollagenous proteins. Cementum is rich in gly-coconjugates, which represent either glycolipids, gly-coproteins or proteoglycans, and harbors a variety ofother proteins.

Like in bone, the predominant noncollagenousproteins are bone sialoprotein and osteopontin (seealso: Differentiation of cementoblasts). Both arephosphorylated (70, 150) and sulfated (62, 133) gly-coproteins. They bind tightly to collagenous matri-ces and hydroxyapatite, participate in the mineral-ization process and reveal cell attachment propertiesthrough the tripeptide sequence Arg-Gly-Asp (RGD)that binds to integrins (42, 65, 189). Since their defi-nite roles in the mineralization process are notknown, their concerted functions in mineralizationare currently being investigated by several researchgroups. As revealed by immunocytochemistry, acel-lular afibrillar cementum and acellular extrinsic fibercementum appear to contain much more of thesetwo glycoproteins than cellular intrinsic fiber ce-mentum (37). The structural organization and themuch slower rate of formation may possibly accountfor the higher immunoreaction and the higher de-gree of mineralization found in acellular extrinsicfiber cementum.

Osteonectin is another glycosylated protein foundin the extracellular matrix of mineralized tissues. Asshown in bone, a close relationship between os-teonectin and collagen seems to exist in the mineral-ization process (60, 64, 157, 208). An immunohisto-chemical study of human cementum showed thatosteonectin is expressed by acellular extrinsic fibercementum- and cellular intrinsic fiber cementum-producing cementoblasts and cementocytes,

whereas the reaction in both cementum varietieswas negative (153).

The two glycoproteins fibronectin (98) and tenas-cin are more widely distributed, high-molecular-weight and multifunctional proteins of the extra-cellular matrix. One function of fibronectin is to bindcells to components of the extracellular matrix. Dur-ing tooth development, fibronectin and tenascin arepresent in the basement membrane of Hertwigs rootsheath at the time of odontoblast differentiation(111,131, 210, 211, 213). Later in development, theyare also found at the attachment site of the peri-odontal ligament to the cementum surface but notin the cementum layer itself (117).

Enamel-related proteins have been detected in ce-mentum by immunohistochemistry and biochem-ical analysis (see: Differentiation of cementoblasts).Since the presence of these proteins in cementum iscontroversial, their functions in cementogenesisawait further clarification by high-resolutionimmunodetection.

The proteoglycans, which are widely distributed inmammalian tissues, consist of a core protein towhich sulfated polysaccharides (that is, glycosamin-oglycans) are covalently linked. Their functions inthe extracellular matrix are manifold. The proteo-glycans of cementum are small proteins (125). Bio-chemical analyses of extracts from human ce-mentum have identified chondroitin sulfate, derma-tan sulfate and hyaluronic acid as the glycosaminog-lycan constituents of these proteoglycans (11).Osteocalcin, a very small protein found in abun-dance in the extracellular matrices of bone, dentinand cementum, appears to be involved in the min-eralization process (92). An immunohistochemicalstudy of rat molars has shown that cellular intrinsicfiber cementum and associated cementoblasts andcementocytes stained for osteocalcin, whereas acel-lular extrinsic fiber cementum and its associated ce-mentoblasts did not (39). Another immunohisto-chemical study of the rat molar (2071, however,found that acellular extrinsic fiber cementum butnot the associated cementoblasts stained positivelyfor osteocalcin, whereas cellular intrinsic fiber ce-mentum and its associated cementoblasts showedmoderate and weak staining, respectively. Althoughthese results are controversial, in both studies aphenotypic difference between cementoblasts hasbeen suggested, with the cellular intrinsic fiber ce-mentum-producing cementoblasts and cemento-cytes expressing a more osteoblast-like phenotype.

The enzyme alkaline phosphatase is believed toparticipate in cementum mineralization (16). Super-

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Human cementumsaturation of phosphate ions, released from organicphosphate esters, would result in the precipitationof calcium phosphate salts (15, 17, 18). Although al-kaline phosphatase exists in a plasma membrane-bound form, part of the enzyme may also be boundto the extracellular matrix (89). In rat molars, the en-zyme is heterogeneously distributed in the peri-odontal ligament, with the highest activity beingfound adjacent to alveolar bone and cementum (88).The enzyme activity adjacent to cellular intrinsicfiber cementum is higher than that to acellular ex-trinsic fiber cementum, and the thickness of the lat-ter correlates positively with the enzyme activity(88).

Mineral componentThe composition of dental cementum has not beenstudied to the same extent as that of other mineral-ized tissues. Cementum is generally less mineralizedthan root dentin from the same teeth (50, 139, 170),although this may not be without exception (72).Acellular extrinsic fiber cementum appears morehighly mineralized than cellular intrinsic fiber ce-mentum and cellular mixed stratified cementum(195).The difference can in part be explained by thepresence of uncalcified spaces, such as lacunae andby the uncalcified core of Sharpeys fibers. In ad-dition, the matrix of acellular extrinsic fiber ce-mentum may be more completely mineralized be-cause its formation is a slow process that allowslonger direct contact of tissue fluids (195). Rockert(156) examined cementum of monkey teeth byquantitative X-ray microscopy and found that theconcentration of Ca varied within a wide range, from0.10 to 0.83 mg Ca/mm3. This variation confirmsthat cementum is not a completely mineralized or ahomogeneously mineralized tissue.

The region of the dentinocemental junction maydemonstrate a variable appearance but commonlyshows a zone of high mineral content and low or-ganic content delineated by zones of low mineralcontent on the dentin side and sometimes on thecementum side as well (2, 61, 73, 155, 223).

Chemical analysis and physicochemical studiesindicate that the mineral component is the same asin other calcified tissues; that is, hydroxyapatite (Ca-,,(P04)6(OH),), with small amounts of amorphouscalcium phosphates present. Transmission electronmicroscopy and electron diffraction analyses haveconfirmed that the mineral crystals are arrangedwith their crystallographic c-axis parallel to the longaxis of the collagen fibril with which they are associ-

ated (173) (Fig. 12). Such studies also indicate thatthe crystallinity of the mineral component is lowerin cementum than in the other hard tissues. Theminute size of the mineral crystals compared withenamel results in a much larger specific surface areaof the mineral component. As a consequence, ce-mentum has a greater capacity for adsorption offluoride and other elements over time but also morereadily decalcifies in the presence of acidic con-ditions.

As in other hard tissues, the hydroxyapatite of ce-mentum is not chemically pure but contains otherelements that have been taken up from the tissuefluid during initial crystallization. The amount ofthese ions initially incorporated into the mineralphase reflects their concentration in the fluid en-vironment during mineralization. Over time, theconcentration may change by additional uptake orsubstitution by other ions.

Thus, cementum contains 0.5-0.9% magnesium(91, 136, 139). Both physicochemical considerationsand analyses of dental hard tissues indicate that theMg ion occupies the place of an equal number ofCa ions in the hydroxyapatite crystal lattice. The Mgcontent in cementum is about half that in dentin.The significance of the low Mg content remains ob-scure, but the finding is in harmony with the notionthat the composition of cementum is more similarto bone tissue than to dentin. The concentration ofMg appears to be lower at the surface than in deeperlayers of cementum (Fig. 14a).

The distribution of fluoride shows the oppositetrend (91, 132, 134, 219, 232) (Fig. 14b). Cementumappears to have a high fluoride content comparedwith other hard tissues. Concentrations up to 0.9%ash weight have been reported, but some authorshave found considerably lower values (40, 82, 134,135, 185, 198, 230, 232). The ash content of ce-mentum is 45-65% of the dry weight. Previously re-ported values must therefore be reduced corre-spondingly in order to arrive a t the dry weight (136).Fluoride concentration in cementum shows a gen-eral increase with age and varies with the nutritionalfluoride supply to the individual (134, 136, 219, 233).An increase in fluoride concentration with increasedfluoride exposure is found in bone, dentin and en-amel as well (40, 81, 100, 219).

Cementum contains 0.1-0.3% sulfur as a constitu-ent of the organic matrix. Sulfur shows a more evendistribution than the inorganic components and doesnot exhibit consistent variation trends within the nor-mal tissue (91) (Fig. 14b). This is consistent with theobservation by microradiography of calcified and de-

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aCalcium

i l lhosphorus1 1agnesiumRoot surface DCJcalciumJRoot surface Sulfur DC J

Fig. 14. 'Isrpical electron microprobe scan through normalcementum from the surface to the dentinocemental junc-tion. a. The concentration of phosphorus closely parallelsthat of calcium. Magnesium exhibits lower concentrationnear the root surface than in the deep layer of the ce-mentum and the underlying root dentin. b. Fluoride ex-hibits a distinct concentrationgradient. Sulfur, which illus-trates the distribution of the organic matrix, shows lessvariation in concentrationthan the inorganic components.

calcified sections that incremental lines do not corre-spond to variations in organic content (2).

A number of trace elements may also be presentin normal cementum in concentrations detectableby electron microprobe analysis, in particular Cu, Znand Na (12, 40); however, their distribution and sig-nificance do not seem to have been studied in anydetail.

Metabolism (turnover) at the tissueand molecular levelsAlthough dental cementum is usually referred to asa bone-like tissue, there are obvious differences in

regard to vascularity, cellular components, and rateof turnover. Bone tissue, including alveolar bone,participates actively in the metabolism of the bodyand constitutes a reservoir of calcium and other ele-ments that the body can draw upon as needed. Thecementum is largely excluded from such processes.

A variety of noncollagenous proteins are stored inthe mineralized matrix of cementum (see: Organicmatrix). Whereas most of these extracellular proteinsare typical matrix constituents of collagen-basedmineralized tissues, others appear specific for ce-mentum. Among these is a cementum-derivedattachment protein that mediates the attachment ofconnective tissue cells (128, 142, 148). Some data in-dicate that this protein is different from both bonesialoprotein and osteopontin. On the other hand, thepossibility cannot be ruled out that this protein mayrepresent a degradation product or a posttransla-tional variant of known proteins. Another proteinbelieved to be specific for cementum is a ce-mentum-derived growth factor (231).During root re-sorption and surgical instrumentation, proteins ex-posed to the root surface and/or released from ce-mentum and dentin could possibly influence theinitiation of the repair process by cell migration, di-vision, attachment and differentiation.

Fluoride accumulates in the surface layer of ce-mentum, which is exposed to the circulating tissuefluids in the periodontal ligament (134, 200). Sincethe F ion reacts aggressively with hydroxyapatite,fluoride concentrates near the surface and showslimited diffusion into deeper layers of the tissue.Thus, the mean fluoride content of the fine layer ofcervical cementum is higher than that of the thickerapical cementum (40, 198).As a consequence of itslongevity, cementum appears to be the most fluor-ide-rich tissue of the body (82, 185,232). By contrast,the bone tissue facing the periodontal ligament isconstantly being remodeled and, therefore, has nochance to accumulate the same amount of fluoride.For the same reason, the fluoride concentration ofalveolar bone is lower than that of the cortical lamel-lae of the maxilla and mandible (232) and of mostother bones in the body. The relatively high fluoridecontent of the surface layer compared with deeperlayers of cementum and root dentin may help ex-plain why any root resorption tends to be of an un-dermining character (Fig. 15). Root caries as wellseems to progress in a similar pattern.

The two opposing hard tissues, dental cementumand alveolar bone, are strikingly different in histo-logical structure, physiological activity, biochemicalcomposition, as well as in age and reactivity of their

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Fig. 15. Root resorption following application of excessiveorthodontic force. After a 4-month remission period, thelesion has been partially repaired by cellular intrinsic fib-er cementum. Photomicrograph. Original magnificationX 150.

organic and inorganic components. These differ-ences may explain why cementum, to a large extent,is excluded from the normal metabolism of the bodyand, consequently, remains unaffected by manypathological conditions which involve bone tissue.

Age changesContinuous depositionCementum formation on the roots of human teethcontinues throughout life unless disturbed by peri-apical or periodontal pathology. When mean valuesof cementum thickness are computed for a largesample, it appears that cementum is deposited at alinear rate (9, 236). More cementum is formed apic-ally than cervically. In addition, cementum thicknessshows characteristic variations among tooth groupsand tooth surfaces (33, 190). There is a tendency forcementum to reduce root surface concavities. Thus,thicker layers of cementum may form in root surfacegrooves and in the furcations of multirooted teeth.Also, great variations in width of incremental layersindicate that the rate of cementum formation mayvary from time to time. The reasons for these vari-ations are not completely clear. However, changes intooth position may exert temporal and spatial vari-ations in pressure and tension on root and bone sur-faces. The biological responsiveness of cemento-blasts to these stimuli may influence the rate as wellas pattern of cementum deposition. This regulatory

mechanism would, in turn, maintain the tooth in itsproper position in relation to its antagonistic andneighboring teeth.

Nonfunctioning, impacted teeth generally appearto have thicker cementum than functioning teeth(91, and the structural architecture is different. In thecementum of impacted teeth, Sharpeys fibers maybe nearly completely absent, and the cementum isbuilt up mainly by intrinsic fibers arranged parallelto the root surface (99) (Fig. 11).In the periodontalligament of such teeth as well, the fiber arrangementmay be predominantly parallel to the root surface.

Physiological activity of cementocytesDeposition of cellular intrinsic fiber cementum ischaracterized by the entrapment of cementoblasts asthey become surrounded by the matrix which theyhave formed. It appears that the number of cells thatbecome incorporated is proportional to the rate ofcementum deposition (29, 67, 146). The density ofcells in cellular intrinsic fiber cementum on humanteeth is, however, much lower than in bone tissue.Also, the system of interconnecting canaliculi thatmight serve to maintain nutritional supply and cellcontacts is more sparse. Cementocytes close to thecementum surface may resemble cementoblasts;however, the amount of cytoplasm is reduced andthey contain less endoplasmic reticulum and fewermitochondria (72,74) (Fig. 16). Characteristically, hemost well-developed cell processes of the ce-mentocytes point toward the root surface (167, 176)(Fig. 17). These observations indicate that the ex-change of metabolites through cellular intrinsic fibercementum is limited. In deeper layers of cellular in-trinsic fiber cementum, more advanced nuclear andcytoplasmic changes may occur (72,74),or the lacu-nae may appear empty. Whether the eventual celldeath is due to starvation or is a consequence of ageis, however, not known.

Although early studies of root permeability haveindicated that the dentinocemental junction repre-sents a barrier against permeation of substances ex-perimentally applied to the root surface, Erasquin &Muruzabal(63) have observed necrosis of cells in thedeep layers of cementum after root canal treatmentin the molar teeth of rats.

Cementum reactions to physiological toothmovement and occlusal forcesThe distribution of cementum on impacted teethtends to indicate that occlusal forces are not

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Fig. 16. Electron micrograph of cementocyte located ap-proximately 30 pm from the cementum surface. Note thepaucity of organelles.A pyknotic nucleus (N), a few mito-chondria (M ) and traces of endoplasmic reticulum (ER)can be seen. The cytoplasm is filled with a structurelessmaterial. The lacunar wall is lined by scattered nonmin-eralized collagen fibrils (CF). Source: Furseth (74) withpermission from the publisher. Original magnificationX 14,000.

necessary to stimulate cementum deposition. Inposterior teeth in the human, cementum is mark-edly thicker on the distal than on the mesial rootsurface, indicating a relationship to mesial drift(56). It has been suggested that cementum isthicker in areas exposed to tensional forces on la-bial and lingual surfaces of incisors (84, 167). Thedeposition of considerably more new cementumhas been noted on the tension side compared withthe pressure side of the root surface of teeth un-dergoing orthodontic tooth movement in rhesusmonkeys (149). This finding correlates with the ob-servation of appositional layers of bone lining thedistal wall of alveolar sockets, and indicates thatcementum, like bone tissue, has the potential tobe dynamically responsive and that its growth maybe stimulated by tensional forces (56).

%es of resorptionAlthough physiological root resorption is a normalphenomenon of deciduous teeth during tooth shed-ding, permanent teeth do not undergo physiologicalresorption. A variety of other factors, however, caninduce root resorption on teeth of either dentition.These factors can be either pathological or not. Inthe former case, infectious and systemic diseases aswell as tumors may cause root resorption. Undernonpathological circumstances, trauma (eithermechanical, chemical or thermal) or sustained over-compression of the periodontal ligament can resultin the resorption of cementum and dentin. In thevast majority of cases, however, idiopathic resorp-tion does occur (127). Root resorption can be furtherclassified by location into internal and external andby the degree of persistence into transient or pro-gressive.

Although it is widely accepted that the root sur-face is more resistant to resorption than alveolarbone, it is also known that the number of teeth re-sorbed and the severity of resorption are markedlyincreased by orthodontic treatment (127). However,the frequency of resorptive defects is likely to behigher than generally believed, since very superficialresorptions are too small to be detected radiographi-cally. In most cases, however, these resorptions arereversible and therefore of minor clinical signifi-cance. When the resorptive activity of odontoclastshas ceased and the stimulus for new odontoclast re-cruitment disappears, they become filled by repaircementum.

RepairThe non-pathologically resorbed root (Fig. 18) is aparticularly good model for studying the repair pro-cess and the adaptation of the adjacent periodontalligament, because complications with an infectiousdisease process do not exist. It can be assumed thatthe origin and differentiation mechanism of the cellsinvolved in the repair process do not differ in thesetwo situations.

Morphological studies have shown that two dif-ferent repair matrices become attached to the re-sorbed root surface (35, 36). Following the detach-ment of odontoclasts from the root surface, ce-mentogenic cells repopulate the Howships lacunae(Fig. 18a) and attach the initial repair matrix to athin decalcified layer of residual and exposed col-

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Fig. 17. Cementocyte lacunae and canaliculi as seen inground section. a. The cells incorporated in cellular ce-mentum are more widely spaced than in bone tissue andexhibit few intercellular connections. Most cell processespoint toward the cementum surface (top).Out-of-focus a-cunae appear as dark dots. b. Detail illustrating canaliculi

extending from two cementocyte lacunae toward the ce-mentum surface which is just outside the top of thephotomicrograph. c. Ground section prepared parallel tothe cementum surface. In this projection the lacunaehave a symmetrical appearance. Original magnification:a: x 150; b, c: x400.

lagen fibrils. These cells and their respective repairtissues reveal remarkable homologies to the initialgenesis of the two major cementum varieties (thatis, acellular extrinsic fiber cementum and cellularintrinsic fiber cementum) on growing human roots(Fig. 18b,e,f and c,g, respectively). In analogy to theformation of the genuine dentinocemental junc-tion, the interdigitation of the newly formed colla-gen fibrils with the residual dentinal matrix fibrilsoccurs before (Fig. 18e,g) the new attachment sitebecomes obscured by electron-dense material (Fig.18h) the globular accumulation of which is indica-tive of mineralization. Eventually, a basophilic andelectron-dense reversal line forms at the fibrillarjunction (Fig. 18d). Subsequently deposited repairmatrix usually resembles cellular intrinsic fiber ce-mentum formed on nonresorbed roots (Fig. 18d).Since the cement line found between old and newbone appears to be laid down before the colla-genous matrix of new bone is deposited (10, 130,2371, the sequential steps of initial bone and rootrepair may be different.

The strong resemblance of the initial formation ofthe two repair matrices with the initiation of acellul-ar extrinsic fiber cementum and cellular intrinsicfiber cementum on the forming root indicates thatrepair cementogenesis recapitulates the events oc-curring during root development, a notion that is in

line with the views of Aukhil(7) and MacNeil &Som-erman (119) but in contrast with the concept holdby Pitaru et al. (147). However, since the precise ori-gin of the cementoprogenitor cells and the molecu-lar factors that trigger their differentiation are notknown, future studies are still imperative to shedlight into the cell dynamics occurring during root re-pair.

Alterations resultingfrom periodontal pathologyEffect of gingival inflammationSubsurface alterations. Cementum may undergoalterations in structure as well as in the compositionof its organic and inorganic components consequen-tial to pathological changes in the immediate en-vironment. Several in-depth reviews of this subjecthave been presented (5,54,80). The cementum mayalso become affected by pulpal pathology and rootsurface caries; however, these processes are not dis-cussed here.

The longstanding presence of an inflammatoryprocess in the gingival connective tissue results ina net loss of collagen and in breakdown of dento-gingival fibers. Although the enzymatic breakdown

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Human cementumof collagen fibers is obvious in the gingival softtissue, the extension of this process into the hardtissue of the root, with loss of collagen cross-band-ing and dissolution of mineral crystals, has alsobeen described (174, 175). This process, however,is rather surface-limited with a diffuse transition tosubjacent unaffected tissue, which explains why ithas been detected and described only by electronmicroscopy.Cervical root resorption. The development of largeroot resorption defects in the cervical region is, mostlikely, also triggered by inflammatory processes inthe adjacent connective tissue. Most frequently, cer-vical resorption is seen in cases of hyperplastic gingi-vitis (152). Such resorption generally has an under-mining character.

It has not been explained why alveolar boneoften is resorbed before the teeth, as is the case inapical and marginal periodontitis, as well as in or-thodontic tooth movement. For the lack of moreplausible alternatives, the epithelial rests of Malas-sez have been ascribed a protective function (116,143, 196, 217). More commonly, immunity to re-sorption has been linked to the presence of an un-calcified, vital layer of precementum on the rootsurface (85, 181). This appears to be an attractive,although somewhat simplistic, explanation basedupon the observation that, on the periodontal sur-face of the alveolar bone, resorption occurs at siteswhere the bone surface is not covered by osteoid,that is, where bone apposition has ended or whereinflammatory breakdown of adjacent collagenfibers has occurred. Studies by Lindskog et al.(115) indicate that the cells lining the root surface

do not respond to parathyroid hormone as do cellslining bone, thus reinforcing previous studies sug-gesting a protective function for these cells againstroot resorption (114). One plausible explanationfor a delay in root resorption is the fact that ce-mentum is not vascularized. As the odontoclastsmost likely take their origin from the bone marrow,these cells cannot attack the root surface as fast asthe osteoclasts reach the bone surface.

Some authors have pointed out that resorptionof alveolar bone often occurs adjacent to the ori-fice of vascular canals where branches of the inter-alveolar vessels enter the periodontal ligament, in-dicating an association with circulatory disturb-ances (152). The latter hypothesis is supported bythe observation that root resorption followingclearing-up of a hyalinized zone consequent to theapplication of excessive orthodontic force is oftenassociated with the ingrowth of a large blood ves-sel close to the root surface (152, and our own un-published material).

Exposure to the oral environmentBacterial contamination. More obvious alterationsmay occur following exposure of the cementumsurface to the environment of the periodontalpocket or the oral cavity. This includes the devel-opment of overt cementum caries, which is notdescribed here. Even the noncarious cementum is,however, sufficiently porous to allow permeation oforganic substances derived from saliva or bacterialplaque, as well as of inorganic ions. Permeation ofparticulate matter, including bacteria, is facilitatedby the occurrence of minifractures and cracks,

Fig. 18. Light (a-d) and electron (e-h) micrographs illus-trating the sequences of repair following the resorptionof human premolar roots. h o different repair matricesbecome attached to the resorbed root surface, one resem-bling acellular extrinsic fiber cementum (b, e, f) and theother one resembling cellular intrinsic fiber cementum (c,g). a. The resorption has penetrated deeply into the dentin(D). The profile of Howships lacunae is seen. b, c. Theinitial repair matrix consists of either a short fiber fringe(FF in b) or a thin layer of cementoid (arrowheads in c).Note that the attachment of the repair matrix can occurside-by-side with odontoclasts (OC in c). d. The bulk ofthe resorptive defect is usually filled with cellular intrinsicfiber cementum (CIFC). Note that a reversal line (RL) sseen at the junction between dentin and cellular intrinsicfiber cementum. e. Cementoblasts (CB) attach the colla-

gen fibrils of the fiber fringe matrix to a thin residual seamof exposed dentinal matrix (star). f. Further developmentof this type of initial repair matrix results in a dense colla-genous fiber fringe resembling the matrix of acellular ex-trinsic fiber cementum. Note that there is no reversal lineseen interfacing the new fibrillar junction. g. Large ce-mentoblasts attach a thin layer of cementoid on the re-sorption lacuna. Note that there is no reversal line seenat the new junction (arrowheads). h. A fine granular andelectron-dense material progressively obscures the fibril-lar junction when the initial repair matrix attachment isestablished on the resorbed root surface. Sources: d-fiBosshardt (35); h: Bosshardt & Schroeder (36) with per-mission from the publisher. Original magnification: a, dx130; b, c: ~ 4 4 0 ; : ~10,000; : X3000; g: X4000; hX 17,500.

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Fig. 19. Microradiograph of a longitudinal section througha periodontally diseased tooth showing hypermineralizedcementum. The highly radiopaque surface zone decreasesin thickness apically and disappears underneath the calci-fied concrement.C: cementum;(2%alculus;D:dentin;E:enamel. Original magnification x 100.

which seem to develop frequently in the exposedcementum. Early observations of histological alter-ations of periodontally involved cementum in-cluded the appearance of pathological granules orvacuoles (3, 4, 13, 20) and the presence of lipidgranules or strong periodic acid-Schiff staining,polysaccharide staining extending 3-12 pm intothe surface of cementum from overlying plaque(55). Armitage & Christie (4) and Armitage (5) con-cluded that the cemental granules representedareas of denatured cemental collagen which havepicked up unidentified substances from the oralenvironment. This hypothesis is supported by thesubsequent observation that similar granules werepresent near the dentinocemental junction in teethwith heavily infected root canals (6).

Bacterial invasion into cementum and root den-tin is a common sequela to chronic periodontddisease (55, 235). Adriaens et al. (1) were able togrow anaerobic bacteria from samples of root den-tin from 87% of periodontally diseased teeth andsuggested that cementum and root dentin mayserve as reservoirs from which recolonization ofmechanically debrided root surfaces can occur, as

well as infection of the dental pulp. More recently,several fatty acids indicating the presence of bac-terial lipopolysaccharide have been detected in the40- to 70-pm-deep surface layer of periodontallydiseased roots (118). Experimental studies in uitro,albeit of short duration, seem to confirm that en-dotoxin does penetrate into cementum but that itsbinding to the cementum surface is weak (138).Hypermineralization. The presence of a highly min-eralized surface layer in the cementum following ex-posure to the external environment has frequentlybeen detected by microradiography (68, 73, 76, 171,177, 224), chemical analysis (136, 197, 198, 221),electron microprobe analysis (178) and nuclear res-onance reaction analysis (52, 160) (Fig. 19). Somestudies have failed to demonstrate a hypermineral-ized surface zone (12, 50). The development of ahypermineralized zone apparently depends on theionic concentration of inorganic elements in the lo-cal environment. Thus, this zone may be more orless generally present on the exposed root surface ormay be completely absent (171). Furseth (75) foundthat healthy cementum experimentally exposed tothe oral environment by a gingivectomy procedureacquired a hypermineralized surface zone within 2 1days. This process could be greatly enhanced bytreating the root surface with a 2% solution of so-dium fluoride for 10 minutes. Similarly, if the originalsurface layer of cementum is removed by root plan-ing, a hypermineralized zone may be re-establishedwithin 4 to 8 weeks (177).

Ultrastructurally, the hypermineralized surfacezone is characterized by the presence of large, atypi-cal crystals, irregular crystal orientation indicative ofdemineralization and remineralization processes,and loss of characteristic collagen cross banding (76,177, 178, 222).

The cementum of periodontally involved teeth,and in particular the hypermineralized surface zone,is also characterized by an increased fluoride con-tent (160, 178,200, 221, 230). This may explain whythe mineral crystals in this zone are extremely resis-tant to acid demineralization in uitro (76). Presum-ably, the high fluoride content of the surface layeralso contributes to the subsurface and underminingcharacter of the demineralization process in ce-mentum caries. Moreover, translocation of mineralions during the caries process may result in the de-velopment of a more densely mineralized surfacezone in the early cementum caries lesion than in theadjacent exposed, noncarious cementum surface(73).


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ConclusionThe periodontal tissues form a functional unit de-signed to maintain tooth support and protection. Inparticular, cementum, by virtue of its structural anddynamic qualities, provides tooth attachment andmaintenance of occlusal relationship. These multiplefunctions are fulfilled by the biological activity andreactivity of cementoblasts, which deposit two colla-gen-containing varieties of cementum with com-pletely different properties. Acellular extrinsic fibercementum, which grows very slowly on the cervicaland middle portions of the root, mainly participatesin tooth anchorage. Cellular intrinsic fiber ce-mentum, on the other hand, may be found eitheralone as a radicular repair tissue at various locations,or in combination with acellular extrinsic fiber ce-mentum as cellular mixed stratified cementum onthe apical root portion and in root grooves and fur-cations. The ability of cellular intrinsic fiber ce-mentum to be rapidly deposited in thick layers dem-onstrates its important function as an adaptive andreparative tissue. Unless disturbed, the cementumcovering of the root increases in thickness through-out life, albeit at a faster rate apically than cervically.Other parameters that less consistently may influ-ence shape and chemical composition of the rootsurface over time include changes in tooth position,resorption and repair, surface exposure to the en-vironment,

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Cementum as a Dyanamic Tissue