Periodontology 2000, Vol. 24, 2000, 28–55 Copyright C Munksgaard 2000Printed in Denmark ¡ All rights reserved

PERIODONTOLOGY 2000ISSN 0906-6713

Molecular and cell biologyof the gingivaP. MARK BARTOLD, LAURENCE J. WALSH & A. SAMPATH NARAYANAN

The healthy periodontium provides the supportnecessary to maintain teeth in adequate function. Itis comprised of four principal components: namely,the gingiva, periodontal ligament, alveolar bone andcementum. Each of these periodontal componentsis distinct in its location, tissue architecture, bio-chemical and cellular composition and yet, theyfunction together as a single unit. Recent researchhas revealed that the extracellular matrix compo-nents of one periodontal compartment can influ-ence the cellular activities of adjacent structures;thus, pathological changes occurring in one peri-odontal component may have significant ramifi-cations for the maintenance, repair or regenerationof other components of the periodontium.

The gingiva, in health, normally covers the al-veolar bone and tooth root to a level just coronal tothe cementoenamel junction (Fig. 1). Anatomically,the gingiva is classified into three distinct domains;the free marginal gingiva, the interdental gingiva andthe attached gingiva (187). Histologically, it is com-posed of two distinct components – the overlyingepithelial structures and the underlying connectivetissue. While the epithelium is predominantly cellu-lar in nature, the connective tissue is less cellular

Fig. 1. Anatomic relationship of gingival tissues to theteeth and alveolar bone. Clinical landmarks include thefree gingiva at the cervical margin of the teeth (FGM); theinterdental papilla (IP); and the mucogingival junction(MGJ), which separates the attached gingiva (AG) from thealveolar mucosa (AM).

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and is composed primarily of an integrated networkof fibrous and nonfibrous proteins, growth factors,minerals, lipids and water. These two componentsare responsible for orchestrating the earliest re-sponses associated with the development of gingi-vitis and periodontitis. This chapter considers thegeneral architecture, cellular composition, biochem-ical attributes and interactive relationship betweenthe gingival epithelium and connective tissue.

Gingival structure, architecture andfunctionGingival epithelium

Terminology

The arrangement of the gingival tissues to the toothcrown and root surfaces is shown in Fig. 1. The gingi-val epithelia, which cover the underlying connectivetissues, can be loosely categorized into at least threedifferent types based on their location and compo-sition. The oral epithelium extends from the mucog-ingival junction to the tip of the gingival crest andis subdivided into the free marginal gingiva and theattached gingiva. The sulcular epithelium lines thegingival sulcus and extends from the tip of the gingi-val crest to coronal most portion of the junctionalepithelium. The junctional epithelium extends fromthe base of the gingival sulcus to an arbitrary pointapproximately 2.0 mm coronal to the alveolar bonecrest and is closely adapted to the tooth surface toform sealing and attachment functions. These threeepithelia differ ultrastructurally (192), and there aredistinct phenotypic differences in their expression ofvarious cytokeratins and cell surface markers (116,221).

Upon completion of the eruption of a tooth, thegingival tissues form a well-defined relationship tothe tooth surface and alveolar bone. The epithelialtissues attach to the tooth via an epithelial attach-

Molecular and cell biology of the gingiva

ment termed the junctional epithelium, which, inhealth, is usually located at, or coronal to, thecementoenamel junction. The gingival tissues attachto the root surface at or below the cementoenameljunction via fiber insertion into the cementum of theroot surface which lies coronal to the alveolar crest.The general dimensions of these structures havebeen described, with the junctional epithelium aver-aging 0.97 mm and the connective tissue attachmentaveraging 1.07 mm (Table 1).

Development

Due to their different anatomic locations, the variousportions of the gingival epithelium can be seen tooriginate from distinct portions of the odontogenicsequence and developing oral mucosal tissues (111,115). The nonkeratinized junctional epithelium orig-inates from the enamel organ, while the nonkera-tinized sulcular and the keratinized gingival epithel-ium originate from the oral mucosa.

The junctional epithelium is formed during thephases of tooth eruption. As the tooth erupts intothe oral cavity, the flattened cuboidal cells cover thenewly formed crown to the level of the cemento-enamel junction. These cells covering the newlyformed crown originate from ameloblasts and cellsof the stratum intermedium of the enamel organ,and are termed the reduced enamel epithelium. Asthe tooth begins to erupt, the epithelial layers cover-ing the tooth crown fuse with the oral epithelium.Following this, the crown becomes exposed to theoral cavity and the developing tooth now becomesfully transgingival. At this stage, the first evidence of

Table 1. Gingival fiber arrangements

Name Distribution

Dentogingival fibers Inserted into the root surface as Sharpey’s fibers and fan out from the root surface subjacentto the junctional epithelium and coronal to the alveolar crest into the gingival tissues

Dentoperiosteal fibers Inserted into the root surface and run over the alveolar crest and insert into the buccal lingualperiosteum

Alveolgingival fibers Arise at the alveolar crest and fan out into the free and attached gingivae

Circular and Are located coronal to the transseptal fibers and run in a circumferential or semicircularsemicircular fibers manner around the teeth

Transgingival and Closely related to the semicircular fibers. Arise in the cementum and splay through theintergingival fibers interdental septum and eventually coalesce with the semicircular fibers of the adjacent tooth

Interpapillary fibers Run in a buccolingual direction through the interdental tissue

Periosteogingival fibers Inserted into the alveolar periosteum and splay out into the attached gingiva

Intercircular fibers Located both bucally and lingualy and run in a mesiodistal manner to join circular fibers ofadjacent teeth

Transseptal fibers Arise in the cementum and traverse the interdental alveolar crest and reinsert in the cementumof the adjacent tooth

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formation of the junctional epithelium is seen. Thenewly breeched oral epithelium appears to havefused to the epithelial covering of the enamel organand forms a continuum of epithelial tissue apicallyalong the crown surface to the level of the cemento-enamel junction. At this stage the bulk of the crownis still submerged and remains covered by its re-duced enamel epithelium. With continuing tootheruption, the conversion of the reduced enamel epi-thelium into junctional epithelium continues, andthe formation of the gingival sulcus begins to be-come apparent. Because the postsecretory amelo-blasts of the reduced enamel epithelium are termin-ally differentiated, they have no capacity to divideand thus do not contribute to future generations ofjunctional epithelium cells. However, the other cellu-lar components of the reduced enamel epithelium,those of the stratum intermedium of the enamel or-gan, retain their ability to proliferate and provide theparent source of future generations of junctional epi-thelial cells. These cells form the basal cells of thejunctional epithelium and give rise to cells whichmigrate coronally towards the gingival sulcus and ex-foliate. This continual renewal process maintains thejunctional epithelium’s structure and relationship tothe tooth surface.

Structure and composition

Oral gingival epithelium. The oral epithelium is astratified squamous keratinizing epithelium com-posed of four layers, namely the stratum basale(basal layer), stratum spinosum (prickle cell layer),stratum granulosum (granular layer) and the stratum

Bartold et al.

corneum (cornified layer) (Fig. 2). On average theoral epithelium of the gingiva is between 0.2 and 0.3mm in thickness (191). The epithelial connectivetissue interface is demarcated by extensions and de-pressions forming the so-called ‘‘rete ridges’’.

The cells of the stratum basale are comparativelysmall and lie in close contact with the basementmembrane, which separates the epithelium from theunderlying connective tissue (see below). Thesecells, often referred to as keratinocytes, proliferatecontinuously to give rise to daughter cells that ma-ture into keratinizing cells and are quite distinctfrom other cells such as the Langerhans cells, mel-anocytes and Merkel cells that reside in the basallayer (see below). The cells of the stratum spinosumare differentiating epithelial cells that are quite largeand polyhedral in shape. This layer is the thickestof all the epithelial layers, with the cells joined viajunctions of the cell processes, which gives thecharacteristic appearance of the spinous layer. To-wards the superficial layers of the stratum spinosum,the cells become flattened and show signs of kera-tohyalin granules within their cytoplasm. The mostsuperficial layer is the stratum corneum, where thecells are markedly flattened, closely aligned andoften void of nuclei and organelles.

Cells, other than those of ectodermal origin, mayalso be found in the oral epithelium of the gingivaand include Langerhans cells, melanocytes and Mer-kel cells (Fig. 3). Langerhans cells are intraepithelialimmunocompetent cells that play an important role

Fig. 2. Histology of human gingival epithelium, demon- of cells in the basal layer, and the polygonal shape of cellsstrating the different cell layers: SB, stratum basale; SS, in the spinous layer. C. Toluidine blue–stained sectionstratum spinosum; SG, stratum granulosum; SC, stratum demonstrating marked rete ridges, and the presence ofcorneum. A. Low-power view (H&E-stained section). non-keratinocytes, which occur as clear cells (arrow).B. High-power view showing the cuboidal morphology

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in protective immunity and generally located in thestratum spinosum. These specialized cells processexogenous antigens and present them to antigen-specific T lymphocytes that, in turn, become acti-vated. During inflammation, quantitative and quali-tative changes to the gingival epithelial Langerhanscells may occur (147).

Melanocytes, which originate from neural crestcells (122), are found in the stratum basale of thegingival oral epithelium (11). These cells have longdendritic processes that are found interspersed be-tween the keratinocytes of the epithelium and pro-duce considerable amounts of melanin, which canproduce a brownish pigmentation of the gingiva.The role of melanocytes and melanin in gingival epi-thelium is obscure since this site is not normally ex-posed to ultraviolet radiation.

Merkel cells are located in clusters at the tips ofrete ridges of gingival oral epithelium (170). Theirorigin remains controversial, with reports suggestingorigins from either neural crest cells or epithelialsources (131, 146). These cells form close associ-ations with intraepithelial nerve endings, formingthe epidermal Merkel cell–neurite complexes (146,170, 237). While it is generally accepted that thesecomplexes are involved in mechano-perception (81),this has been disputed (51).

Oral sulcular epithelium. The sulcular epithelium isthe epithelial lining of the gingival sulcus, which inhealth is a small crevice of approximately 0.5 mm

Molecular and cell biology of the gingiva

Fig. 3. Non-keratinocytes in human gingival epithelium. giva. Faint deposits of melanin pigment can be seen in theA. Langerhans cells (CD1a positive) are predominantly su- adjacent keratinocytes. D. Merkel cells (which are CD56pra-basal in location. Many cross sections of dendrites are positive) are present in the stratum basale of the palatalvisible. B. High-power view of gingival Langerhans cells gingiva. The cell processes insinuate between nearbyshowing cell bodies and dendrites. C. Pigment-containing keratinocytes.melanocyte in the supra-basal epithelial layer of the gin-

in depth running circumferentially around the teethbetween the tooth surface and the gingival margin.The base of the gingival sulcus is where the cells ofthe junctional epithelium are exfoliated. The cellularstructure and composition of the sulcular epitheliumis very similar to the oral gingival epithelium, beingmultilayered and often parakeratinized. In health,the epithelial connective tissue interface is demar-cated by rete pegs, and with developing inflam-mation they become elongated.

In contrast to the junctional epithelium, the sulcu-lar epithelium is not heavily infiltrated by polymor-phonuclear leukocytes, and it appears to be less per-meable.

Junctional epithelium. The junctional epitheliumforms the tissue attachment of the gingiva to toothstructures. It differs from the gingival oral epitheliumand sulcular epithelium in both its origin (see above)and its structure. This specialized epithelium rangesin thickness from a few cells at its most apical por-tion to between 15–30 at its most coronal portionadjoining the sulcular epithelium, and the cells tendto align themselves in a plane parallel to the toothsurface. Only two morphotypes of epithelial cell areevident in the junctional epithelium. The cells of thestratum basale proliferate rapidly, while those of thesuprabasale layer have no mitotic capacity. The epi-thelial connective tissue interface is not character-ized by rete ridges and tends to be straight with onlymild undulations apparent in the more coronal por-tions. Within the junctional epithelium, the gaps be-

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tween cells appear to be larger than in either the oralgingival epithelium of sulcular epithelium (190).Throughout the junctional epithelium, numerousmigrating polymorphonuclear leukocytes are evi-dent. These migrating leukocytes are present inhealth but dramatically increase in number with theaccumulation of dental plaque and are closely as-sociated with the development of gingival inflam-mation. Lymphocytes (particularly T lymphocytes)are also found with an intraepithelial distribution inthe junctional epithelium (225).

Intercellular extracellular cellular matrix

Since the epithelia of the gingiva are composed pri-marily of cells in close apposition, there is very littleextracellular space (Fig. 4). In contrast to the under-lying connective tissues, these epithelia do not con-tain any fibrous proteins in their extracellular matrix,although anchoring fibrils do form part of theattachment complex between epithelial cells of thestratum basale, the basement membrane and thesuperficial layers of the connective tissue (see be-low). In addition, type VIII collagen has been notedin the epithelial attachment apparatus of the junc-tional epithelium and may contribute to the attach-ment of this tissue to tooth surfaces (182).

The spaces between cells of the gingival epitheliavary with the junctional epithelium having thewidest gaps and presumably the greatest per-meability. The intercellular material of gingival epi-thelium has been relatively poorly studied, although

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Fig. 4. Expression of glycoproteins and adhesion mol- inin, which is a normal component of the basement mem-ecules by keratinocytes. A. Syndecan-1. B. HLA-DR. C. La- brane of the epithelium as well as the blood vascular andminin receptor (alpha-6 integrin subunit; CD49f). D. Lam- neural networks.

it is recognized that it does contain a variety of gly-coproteins, lipids, water and proteoglycans and ex-tensions of intercalated cell surface molecules (20).Of these, the proteoglycans have probably been thebest studied, with hyaluronan, decorin, syndecanand CD-44 being identified on epithelial cell surfacesand within the intercellular spaces (71, 216). In ad-dition, numerous cell surface glycoproteins involvedin cell adhesion have been identified including thoseof the b1 integrin family (a2b1, a3b1, a9b1) and in-tercellular adhesion molecule-1, which are stronglyexpressed by gingival epithelial cells (44, 103). Inter-estingly, intercellular adhesion molecule-1 is selec-tively expressed by the junctional epithelium andoral gingival epithelium, but not the oral sulcularepithelium; the significance of this is unclear. Otherintegrins, such as a6b4, are significant componentsof hemidesmosomes. This integrin is involved in theattachment of the cells to the basement membraneand participates in the transduction of signals fromthe extracellular matrix to the interior of the cell tomodulate cytoskeletal organization, proliferation,apoptosis and differentiation (29).

In general, the intercellular matrix of the gingivalepithelia, not only serves roles in cell adhesion, ad-hesion to the tooth surface and basement mem-brane but also plays a very important role in theregulation of diffusion of water, nutrients and toxicmaterials (antigens and other plaque metabolites)through the epithelium (9, 15).

Functions

By virtue of its surface coverage, the gingival epithel-ium performs a number of very important protective

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and defense functions. While the oral gingival andoral sulcular epithelia are largely protective in func-tion, the junctional epithelium serves many moreroles and is of considerable importance in regulatingtissue health. The junctional epithelium not onlyforms the epithelial attachment apparatus to thetooth surface but also provides a vehicle for the bidi-rectional movement of substances between the gin-gival connective tissue and the oral cavity. In ad-dition, the junctional epithelium plays both an in-structive and communicative role in host defenseagainst bacterial infection (155).

The permeability of the junctional epitheliumwith respect to egress of sulcular fluid and ingress offoreign particles is reasonably well established (195).However, how material enters and permeates (pass-ively) through this tissue into the gingival connectivetissue is unclear. Furthermore, with the continualpassage of leukocytes and the rapid turnover of thistissue, the logistics of permeation of substances in-wards is difficult to understand. Nonetheless, suchpermeability has been considered to be one of theprincipal events associated with the establishmentof disease. Indeed, the so called ‘‘wider’’ intercellularspaces of the junctional epithelium have alwaysbeen considered a ‘‘weak link’’ allowing permeationof bacterial products into the gingival connectivetissue and initiating an inflammatory response.

It is now recognized that epithelial cells are notpassive bystanders in the periodontal tissues, butrather are metabolically active and capable of re-acting to external stimuli by synthesizing a numberof cytokines, adhesion molecules, growth factors andenzymes. More importantly, the epithelial cells gen-erate a family of potent antimicrobial peptides for

Molecular and cell biology of the gingiva

protection against infection. These peptides, whichare cationic and called b-defensins, appear to workin concert with other host defense mechanisms tocombat multiple microbial species and form a firstline of host defence (24, 49, 50, 55, 61, 73, 74, 130,188, 193). The b-defensins identified in epithelialcells include hBD-1, hBD-2 and LL-37, and thesehave been identified in the gingival epithelium. Withthe accumulation of bacterial plaque, the epithelialcells also begin to overexpress adhesion moleculessuch as intercellular adhesion molecule-1 and cyto-kines such as interleukin-1b and interleukin-8,which are involved in neutrophil recruitment andmigration. Thus, the gingival epithelium is an im-portant initiator, regulator and mediator of the hostimmune response against periodontal pathogens.

Epithelial repair and regeneration

Epithelium has a remarkable capacity to regeneratefollowing injury. Indeed, this property of all epitheliais fundamental to cutaneous and mucosal tissues. Ir-respective of whether healing is to occur via primaryor secondary intention, the processes of re-epi-thelialization are the same.

Within hours of injury the epithelial cells, whichare ‘‘labile’’ cells, begin to migrate with the expresspurpose of covering the exposed connective tissuesurface. For both gingivectomy and incisionalwounds of the gingiva, undamaged epithelial cellsfrom the wound margins (arising from the stratumbasale) commence migration within hours of injury(58, 78) under stimuli provided by locally releasedfactors such as epidermal growth factor, platelet-de-rived growth factor-AA and platelet-derived growthfactor-AB, fibronectin and other cytokines (65, 213).These cells undergo marked phenotypic alterations(especially the basal cells), losing their desmosomesand producing cytoplasmic actin filaments necess-ary for locomotion (150). The interaction betweenthe cells and the wounded substratum is important.Normally the basal cells reside in contact with abasement membrane interspersed between the cellsand the lamina propria of the underlying tissue.Upon injury, the cells cease to produce componentsof the basement membrane and migrate over the ex-posed connective tissue surface composed of an in-terim matrix of fibrin and fibronectin. In addition,collagenase and plasminogen activator are releasedto enhance collagen remodelling and dissolution ofthe fibrin clot (42, 213). As the cells migrate, newhemidesmosomes form in association with depo-sition of a new basement membrane (100, 218). Dur-

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ing the initial phases (days 1–2) of healing of gingi-vectomy wounds, the migrating epithelial surfacecovering is only 2–3 cells thick and forms a stratumbasale. By day 5, the wound appears fully covered,and by day 7 the epithelium has matured and a newstratum corneum is usually evident (65). For openwounds or incisional wounds that have two epi-thelial edges, this process will continue until thecells from either side of the wound meet.

Epithelialization of a periodontal flap is compli-cated by the fact that there is only one epithelialedge to the wound (210). In these wounds the epi-thelium migrates apically along the root and over thewound surface. Epithelial migration will continuealong the root surface as long as there are no at-tached collagen fibers on the root surface. Apical mi-gration of the epithelium ceases as soon as suchfibers are encountered. This process is a fundamen-tal principle of periodontal wound healing and ex-plains the formation of a ‘‘long junctional epithel-ium’’. Since epithelium migrates at a much faster ratethan the formation of new connective tissue attach-ment to a debrided root surface, the epithelialattachment forms at the expense of new connectivetissue attachment. Because some new connectiveattachment will form at the base of the incision (onlybecause of the time it takes the epithelium to reachthe apical portion) the biological width (see above)is re-established. Perhaps of more fundamental im-portance is that, since some new connective tissueattachment and new cementum formation can oc-cur in the apical region of the wound, this clearlyindicates that these tissues, if given appropriate timeand environment, can regenerate. Thus, means ofexcluding or delaying rapid re-epithelialization ofthe flap wound forms an essential requirement toachieving periodontal regeneration (see section onperiodontal regeneration).

Interface between epithelium andconnective tissue

The interface between basal epithelial cells and theunderlying connective tissue is a highly specializedanatomical site termed the basement lamina (Fig. 5).It serves as a barrier to the exchange of cells and somelarge molecules across the junction. Ultrastructurally,the epithelial-connective tissue interface is composedof four elements. These have been identified as: (1)the basal cell plasma membrane with its specializedattachment devices (hemidesmosomes); (2) an elec-tron lucent region 30–50 nm in width called the lam-ina lucida; (3) an electron-dense region 30–60 nm in

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Fig. 5. Schematic representation of the electron micro-scopic appearance of basement membranes. The basalsurfaces of the epithelial cells are located superficially tothe lamina lucida which is less electron dense than theunderlying lamina densa. The reticular zone, immediatelysubjacent to the lamina densa, is a poorly defined area offibrillar and nonfibrillar matrix components that formsthe junction between the basement membrane and theunderlying connective tissue.

width called the lamina densa; and (4) a reticular layercontaining a fine band of specialized connectivetissue containing a variety of fibrillar and nonfibrillarproteins (35). The lamina lucida, the lamina densaand the anchoring fibrils are considered to be epi-thelial cell products (34, 172), while the reticular layeris of connective tissue origin.

The anchoring fibrils of the basement membranewere first noted in gingival epithelium as short curv-ing fibrils of approximately 20–40 nm thick and tra-verse the lamina densa and lamina lucida near thehemidesmosomes (128, 196, 215). These fibrils ap-pear to terminate in the connective tissue in elec-tron-dense patches termed anchoring plaques (172).Anchoring fibrils have been measured at 750 nm inlength from their epithelial end to their connectivetissue end, where they appear to form loops aroundcollagen fibers (92).

The chemical composition of most basementmembranes appears to be relatively consistent, al-though some minor quantitative differences may oc-cur depending on the location and function of thetissue. While the basement membrane of gingivalepithelium has been poorly studied, it is very likelythat it would be very similar to skin, where the majorconstituents are type IV collagen, laminin and theheparan sulfate proteoglycan perlecan. In addition,other recognized components of basement mem-branes include the hemidesmosomes, which containbullous pemphigoid antigens (110); anchoring fila-ments which contain kalinen (176) and K-laminin

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(118); anchoring fibrils which contain type VII colla-gen; entactin/nidogen, which forms complexes withlaminin; and several poorly characterized chondroi-tin sulfate and heparan sulfate proteoglycans (123,231).

Type IV collagen is a major component of base-ment membranes. It is found mainly in the laminadensa, and also within the anchoring plaques of thereticular layer. Type IV collagen has some uniquestructural features, including interrupted Gly-X-Y se-quences that allow point flexibility of the molecule,and a short globular domain at the C-terminus,where intra- and inter-molecular disulfide bondslink collagen chains together in a head-to-headmanner. These features allow collagen type IV to or-ganize into a three-dimensional meshwork (222).Gingival epithelium (oral, sulcular and junctional)express this protein ubiquitously (141, 173).

Laminin appears to be ubiquitous in basementmembranes and, in conjunction with nidogen (en-tactin), forms an important complex within the ma-trix. At least seven different forms of laminin, whichhave selective distributions and functions, have beendescribed. Laminin is composed of three polypep-tide chains which aggregate in a crucifix-like struc-ture. With nidogen acting as an intermediary agent,laminin is able to interact with type IV collagen andcontribute to the sieve-like network organization ofbasement membranes. In addition to its interactionwith nidogen and type IV collagen, laminin mediatescell adhesion through cell surface integrins (in par-ticular a6b1) and may also play a regulatory role incell proliferation and migration of epithelial cells.Immunolocalization studies have shown the lamininto be uniformly distributed in the basement mem-brane of gingival epithelia, with both laminin-1 andlaminin-5 being identified in these tissues (83, 241).

Type VII collagen is the predominant component ofthe anchoring fibrils of basement membranes (180).This specialized collagen is secreted in a soluble formby basal epithelial cells and is composed of a centralhelical portion with a globular domain at the C-ter-minal and a small nonhelical domain at the N-ter-minal (172). These triple helical molecules align at theC-terminus as antiparallel dimers of approximately450 nM each in length. One end of the molecule be-comes embedded in the lamina densa of the base-ment membrane, where it interacts with type IV colla-gen, and the other end attaches to anchoring plaques(92). Type VII collagen has been localized in the base-ment membrane of gingival oral epithelium (173).

Proteoglycans of basement membranes are gener-ally very rich in the glycosaminoglycan heparan sul-

Molecular and cell biology of the gingiva

fate. At least two differently sized heparan sulfateproteoglycans have been identified in basementmembranes and appear to localize to the laminadensa and connective tissue immediately below thelamina densa (82). The larger of these two proteolgy-cans is now termed perlecan and has been well char-acterized (88, 148, 149). Perlecan is synthesized bymesenchymal cells and interacts within the base-ment membrane with laminin, type IV collagen and,to a lesser extent, nidogen (88). Other heparan sul-fate proteoglycans, as well as proteoglycans contain-ing chondroitin sulfate, have been localized in base-ment membranes (123, 231). Within the gingivalepithelium basement membranes, several proteo-glycans have been identified but have been poorlycharacterized (96, 181, 185, 201).

The junctional epithelium differs from both theoral sulcular epithelium and the oral gingival epi-thelium in that it is associated with two basementmembranes, one involved in the dento-epithelialcomplex and the other with the underlying connec-tive tissue. The basement lamina facing the toothsurface is termed the internal basal lamina, whilethat facing the gingival connective tissue is termedthe external basal lamina (193). Although it has beensuggested that the internal basement lamina doesnot appear to have an identifiable lamina lucida andlamina densa (193), a recent report has indicated thelamina lucida to be present and somewhat thickerthan other basement membranes (185). It has beenproposed that the internal basal lamina of the junc-tional epithelium is structured for mechanicalstrength and the provision of a tight seal around thetooth surface for protection of the periodontaltissues from the oral environment (185). While theinternal basal lamina is also unique in that it lackstype IV collagen and prototypic laminin (laminin-1),two common components of basement membranes(83, 186), laminin-5 appears to be a major compo-nent of the internal basal lamina in human tissues(83, 132, 221). In most other respects, the interactionof the junctional epithelium with its two basal mem-brane structures is similar to other epithelia beingadjoined by hemidesmosomes and a6b4 integrins,together with fibrillar and nonfibrillar matrix pro-teins (84, 185).

Gingival connective tissue

Development

The gingival connective tissue is largely a fibrousconnective tissue that has elements originating di-

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rectly from the oral mucosa connective tissues aswell as some fibers (dentogingival) that originatefrom the developing dental follicle. As the develop-ing tooth begins to erupt, it emerges from its bonycrypt with the most coronal Sharpey’s fibers alreadyembedded within the cementum of the root surface.These fibers have their origin from cells and tissuesarising from the dental follicle. The tooth emergesthrough the submucosal tissues of the oral mucosaand eventually penetrates the oral epithelium withsubsequent formation of the epithelial attachmentapparatus (see above). As the tooth erupts, thetissues originating from the dental follicle and oralmucosa coalesce. However, it is of interest to notethat the tissues immediately subjacent to the gingi-val epithelia appear to have some instructive roleover the superficial epithelium dictating its form andstructure. Whether this reflects the embryologicalorigin of these tissues remains to be established.

Structure and composition of gingiva

Fibroblasts. Fibroblasts are of mesenchymal originand play a major role in the development, mainten-ance and repair of gingival connective tissues. His-torically, these cells have been considered to be ofuniform nature and rather passive contributors tothe tissues, responding only when required andbeing of limited variability due to their differentiatedstate. However, recent evidence indicates that thesecells have many subtleties relating to their pheno-type, responsiveness to various cytokines and growthfactors, as well as their general role in the tissues.

The principal function of fibroblasts is to synthes-ize and maintain the components of the extracellularmatrix of the connective tissue. This feature seemsto be consistent for all ‘‘types’’ of fibroblasts, withvariation in the types and amounts of matrix pro-teins synthesized occurring according to the tissueof origin, as well as localized functions of the cellswithin the tissues.

The morphology and ultrastructure of fibroblastshave been studied extensively and, in general,fibroblasts in vivo have been noted to have a typicalelongated or spindle shape and, consistent with theirhigh level of synthetic activity, have prominentrough endoplasmic reticulum and Golgi apparatus.Their cytoplasm is usually rich in numerous mito-chondria, vacuoles and vesicles. Intracellular micro-filaments are sparse in non-motile fibroblasts butbecome prominent upon activation of cell mi-gration. In vivo, fibroblasts are rarely seen in contactwith one another; rather they tend to exist in iso-

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Fig. 6. Role of fibroblasts in maintaining tissue homeo-stasis. Fibroblasts may respond to a variety of stimulantsvia production of cytokines, enzymes, enzyme inhibitorsor matrix macromolecules. LPS: lipopolysaccharide.TIMPs: tissue inhibitors of matrix metalloproteases.MMPs: matrix metalloproteases. PGs: prostaglandins.PGE2: prostaglandin E2.

lation attached to a surrounding matrix of collagensand other glycoproteins. In vitro, these cells mayoften vary slightly in their appearances, which in-clude greater expression of intracellular microfila-ments, and fibers and intimate cell-cell contactthrough gap junctions (163). More recently it hasbeen recognized that such generalizations may notbe valid with considerable morphological and ultra-structural heterogeneity being observed for popula-tions of fibroblasts residing in a number of tissues,including the periodontium (97, 107, 184).

Fibroblast heterogeneity is now a well-establishedfeature of fibroblasts residing within the periodon-tium (28, 77, 124, 189). Indeed, differences in popu-lations of fibroblasts identified within the tissues, aswell as cultured from the tissues, indicate a widerange of variable features including morphology, ul-trastructure, proliferation, migratory behavior, ma-trix synthesis, and responsiveness to growth factorsand cytokines. Although the biological and clinicalsignificance of such heterogeneity is not yet clear, itseems that such functions are necessary for the nor-mal functioning of tissues in health, disease and re-pair. For example, heterogeneous populations mayrepresent subgroups of cells responsible for activitiesas wide ranging as fibrogenesis, formation of hardand soft connective tissues, intercellular communi-cations and endocrine/autocrine control of connec-tive tissue metabolism (97, 107). These features areconsidered to be of importance in the context of di-recting regenerative or homeostatic events withinthe periodontium and are considered in more detailin the chapter on periodontal regeneration.

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While fibroblasts are considered primarily respon-sible for synthesis of the extracellular matrix, theyare also involved in a number of regulatory pro-cesses necessary for maintenance of tissue homeo-stasis (Fig. 6). Fibroblasts are specifically involved inthese processes via phagocytosis and the secretion ofcollagenases. Phagocytosis of collagen by fibroblastsduring tissue turnover and remodeling has been pro-posed as one of the principal mechanisms throughwhich tissues can be remodeled and allow changesin shape or structure without impairing function(219). Fibroblasts (including gingival fibroblasts)synthesize a wide range of matrix metalloproteinasescapable of degrading collagens, proteoglycans andother matrix components (26, 27). These enzymes,together with their inhibitors (tissue inhibitors ofmatrix metalloproteinases), allow for a very regu-lated control of matrix degradation for remodellingor turnover purposes (154). If, for one reason or an-other, these regulatory mechanisms are deranged,then a net gain or loss of connective tissue can resultleading to overgrowth or tissue destruction.

In light of the above, it is clear that fibroblasts arevery sensitive to their immediate surroundings andwill respond depending upon the messages being re-ceived. Accordingly, fibroblasts are particularly sen-sitive to changes in the surrounding matrix, growthfactors or cytokines. These cells have the ability notonly to respond to paracrine signals but may alsosynthesize and secrete a number of growth factors,cytokines and metabolic products that further dic-tate cell activity in an autocrine manner. A more de-tailed description of the effects of soluble mediatorson fibroblast function is provided later in thischapter.

Apart from regulating matrix synthesis and re-modeling, fibroblasts possess two other propertiescritical to their function. These are an ability for sitedirected migration (chemotaxis) and attachment tovarious substrata. Following injury to tissues, woundhealing requires the recruitment of cells with regene-rative capacity to the site in order for tissue repairor regeneration to occur. The ability of fibroblasts tomigrate has received considerable attention, and themechanisms involved are reasonably well docu-mented. During migration, fibroblasts elongate andsend out small extensions (lamellopodia) thatadhere to the substratum. Through the subsequentinteraction between cell surface integrins and theunderlying matrix a rearrangement of microtubules,myosin as well as vimentin and actin filamentswithin the cytoplasm occurs and the cell is able topull itself in the direction of the newly placed lamel-

Molecular and cell biology of the gingiva

lopodia and thus migrate (1, 25, 165). As a result ofthese studies concerned with general mobility offibroblasts, chemotactic behavior of fibroblasts (in-cluding gingival fibroblasts) has been studied (4, 33,86, 166, 200). Up to ten different classes of chemo-attractants for fibroblasts have been identified, manyof which are present in abundance at inflammatorysites (167).

Once recruited to the site of trauma, the fibroblastmust then be able to immobilize itself and commencematrix synthesis. The attachment of fibroblasts tovarious substrata has been the topic of considerableinterest as it has ramifications not only for repair andregeneration, but also malignant transformation. In-deed, the attachment of cells to the extracellular ma-trix is critical for maintaining appropriate cell shape,cell function and tissue integrity. In the context offibroblast-matrix interactions, the cell surface recep-tor molecules known as integrins are the best studied(87). A detailed discussion of integrins is beyond thescope of this chapter, and the reader is referred to sev-eral excellent reviews (45, 62, 80, 228). The mechan-isms involved in cell-matrix attachment require clus-tering of adhesion receptors and subsequent re-arrangement of cytoskeletal proteins and involveboth intracellular and extracellular processes (68).The interaction between adhesion molecules andtheir receptors leads to activation of a variety of signaltransduction pathways that are crucial for controllingevents as diverse as cell adhesion, migration,apoptosis and gene regulation (45, 80, 90, 178).

Matrix composition. The composition of extracellu-lar matrix in the gingival connective tissue has beenreviewed in greater detail elsewhere (13, 139); it willtherefore be discussed only briefly here. Collagenousproteins account for the bulk of the matrix proteinsin the periodontal tissues (144). Ultrastructuralstudies by electron microscopy and immunocytoch-emistry have revealed that collagen fibers are organ-ized into distinct architectural patterns in the peri-odontal tissues. These have been classified accord-ing to their location, origin and insertion (187, 191)and are listed in detail in Table 1.

Type I collagen is the main collagen species in alllayers of gingival connective tissues (140, 171). Thecollagen fibers are arranged in two patterns of or-ganization, one consisting of large, dense bundles ofthick fibers, and the other, a loose pattern of shortthin fibers mixed with a fine reticular network (41).These fibers contain both type I and III collagens.Type I collagen is preferentially organized intodenser fibrils in the lamina propria. Although it is

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not restricted to any particular region, type III colla-gen appears to be preferentially localized as thinnerfibers in a reticular pattern near the basement mem-brane at the epithelial junction (141, 233). Type IIIcollagen has a more diffuse pattern in lamina pro-pria. Immunostaining studies have revealed thattype V collagen has a parallel filamentous pattern,and this collagen appears to coat dense fibers com-posed of type I and III collagens (141, 174). The gin-gival connective tissue contains type VI collagen aswell, which is present in diffuse microfibrillar pat-tern. This collagen is present near basement mem-branes in the rat, but not in marmosets (174, 175).In the gingiva, basement membrane is present atjunctions of connective tissue with epithelium, inrete pegs and around blood vessels and nerves, andcontains type IV collagen, laminin and heparan sul-fate (41, 141, 174, 175). The presence of type IV colla-gen in the rat appears to be restricted to externalbasal lamina, while both external and internal lam-inae contain laminin (186).

Proteoglycans are also ubiquitous constituents ofthe periodontal tissues (75). At present proteo-glycans are broadly classified into three groups de-pending upon their location and include (i) matrixorganizers and tissue space fillers; (ii) cell surfacecomponents; and (iii) intracellular proteoglycans ofthe hemopoietic cells (60). Early identification andlocalization of proteoglycans in periodontal tissueswere based on analyses of the total uronic acid con-tent and constituent glycosaminoglycan species. Ap-proximately 0.3% of the total dry weight of gingiva isuronic acid. Within the gingival connective tissues,approximately 60% of total glycosaminoglycans isdermatan sulfate, an additional 30% of the glycosa-minoglycans is chondroitin sulfate and the remain-ing 10% is accounted for by roughly equal pro-portions of hyaluronan and heparan sulfate (14). Incontrast to the connective tissue, heparan sulfate isthe predominant glycosaminoglycan in the gingivalepithelium. Apart from differences in sulfation andcharge, these glycosaminoglycans have great hetero-geneity in their molecular size, ranging from 15,000for heparan sulfate to 340,000 for hyaluronan (20).Biochemical studies of intact proteoglycans isolatedfrom homogenates of gingival epithelium and con-nective tissue have identified numerous pools ofproteoglycans which differed in size and glycosami-noglycan content (20, 21, 105). Using specific anti-bodies and cDNA probes, several proteoglycan spe-cies have been identified to be associated with gingi-val tissues and these include decorin, biglycan andversican (31, 91, 104).

Bartold et al.

With the development of specific antibodies tovarious proteoglycans, several studies have not onlyconfirmed the presence of proteoglycans within theperiodontal tissues but also determined the distri-bution of various proteoglycans throughout thetissue compartments (18, 71, 181, 201). Dermatansulfate appears to be localized closely associatedwith collagen fibers and is particularly evident at theepithelial connective tissue interface. In contrast, he-paran sulfate is localized primarily in the basementmembranes of epithelium and capillary endo-thelium (57). Specific proteoglycans have also beenidentified by immunohistochemistry within the gin-gival tissues and include decorin, biglycan, versican,syndecan, CD-44 and perlecan (71, 152). Decorin ispresent within the gingival tissues closely associatedwith bundles of collagen fibers, especially in the sub-epithelial region. Biglycan is a relatively minor con-stituent of gingiva, and it appears to be present infilament-like structures in the matrix near oral epi-thelium (18, 71, 201).

Fibronectin is distributed throughout the gingivalconnective tissues and is localized over collagenfibers (141, 164, 211). Gingiva also contains os-teonectin, vitronectin, elastin (19, 183, 211) and ten-ascin, which is present diffusely in the connectivetissue and prominently near the subepithelial base-ment membrane in the upper connective tissue andcapillary blood vessels (23). Elastin is a minor com-

Fig. 7. Distribution of vascular networks within humangingival epithelium. The primary vascular supply is sup-plied via vessels located in the periodontal ligament (1),the gingiva (4), and the alveolar bone (5). Subjacent tothe junctional epithelium (3), the vasculature resemblesanastomosing postcapillary venules, while subjacent tothe oral epithelium (4) the vasculature is composed of art-erio-venous anastomoses of a glomerular-like arrange-ment.

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ponent of gingival connective tissue accounting forapproximately 6% of the total tissue protein (40). Im-munohistochemical studies have shown that elastinis more prominent in the submucosal tissues of themore moveable and flexible alveolar mucosa (19).

The interaction of cells with their surroundingmatrix is usually mediated through specific cell sur-face-associated molecules. Of these, the integrins areof central importance in the regulation of cell ad-hesion and migration (3, 177). Integrins are hetero-dimeric molecules composed of a- and b-subunitsand are classified on the basis of their b-subunitcomposition. These integral cell membrane compo-nents are implicated in the migration of leukocytes,epithelial cells and fibroblasts as well as T-cell andmacrophage interactions and clot formation and areexpressed in high proportions during wound healing(36, 179). The integrins expressed by fibroblasts bindprincipally to matrix proteins such as fibronectin, vi-tronectin, collagen, laminin and fibrinogen (179).Studies regarding the distribution of integrins in gin-gival tissues have focused mainly on the epithelialcomponents in which the b1, b4 and a6 integrins areexpressed by cells of the epithelium and basal lam-ina (84, 102). Fibroblasts within gingival connectivetissues express a1, a2, a5, av, avb3 which serve as re-ceptors for vitronectin, fibronectin and collagens(85, 211).

Neurovascular composition. The neurovascularcomposition of the gingivae has been studied insome detail. The gingiva has one of the largest endorgan blood supplies in the body and, as such, maybe susceptible to any factor that compromises bloodflow. The vascular supply to the gingiva forms twodistinct networks (Fig. 7), one bounded by the oraland sulcular gingival epithelia and the other sub-jacent to the junctional epithelium (197). The ar-rangement of the vasculature in each of these re-gions varies (54, 93). Adjacent to the junctional epi-thelium, the vascular plexus is composed ofanastomosing postcapillary venules termed the gin-gival plexus. Elsewhere in the gingiva, the capillaryloops consist of an ascending arterial and de-scending venous component. At the arterio-venousanastamoses, a glomerular-like structure has beenreported (207). The subtle differences between thevessels associated with the junctional epitheliumand the other gingival epithelia is of great signifi-cance with regards to initiation of gingival inflam-mation (see below).

Neural elements are extensively distributedthroughout the gingival tissues (114, 119). Within the

Molecular and cell biology of the gingiva

gingival connective tissues most nerve fibers are my-elinated and are closely associated with the bloodvessels (114). Many of these nerve fibers are im-munoreactive to a number of neuropeptides includ-ing calcitonin gene-related peptide, substance P andneuropeptide Y (79, 113) (Fig. 8). Apart from playinga sensory role, the presence of nerves and associatedneuropeptides are considered to contribute a neuro-genic component to the inflammatory alterationscaused by mechanical, chemical and possiblyemotional stimuli (32, 70, 98). Nerve fibers originat-ing in the subepithelial connective tissue may alsopenetrate into the junctional epithelium (117). Theintraepithelial nerves are unmyelinated but do haveendings containing a number of neuropeptides (117,217). The function of intraepithelial nerves may notonly be for sensory purposes, but may form net-works of communicative pathways between the epi-thelium and underlying connective tissues as seenin skin where dermal mast cells appear to be linkedto epidermal Langerhans cells via nerve fibers (52).

Function of gingival connective tissue

The gingival connective tissue serves primarily toprotect the root surface and alveolar bone from theexternal oral environment. In addition, it aids in thesupport and fixation of teeth within their alveolarhousing and provides adequate support for the epi-thelial tissues.

In carrying out its protective role, the gingivaltissues provide the stage upon which the host re-sponse acts out its role of surveillance, interceptionand removal of foreign materials. In doing so, theresponse may become destructive rather than pro-tective, in which case the gingival tissue architecture

Fig. 8. Neuronal networks within human gingivae. proximity of these two networks in both vertical and hori-A, B. Nerves in close proximity to the basal epithelial layer, zontal sections (upper and lower panels, respectively).and penetrating the basement membrane (NCAM stain- D. Double-stained section showing nerve forming directing). C. Double staining for nerves (NCAM, black color) contact with a Langerhans cell (arrow).and Langerhans cells (CD1a, brown color) reveals the

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is significantly affected and tissue function is com-promised (see below). If, however, the host defenseis successful in eradicating the foreign materials in-ducing the inflammatory reaction, or, the level ofabuse is minimal, the gingival tissues are able to re-pair themselves and maintain adequate tissuehomeostasis and normal function. Thus, underhealthy conditions, there is a delicate balancing actplayed out in the gingival tissues involving bothtissue repair and tissue destruction.

Repair of gingival connective tissue

Due to their high turnover rate, the connectivetissues of the gingivae have remarkably good healingand regenerative capacity (127). Indeed, it may beone of the best healing tissues in the body and gen-erally shows little evidence of scarring following sur-gical wounding (Fig. 9). Although surgical woundingof skin usually results in scar tissue formation, simi-lar wounding to the gingival tissues normally resultsin rapid reconstitution of the fibrous architecture ofthe tissues and generally very little, if any, scarringresults (127, 226, 227).

Although the gingival connective tissue has rapidturnover, it is not as great as the reparative capacityof periodontal ligament and it is not like the epi-thelial tissues (see above). Following initial injury,the gingival connective tissue commences its repara-tive efforts in a manner similar to most other tissuesinvolving a demolition phase followed by synthesisof granulation tissue, organization, contraction andremodeling (42, 65). These processes involve an in-tricate interplay between inflammatory cells, fibro-blasts, and the newly synthesized matrix. The role ofthe inflammatory cells in wound healing is to secrete

Bartold et al.

Fig. 9. Scar tissue in skin (A) following surgery and theabsence of scar tissue in the gingiva (B), despite scarringof the alveolar mucosa (arrow) following a frenectomy

polypeptide mediators that act as agents for the re-cruitment of cells to the site to commence repair andstimulation of these cells to commence new matrixsynthesis. Angiogenesis is also a feature of healinggingival wounds, with the microvascular endothelialcells being responsible for this activity. The cells re-sponsible for production of the new granulationtissue matrix are myofibroblasts, which most likelyoriginate from the gingival fibroblast population(72).

Early healing events at the dentogingival interfacehave been studied in detail in an animal model(235). Within hours, the wound site is stabilized bythe formation of a fibrin clot that adheres to the rootsurface, and there is a heavy infiltrate of neutrophils.Within 3 days, granulation tissue becomes evidentat the wound site and, although fibroblasts can beidentified within the wound, the site is still heavilyinfiltrated by inflammatory cells. During this phasethe fibrin clot is slowly degraded. By day 7, the siteis rich in newly formed granulation tissue and thecollagen fibers appear to align in a parallel arrayalong the root surface. The matrix continues to re-

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model, and by day 14 the collagen fibers may showsome signs of attachment to the root surface, withsubsequent cementum formation not appearing un-til the third week after wounding.

For fully functional connective tissue attachmentto a root surface through the reformation of Sharpe-y’s fibers, a minimum of 3 weeks of healing is re-quired. This being the case, it is not surprising thatthis rarely occurs to any significant extent becausethe more rapid migration of the gingival epitheliumresults in epithelial coverage of the dentogingivalwound area long before the connective tissue has anopportunity to realign itself with the root surface.

Changes to the gingival tissues withonset of inflammationEpithelial changes

With accruing knowledge, the epithelium can nolonger be considered a silent partner in the patho-genesis of the inflammatory periodontal diseases(214). While this tissue was originally considered toprovide an elementary protective role that eventuallypermitted some passage of antigens to the connec-tive tissues leading to destructive inflammation, thisconcept must now be considered naive. The gingivalepithelium, and in particular the junctional epithel-ium, is involved at the earliest phases of the in-flammatory response and is very likely to be instruc-tive with regards to the establishment of disease.While epithelial permeability is important, many cel-lular signaling events occur as a rapid response tothe accumulation of dental plaque (Fig. 10). Theseevents, together with the later molecular, cellularand tissue changes, need to be considered in thecontext of current information.

Before the clinical signs of gingivitis develop in re-sponse to plaque accrual, the potential for epithelialcell activation is high. Epithelial cells can producenumerous cytokines including interleukin-1 and in-terleukin-8, both of which may be involved in neu-trophil trafficking (224), and growth factors such asplatelet-derived growth factor-AA and platelet-de-rived growth factor-AB. In addition, epithelial cells(junctional epithelia in particular) express intercellu-lar adhesion molecule-1 and E-selectin, two ad-hesion molecules involved in neutrophil binding (43,162). The expression of intercellular adhesion mol-ecule-1 appears to increase with increasing neutro-phil migration, although once the infiltrate becomesdense, the expression level of this molecule de-

Molecular and cell biology of the gingiva

Fig. 10. Epithelial changes associated with inflammation. in appearance. B. Medium-power view of the junctionalA. Low-power view of attached gingiva shows widened in- epithelium. C. High-power view of junctional epithelium.tercellular spaces within the junctional epithelium (left Infiltrating leukocytes are a prominent feature.side), whereas the oral epithelium (right side) is normal

creases (63). By this stage, the need for intercellularadhesion molecule-1 is diminished and other factorsassociated with the general pathological state of thetissues act as ongoing recruitment factors for theneutrophil infiltrate. The presence of E-selectin ingingival epithelial tissue is interesting since this isnormally associated with neutrophil adhesion to en-dothelial cells. Whether this molecule is involved inneutrophil adhesion within the epithelium or is as-sociated with other functions such as binding ofLangerhans cells or lymphocytes to keratinocytes re-mains to be established (162). Nonetheless, it is clearthat at a very early stage, the epithelium is capable ofproducing a number of instructive messages, whichhave the potential to lead to neutrophil recruitment,chemotaxis and adhesion. That intercellular ad-hesion molecule-1 appears to be constitutively ex-pressed by gingival epithelium further enhances thisargument since it is well established that a constantflow of neutrophils occurs through the junctionalepithelium during ‘‘health’’.

Concomitant with the expression of cytokinessuch as interleukin-1 and interleukin-8 and ad-hesion molecules such as intercellular adhesionmolecule-1 and E-selectin within the epithelium, theintercellular spaces of the junctinal epithelium beginto widen and serve as a primary pathway for theegress of the inflammatory exudate from the gingivato the gingival sulcus. This feature seems to be re-stricted to the junctional epithelium since neitherthe sulcular nor oral epithelium show this response.These enlarged spaces do not appear to be artifactsof tissue processing for histology (193, 197) since theelectron-dense (possibly proteoglycans and other

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glycoproteins) intercellular matrix remains identifi-able and the intercellular attachments (desmosom-es) remain intact – despite the enlargement (198).The mechanism causing the enlargement is poorlyunderstood but may be related to the increased hy-drostatic pressure resulting from the accumulatinginflammatory exudate, which creates a pressuregradient from the connective tissues, forcing fluidinto the epithelial intercellular spaces (159, 198).

Tissue destruction at the epithelium–connectivetissue interface may be associated with changes ininteractions between these two tissues and could bemediated via a number of integrins (69). For ex-ample, a more widespread distribution of the inte-grins a2b1 and a3b1 in pocket epithelium may beassociated with keratinocyte proliferation and mi-gration, while the weaker expression of a6b4 in thepocket epithelium may be related to epithelial de-tachment from the tooth surface (69, 103).

The permeability of the junctional epithelium isalso a critical factor in the establishment of gingi-vitis. While the oral and gingival epithelia appear tobe relatively impervious, the junctional epitheliumdoes permit permeation of molecules from the ex-ternal surface towards the connective tissue (53, 125,223). As a result, a chemotactic gradient is estab-lished that further facilitates the migration of neu-trophils. It is not until there is a significant influx ofneutrophils that the intercellular spaces of the junc-tional epithelium become pathologically altered,with disruption to the intercellular junctions and in-creasing widening of the intercellular spaces. Oncethis level of infiltration has occurred, and if the driv-ing stimulus (dental plaque) remains, then the nor-

Bartold et al.

mal rapid turnover of the junctional epithelium isinsufficient to restore health and the pathway to on-going tissue damage is established.

With continuing plaque accrual, neutrophil mi-gration and early activation of macrophages andlymphocytes within the gingival connective tissue,the junctional epithelium can be seen to commencemigration in an apical direction and result in theearliest formation of a periodontal pocket. While thejunctional epithelium is not invasive per se, cells ofthe basal layer are capable of producing collagenasesthat can degrade the underlying collagen sub-stratum; thus, a mechanism exists for matrix degra-dation followed by epithelial migration (229). Of par-ticular interest is the fact that gingival epithelial cellsare stimulated to produce collagenase-3 (matrixmetalloproteinase-13) by tumor necrosis factor-a,transforming growth factor-b and keratinocytegrowth factor – all of which are likely to be in abun-dance within the gingival tissues during the early in-flammatory response (112, 229).

It is clear that the gingival epithelium can ‘’sense’’the presence of the developing dental plaque biofilmand subsequently illicit signals to the underlyingconnective tissue. Whether this signalling is via thesecretion of cytokines such as interleukin-1 and in-terleukin-8 as described above or alternate pathwayshas not been established. Communicative pathwaysmay be established through a variety of cells knownto reside in the epithelium. For example, Langerhanscells, which act as antigen-presenting cells, havebeen noted to increase in number with increasinginflammation. In addition, other mononuclear cells,specifically T lymphocytes, have been noted in gingi-val epithelium. The observation of nerve fibers con-necting epithelial tissues to connective tissue ele-ments is also of interest in the context of gingivalinflammation. These small fibers may provide amechanism whereby the earliest of chemical stimulimay activate cells within the underlying connectivetissue, leading to vascular responses and subsequentfluid and cellular extravasation.

Connective tissue matrix changes

Qualitative and quantitative changes in periodontalconnective tissues, especially in the gingiva, areprominent features of the periodontal diseases (Fig.11). Gingivitis is one of the most common chronicinflammatory conditions affecting humans. Sub-sequent to the initial inflammatory response, con-nective issue destruction occurs within 3 to 4 daysafter plaque accumulation (160). The destruction be-

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gins at perivascular collagen bundles, and approxi-mately 70% of collagen within the foci of inflam-mation is lost. The major inflammatory cells respon-sible for the destruction are polymorphonuclearlymphocytes and macrophages (156). As the in-flammatory process develops, the destruction mayexpand deeper towards the periodontal ligamentand alveolar bone resulting in tooth mobility and,ultimately, tooth loss if the disease is left untreatedand continues to progress. Simultaneously with de-struction, fibrosis and scarring may coexist at foci ofinflammation. Gingival fibrosis, manifested by scar-ring of gingival tissues, is seen in slowly progressiveperiodontitis in humans, baboons and chimpanzees;however, fibrosis is not seen in dogs, rodents, minksand marmosets (157, 187).

In periodontitis, numerous quantitative and quali-tative changes occur to the gingival collagens (142,145). For example, the gingival collagens becomemore soluble, and the ratios of collagen types arealtered. Furthermore, the amount of type V collagenincreases and a new collagen, type I trimer, may ap-pear (141, 143). However, in spite of these quantitat-ive changes, there does not appear to be a change inlocalization and distribution of constituent collagentypes. Quantitative changes also occur in noncolla-genous gingival constituents in beagle dogs, whichare lost from diseased gingiva (145).

The gingival proteoglycans manifest fewer quanti-tative and qualitative changes than do the collagens.Early histochemical studies demonstrated thatproteoglycans appeared to be lost from the center ofinflammatory foci but were present in higher con-centrations around the periphery (126). These earlystudies implied that, not only did the fibroblasts atthe periphery of the inflammatory lesion show anincreased capacity to synthesize proteoglycans, butthe cells of the inflammatory infiltrate also stainedstrongly for the histochemical dyes used to locatethe proteoglycans (126). Subsequent biochemicalanalyses of homogenates of inflamed human gingi-vae demonstrated that the amount of dermatan sul-fate decreases while the content of chondroitin sul-fate increases. In addition, degradation of bothproteoglycan core proteins and hyaluronic acid arecharacteristic features of inflamed gingival connec-tive tissues (16). Despite these qualitative and struc-tural changes in inflamed tissues, no conclusive evi-dence of depletion of proteoglycans from inflamedgingival tissues has been demonstrated. Such a con-trasting finding to the massive loss of collagen fromthe same tissues may be explained by the contri-bution of stimulated fibroblasts at the periphery of

Molecular and cell biology of the gingiva

Fig. 11. Connective tissue changes with gingival inflam- cytes beneath the junctional epithelium. F, G. Activatedmation. A, B. Foci of infiltrating leukocytes at medium post-capillary venular endothelium expressing the selec-and high magnifications (H&E). C. Alcian blue staining tion leukocyte adhesion molecules CD62E (endothelialdemonstrating an increase of Alcian blue positive matrix cell leukocyte adhesion molecule-1) (F), and CD62Pmaterial associated with localized foci of inflammation. (GMP-140) (G). H. Degranulated mast cells (expressingD. Masson’s trichrome demonstrating loss of collagenous tryptase) within the gingival connective tissues.material at inflammatory foci. E. CD3 positive T lympho-

the inflammatory lesion, together with the contri-bution of proteoglycan content and products of theinflammatory cells. Indeed, the principal proteo-glycan synthesized by inflammatory cells containschondroitin sulfate (17); thus the presence of suchproteoglycans could explain why chondroitin sulfatelevels increase at the expense of dermatan sulfate ininflamed gingival tissues.

As discussed earlier, the vascular anatomy ad-jacent to the junctional epithelium is unique consist-ing of the gingival plexus which contains primarilypostcapillary venules. As the inflammatory responseis evoked, the glomerular nature of this plexus in-creases with an increase in the number and size ofthe capillary loops (94, 168). During this process, thepost capillary venules adopt the appearance of highendothelial venules (239, 242) which facilitates emi-gration of lymphocytes (59). A peculiar feature of thehigh endothelial cell venules found in gingival tissueis their preference for polymorphonuclear leukocyte

43

emigration rather than lymphocyte migration (242).These structures, together with their ability to ex-press many leukocyte adhesion molecules duringthe initial inflammatory response, allows mar-gination and diapedesis of leukocytes from the bloodvessels into the connective tissues. The major ad-hesion molecule expressed by venules and high en-dothelial venules of the gingival plexus include, en-dothelial cell leukocyte adhesion molecule-1, inter-cellular adhesion molecule-1, leukocyte function–associated antigen-3, vascular cell adhesion mol-ecule-1 and platelet-endothelial cell adhesion mol-ecule-1. Although these molecules appear to beconstitutively expressed in the venules of the gingi-val plexus to facilitate the emigration of small num-bers of neutrophils seen in healthy tissues, their ex-pression is rapidly upregulated in the high endo-thelial venules that appear upon initiation of theinflammatory response (243).

Studies on innervation of gingivae obtained from

Bartold et al.

periodontitis-affected sites has provided some usefulinformation with regard to the potential role of aneurogenic contribution to periodontal inflam-mation. In particular, the role for locally releasedneurogenic peptides as inflammagens cannot be dis-counted. Substance P, which is released from pri-mary sensory afferent nerves, has significant pro-in-flammatory actions (108) and has been proposed toplay a role in neurogenic inflammation of the peri-odontal tissues (12). Many neurogenic peptides havebeen identified in inflamed gingival tissues andnoted to localize throughout the connective tissuesand around blood vessels (113). Although theseneurogenic peptides are present in healthy tissues,an upregulation of these potent bioactive moleculescould have a significant impact on the initiation andestablishment of the inflammatory response.

The mechanisms by which matrix changes arebrought about are discussed in a separate chapter inthis volume.

Role of connective tissue changes ininflammatory reactions

Factors regulating fibroblast function

Under healthy conditions, the fibroblasts are embed-ded in a matrix composed of collagen and noncolla-genous components. They are sparsely distributedand the cells have a flattened morphology indicatinglow metabolic turnover. Following injury and in-flammation, the matrix scaffolding of the gingivalconnective tissues is disrupted and the fibroblastsmigrate to the wound site, divide and produce newmatrix. The cells at this stage may assume thephenotypic characteristics of smooth muscle cellsand become myofibroblasts. A variety of factorspresent in the local environment dictate the activi-ties of fibroblasts; these include degradation prod-ucts of the matrix and blood plasma and numerouscytokines and growth factors derived from inflam-matory cells. These substances affect the migration,adhesion, proliferation and fibroblasts and their ma-trix synthesis (42).

The most prominent mitogen for fibroblastsunder inflammatory conditions is platelet-derivedgrowth factor, which may consist of homo- andheterodimeric form of platelet-derived growth factor‘‘A’’ and ‘‘B’’ chains. All three dimeric forms are se-creted by platelets and macrophages; however, re-cent studies indicate that the gingival epitheliummay also be a major source of these growth factor inthe gingiva (6, 8). Various forms of platelet-derived

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growth factor can localize to cell surfaces to act inan autocrine or juxtacrine manner. Alternatively,platelet-derived growth factor may accumulate inthe extracellular matrix to become available at a laterstage to induce cell proliferation and migration.Regulation of the cellular responses to platelet-de-rived growth factor is via receptors on the surface oftarget cells. Two receptors have been identified; thea-receptor binds both the A- and B-containing formsof platelet-derived growth factor, while the b-recep-tor binds only platelet-derived growth factor-B (76).Variability in biological response to the variousforms of platelet-derived growth factor may be re-lated to signal transduction through the a- and b-receptors. For example, platelet-derived growth fac-tor-AA is not as potent as platelet-derived growthfactor-BB with respect to mitogenesis (38, 169) andcell migration appears to be mediated through theb-receptor (56). Platelet-derived growth factor-AA isthe major isoform present during early wound heal-ing (8, 65, 209). Platelet-derived growth factor iso-mers containing the B chains appear to be more po-tent in chemotaxis towards polymorphonuclearlymphocytes, monocytes and fibroblasts and inmitogenic stimulation of fibroblasts (66, 212). Theplatelet-derived growth factor-BB and -AB chains aremitogenic to gingival and periodontal ligamentfibroblasts, and these cells respond only weakly toplatelet-derived growth factor-AA (30, 120, 240). Hu-man gingival fibroblasts contain messenger RNA forthe platelet-derived growth factor-A chains, and itstranscription is activated by serum and by manygrowth factors (240). The platelet-derived growthfactor isoforms also affect the synthesis of collagensby periodontal fibroblasts (145).

The major mediator that influences the synthesisof collagen and other matrix components by fibro-blasts is transforming growth factor-b. This polypep-tide, also secreted by platelets and macrophage, isbelieved to be responsible for accumulation of ma-trix elements during fibrosis. Transforming growthfactor-b affects matrix accumulation through activ-ating the transcription of genes of type I, III, IV, VIand VII collagens and proteoglycans (91, 153, 232),and by inhibiting the synthesis of collagenase (238).Collagen synthesis is inhibited by prostaglandin E2,interferon-g, and tumor necrosis factor-a. Thesesubstances affect collagen synthesis at the transcrip-tional level, while tumor necrosis factor-a also acti-vates the transcription of collagenase gene (106).

Another major cytokine regulating matrix compo-sition is interleukin-1, which is produced by manycell types, including fibroblasts and monocytes. In-

Molecular and cell biology of the gingiva

terleukin-1 is a major cytokine involved in matrixdegradation in rheumatoid arthritis, periodontitisand other inflammatory diseases and during boneresorption. The mode of action of interleukin-1 ap-pears to be through induction of genes for collagen-ase, gelatinases and stromelysin-1 (26, 27, 48, 220).Inflamed gingival tissues and gingival crevicularfluid contain interleukin-1a, interleukin-1b, tumornecrosis factor-a, interleukin-6, interleukin-8 and in-terferon-g, and the presence of these cytokines is be-lieved to contribute to higher levels of matrix-de-grading enzymes in the gingival crevicular fluid (5).

Cytokines and growth factors influence cellularactivities in several ways (95). These substances firstbind to specific cell surface receptors; the bindingactivates a variety of signaling events that are re-quired for cell migration, attachment, DNA synthesisand other cell functions (39, 158). Frequently the ex-pression of integrins and other cell surface receptorsis affected (179), resulting in modification of cell-matrix and cell surface interactions. The target genescould also be those of other cytokines or growth fac-tors, which in turn influence and regulate the activi-ties of cells and cell to cell interactions (22, 169, 205,240). The various substances that affect matrix syn-thesis and degradation have been recently reviewed(208).

The manner in which fibroblasts respond to vari-ous agents depends upon several factors such as thestage of cell cycle and age of the cells. Another factoris the local environment. Cell geometry is dictatedby the matrix in which the cells are embedded andthis determines the cellular response. Frequently theresponse of monolayer cultures is opposite of that ina three-dimensional matrix (89). For example, matrixsuppresses cell division and promotes differentiationwhile cells continue to divide in the absence of amatrix (206). The presence of more than one me-diator also affects the cellular response, and theircombined effect may be complimentary, contradic-tory, or additive. Thus, the type and concentration ofvarious substances present in the local environmentdetermine the manner in which cells respond andregulate the progression of healing and repair events.

The response to specific cytokines may vary fordifferent molecules and may depend on the celltype. For example, interleukin-1 enhances type VIIcollagen synthesis significantly, while type I is notaffected. Evidence indicates that fibroblast culturesobtained from some tissue explants consist of sub-types which differ in functional properties such asgrowth rate and collagen synthesis, and they re-spond differently to transforming growth factor-b,

45

interferon-g, prostaglandin E2 and other substances(2, 28, 124, 161). Selective interactions betweenfibroblast subpopulations and inflammatory me-diators have been shown to give rise to selection andenrichment of fibroblast subtypes, and the presenceof such subtypes is believed to be one factor con-tributing to disease phenotypes in inflammation andfibrosis (109, 145).

The activities of fibroblasts under healthy andpathological conditions may be influenced by factorsderived from epithelium. Fibroses and overgrowthsare often associated with enlarged epithelia, howeverthe interactions between epithelium and underlyingconnective tissue are poorly understood. Epitheliumappears to be a significant source of platelet-derivedgrowth factor and transforming growth factor-b inhealthy and wounded dermis and in gingiva (8, 65,99).

Role of matrix on inflammatory cell function

The inflammatory response involves an intricate in-teraction between the cells and the surroundingextracellular matrix. In addition to cell surface mol-ecules that facilitate the adhesion of neutrophils andlymphocytes to other cells (endothelial cells, fibro-blasts and keratinocytes), these cells must also inter-act with the molecules which comprise the extracel-lular matrix. To date, a number of extracellular ma-trix receptors have been identified and includemembers of the integrin family and other adhesionmolecules such as the cellular adhesion moleculesas well as molecules such as CD44. As these cellsmove through the extracellular matrix, molecularmechanisms (some of which remain poorly under-stood) require a system for recognition of matrixcomponents compatible with adhesion or migrationas well as the cellular machinery to allow the cells torespond to their environment. The role of intact ver-sus degraded extracellular matrix on inflammatorycell function is poorly understood. Nevertheless,changes in the composition of the matrix are likelyto have very significant effect not only on cell ad-hesion or migration but also cell functions as it re-sponds to its ever-changing environment.

Fibroblast–inflammatory cell interactions

During the early phases of gingival inflammation,the connective tissues are infiltrated by both neutro-phils and lymphocytes. Thus, the potential for inter-actions between these cells and the resident fibro-blasts is high. Lymphocyte fibroblast interactions

Bartold et al.

Fig. 12. Schematic representations of the possible interac-tions between fibroblasts and lymphocytes. Interactionsmay be either (A) via release of soluble mediators such ascytokines and prostaglandin E2 or (B) of a cognate (physi-cal) nature involving the expression of cell surface inte-grins, cell-cell contact and stimulation of cytokine pro-duction. Regardless of these mechanisms, it is clear thatthe fibroblasts participate in a significant manner at thelocal site of inflammation and as such must be considereda major player in the inflammatory response.

may contribute to the inflammatory reaction via therelease of soluble mediators following interactiveprocesses (Fig. 12). Early studies concerning thepathogenesis of periodontitis indicated that lympho-cytes exerted significant cytotoxic effects on gingivalfibroblasts either through the release of soluble me-diators or via direct cell-cell contact (194). More re-cently, studies have shown that adherence oflymphocytes to gingival fibroblasts induce the ex-pression of messenger RNA for interleukin-1a andinterleukin-6 (135, 138). Thus, provided the appro-priate cell surface receptors are available, the poten-tial exists for one cell to influence the other in eitheran autocrine or juxtacrine manner (134).

Fibroblast/lymphocyte interactions may also beinitiated by fibroblasts. For example, it has been

46

shown that ‘‘activated’’ dermal fibroblasts can regu-late the proliferative responses of T lymphocytes inboth a negative and positive manner (47, 64, 121,230). An inhibitory effect on T-lymphocyte prolifer-ation by interferon-g-stimulated gingival fibroblastshas also been reported (202). Furthermore, culturedhuman gingival fibroblasts have been found to ex-press HLA-DR antigens in vitro (10). These obser-vations led to speculation that gingival fibroblastsmay be able to act as antigen-presenting cells (203,234). Although exposure of gingival fibroblasts to in-terferon-gamma induces HLA-DR and intercellularadhesion molecule-1 expression, these cells wereunable to induce a proliferative response in alloreac-tive T lymphocytes. It was proposed that this wasdue to an inability of gingival fibroblasts to expressCD80, which normally facilitates the activation of Tlymphocytes (101, 203). Nonetheless, it is clear thatan interactive relationship between gingival fibro-blasts and lymphocytes does exist and these mayplay an important role in regulating lymphocytefunction.

Direct adhesive interaction between fibroblastsand lymphocytes may be one mechanism by whichlymphocytes within the gingival tissues becomelodged and contribute to ongoing tissue destruction.The adhesive interactions between lymphocytes andgingival fibroblasts has been studied and found tobe mediated, at least in part, by a combination ofvery late activation antigen integrins, CD44/hyalu-ronan and lymphocyte function–associated antigen/intercellular adhesion molecule-1 (134, 136–138).Another mechanism for lymphocyte/fibroblast inter-action has been proposed in which CD40 expressionby gingival fibroblasts allows interaction with CD40-ligand-expressing B cells and subsequent synthesisof interleukin-6 by the fibroblasts (199).

Interactions between fibroblasts and neutrophilshave been studied in other systems, but have beenlargely neglected within the periodontal environ-ment. As for lymphocytes, since neutrophils arelikely to come into contact with fibroblasts duringtheir passage through the tissues, it would seemimportant to establish and understand the natureof the interaction. Adhesion of neutrophils tofibroblasts has been demonstrated in severalmodel systems (37, 67, 204). Although some spon-taneous adhesion between resting fibroblasts andneutrophils occurs in vitro, this interaction can besignificantly stimulated through the addition of in-terferon-g, interleukin-1 and interleukin-6 (67). Theprincipal component involved in these interactionsseems to be the b2 (CD11/CD18) integrins with in-

Molecular and cell biology of the gingiva

tercellular adhesion molecule-1 playing only a mi-nor role (37, 204). The significance of such interac-tions is still unclear although short term co-culti-vation of fibroblasts with neutrophils does lead tocytotoxic effects mediated primarily through thegeneration of oxygen-derived free radicals (129).Lipopolysaccharide has been noted to promote ad-herence of neutrophils to periodontal fibroblasts,and this may be an important mechanism leadingto neutrophil mediated damage to periodontalfibroblasts (46).

Conclusion: gingiva – protector,regulator or harbinger of bad news?

Gingivitis and periodontitis are two of the most com-mon chronic inflammatory diseases affectinghumans as well as several, but not all, animal spe-cies. These diseases are the result of an induction ofhost inflammatory responses to the accumulation ofbacteria on tooth surfaces adjacent to the supragin-gival and subgingival tissues.

Initially, gingivitis represents a generalized acuteinflammatory response to the bacteria that colonizeon the tooth surface adjacent to the gingiva. Withtime, gingivitis may become well established but stillconfined to the superficial gingival connectivetissues and may manifest all the classic features ofa chronic inflammatory lesion. If the inflammatoryresponse contained within the gingivitis lesionspreads to the deeper periodontal tissues and al-veolar bone is lost, then the resultant lesion istermed periodontitis. The precise mechanisms gov-erning the progression of gingivitis to periodontitisare unclear. In some cases, gingivitis may representthe early stage in the evolution of periodontitis.However, in some individuals, gingivitis may exist asan independent clinical condition without pro-gressing into periodontitis (236). Indeed, the possi-bility exists that gingivitis and periodontitis are quiteseparate diseases.

Periodontitis is a family of related diseases thatdiffer in their causation, rate and pattern of pro-gression, natural history and response to therapy.Such variability can be attributed to differences incomposition of the microbial flora, together with thepresence of factors that might modify the host re-sponse to microbial assault as well as factors whichmay predispose the individual to bacterial coloniza-tion at specific sites. Of these, it seems that the mi-croflora composition, and the host modifying fac-

47

Fig. 13. Schematic representation of interactive processesbetween gingival epithelium and connective during theinitiation of gingival inflammation. PMN: polymorpho-nuclear lymphocytes. IL: interleukin. ICAM: intercellularadhesion molecule.

tors, are the most important with regard to manifes-tation of the various periodontal diseases.

While the host response and environmental fac-tors that affect this response are important for dis-ease manifestation, gingivitis and periodontitis can-not commence without the presence of bacteria.Nevertheless, it must be noted that, although bac-teria are necessary for disease initiation, they are notsufficient to cause disease progression unless thereis an associated inflammatory response. The latteroverrides its protective role and permits destructionto occur (151, 155).

A large number of bacterial species colonize theteeth in the supragingival and subgingival dentalplaque. For gingivitis to develop, the type of bacteriapresent is relatively inconsequential since gingivitisis a nonspecific inflammatory response to dentalplaque. However, approximately 20 microbes that in-habit the subgingival environment are considered tobe significantly pathogenic to be associated withvarious forms of periodontitis. The most significantbacteria associated with periodontitis are Actino-bacillus actinomycetemcomitans, Porphyromonasgingivalis and Bacteroides forsythus (7). An import-ant emerging concept with respect to the subgingivalmicroflora is that it behaves as a ‘‘biofilm’’ that per-mits the occupants to survive as a community andresist common host defense mechanisms as well asantibiotic exposure during therapy.

It is in the context of the above that the role ofthe gingival tissues becomes apparent (Fig. 13). Al-though the bacteria may be necessary for disease

Bartold et al.

induction, it is the manner in which the gingivalsulcular epithelium not only provides a barrierprotection role but, more importantly, initiates theearliest of signals of impending bacterial assault tothe underlying connective tissues that is critical.Through the release of these soluble messages,vascular changes are induced and neutrophils arerecruited to the site to help battle the accumulat-ing bacteria. In this sense the gingiva serves aprincipal protective role. However, with ongoingbacterial accumulation, the intercellular spaces ofthe sulcular epithelium widen and may provide anavenue for either bacterial penetration of thetissues or a means of permeation of the tissues bysoluble products produced by the bacteria. Duringthis phase, continuing communication signals areproduced by the epithelium, including the furtherproduction of cytokines and adhesion molecules,all of which act to further upregulate the underly-ing developing inflammatory response. By thisstage the epithelium has become the ‘‘harbinger’’of bad news and now dictates a more concerteddefense mechanism to be activated within the gin-gival connective tissues. The reactions occurringwithin the gingival connective tissues during thedevelopment of gingivitis include the classic fea-tures of chronic inflammation, with a balanceexisting between tissue destruction and tissue re-modelling. For the most part, the inflammatory re-sponse can be contained within the gingivaltissues and in this sense the gingiva again acts ina protective role. However, should the balance tiptowards uncontrolled tissue destruction, then re-sorption of the alveolar bone, together with loss offibrous attachment to the root surface and apicalmigration of the junctional epithelium ensues. As aresult, periodontitis develops and, once again, thegingiva becomes the harbinger of bad news.

The ever-present and ongoing interactions be-tween the gingival sulcus and the underlying con-nective tissues are most likely under the control ofthe gingival sulcular epithelium. The ‘‘decision’’ ofhow the lesion develops is played out in the com-plex milieu of the gingival connective tissue. To-gether these two tissues play the roles of protectorand regulator and provide many of the messagesto indicate impending damage. It is within thisframework that opportunities exist to consider howtherapeutic strategies (chemical, molecular or cel-lular) could be initiated to block, control or other-wise regulate these messages to aid in the manage-ment of development of the inflammatory peri-odontal diseases.

48

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