Tissues

Periodontology 2000, Vol. 3, 1993, 9-38 Printed in Derirnnrk . All rights reserved

C o p y r i g h t Q Miriiksgnard 1993

PERIODONTOLOGY 2000 ISSN 0906-6713

Tissues and cells of the

THOMAS M. HASSELL

Introduction

New knowledge in periodontology has evolved extremely rapidly in the past decade, including new knowledge about the structure and function of periodontal tissues. However, many questions remain unanswered and many problems unresolved. These relate mainly to understanding disease pathogenesis and therefore also to diagnosis and to therapy.

The periodontium is a dynamic structure that is served throughout life by a unique vascular arrangement, a lymphatic system and a highly specialized network of nervous elements. The anatomic features of the periodontium, as described early on by G. V. Black (16) and even earlier by others, anticipate this organ’s varied and important functions. Contempor- ary research directed toward elucidation of these functions represents a major endeavor of oral science today.

Ultimately, all biological research and all biological science, however esoteric it may appear to the layperson or beginning student, must relate itself to clinical observations and to clinical situations. If periodontal disorders are to be diagnosed and treated and ultimately prevented, a thorough understanding of the origin, development and structure of the periodontium must be gained. Only knowledge of structure will allow function to be evaluated or even perceived. Only the combined knowledge of structure and function will enable possible pathological deviations in oral tissues to be recognized.

Although dental investigators have progressed beyond simple observation and recording and into hypothesis-based examination of cause-and-effect relationships among the myriad of factors that control tissue homeostasis and therefore normalcy within the periodontium, the clinical relevance of the effort is the possibility that it may enhance the practitioner’s ability to cure and to do no harm (pr i - mum nil nocere). This remains the societal basis for the great cost associated with continuing the search

for understanding of periodontal structure and function beyond histology and physiology, into cellular and molecular levels of inquiry.

The purpose of this chapter is to revisit the basic structural configuration of the periodontium and to portray the cellular elements whose normal functions maintain the homeostatic balance known clinically as periodontal health. This chapter presup- poses basic knowledge of human histology, biochemistry and physiology, but not of pathology.

External anatomic features

The periodontium is defined simply as “the tissues investing and supporting the teeth” (30). Thus, the periodontium is composed of the following tissues: alveolar bone, root cementum, periodontal ligament and gingiva. Of these, only the gingiva is visible clinically upon inspection of the healthy oral cavity. The normal gingiva (Fig. 1) is pink in color (salmon or coral pink) and is demarcated apically from the oral mucosa (usually deep red in color) by the mucogingival line, which is more or less obvious clinically depending on the degree of keratinization and pigmentation of the gingiva. The accumulation of melanin pigmentation in the gingiva is normal (Fig. 2), varies from individual to individual and is more frequently observed in blacks and Asians than in Cau- casians. The attached keratinized gingiva extends coronally from the mucogingival line and is firmly bound to the underlying periosteum by collagen fibers. The coronoapical width of attached gingiva can vary significantly from tooth to tooth (Fig. 3) and among different individuals; it tends to become wider with age ( 2 ) . There is no absolute minimum width of attached gingiva that is required for periodontal health (124). The surface of the gingiva frequently exhibits an orange peel-like appearance, referred to as stippling (Fig. 4); previously, many clinicians regarded unstippled gingiva as a sign of

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Fig. 1. Healthy gingiva of a 24-year-old woman. The free gingival margin (1) courses parallel to the cementoenamel junction. The gingival groove (2) can be discerned in some areas as a shallow linear depression demarcating the free gingival marginal from the attached gingiva (3). Visible also are the non-keratinized oral mucosa (4), the mucogingival line (5), interdental gingiva (6) and stippling (7).

Fig. 2. Melanin pigmentation in normal human gingiva. A. This 35-year-old black man exhibits moderate pigmentation in both maxilla and mandible, concentrated at and near the mucogingival line, with several “fingers” of pigmentation projecting incisally in a more diffuse presen- tation, such as between the maxillary central incisors and immediately mesial to both mandibular cuspids. B. Ex- tremely dense pigmentation in the attached and marginal gingiva surrounding the maxillary anterior teeth in this young black individual. Note the extremely sharp line of demarcation between the pigmented attached gingiva and the oral mucosa. Even the interdental papillae are densely pigmented.

Fig. 3. Diagram depicting the average coronoapical dimension of human keratinized (attached) gingiva. In the maxilla, the facial gingiva in the area of the incisors is expansive, but narrow around the cuspids and bicuspids. On the palatal aspect the attached gingiva is continuous with the keratinized palatal mucosa without any visual demarcation. In the mandible, the lingual gingiva in the area of the incisors is narrow, but wide on the molars. On the facial aspect the gingiva around the cuspids and first bicuspids is narrow, but wide around the lateral incisors.

Fig. 4. Clinical appearance of stippling in normal, healthy gingiva in a middle-aged Caucasian woman. The irregular pattern of depressions is everywhere apparent, even on the interdental papillae. The orange peel-like appearance is also evident on the free gingival margin; the gingival groove is clearly visible.

Fig. 5. Schematic depiction of the approximate location and dimension of the col in molar (M), premolar (P) and incisor (I) teeth in humans. The col resides immediately apical to the contact area between adjacent teeth, and is considerably broader (buccolingually) in posterior segments. Col tissue is absent if diastemata exist.

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Tissues and cells of the periodontiurn

incipient pathological alteration, but today it is widely accepted that unstippled gingiva can exhibit a histological picture identical to that of perfectly healthy and stippled gingiva. The free marginal gingiva, which is normally about 1.5 mm in the coronoapical dimension, surrounds but is not attached to each tooth, and its internal surface thus forms one lateral aspect of the gingival sulcus (see below). The free marginal gingiva can often be visualized clinically by the presence of the gingival groove, a shallow depression on the facial gingival surface that roughly corresponds to the base of the gingival sulcus. In health the gingiva completely fills the em- brasure spaces between teeth, as the interdental gingiva or gingival papilla. The coronal height of the papilla resides immediately apical to the contact area of any two adjacent teeth. In the posterior segment of the mouth, where the contact area between teeth is usually broad, the interdental gingiva consists of two papillae, one vestibular and one oral, connected by the col (Fig. 5). The col is non-keratinizing epithelium, representing essentially the fusion of the interproximal junctional epithelia of the two adjacent teeth. It is a structure that is particularly susceptible to noxious substances or physical trauma and, as such, represents a most frequent site for the initiation of the pathological breakdown of the tooth-supporting apparatus. Where adjacent teeth do not contact each other or when a tooth is missing from the arch, keratinized attached gingiva courses uninterruptedly across the alveolar ridge fa- ciolingually; in such locations, interdental papillae and the col are absent.

Microstructural anatomy of gingival epithelium

Oral gingival epithelium

Gingival epithelium provides the protective integument of the periodontium. The histological appearance of human gingival epithelium has been studied in great detail and described by numerous authors (74, 97, 101). Free marginal and attached gingivae are covered by a typical keratinizing stratified squamous epithelium consisting of the 4 classical epithelial strata: basale, spinosum, granulosum and corneum (Fig. 6). The stratum basale represents the germinative layer; its cells are mainly cuboidal in appearance and are attached to the underlying basal lamina by means of hemidesmosomes. The basal cells are also attached laterally to each other via both gap junctions and hemidesmosomes; elaborate

interdigitations of the plasma membranes of basal cells often exist along the lateral cell-cell approxi- mations. Some but not all cells of the stratum basale migrate through the entire epithelial thickness and eventually keratinize; these are known as keratinocytes. As with cells in other mammalian epithelia, following cell division in the basal layer and the re- sultant birth of a new keratinocyte, about 10 days are required for the new cell to traverse the epithelium to reach the stratum corneum (112). This tem- poral interval is known as the epithelial cell turnover time. Another cell type typically observed in the stratum basale is the melanocyte (98), a stellate cell with numerous dendritic processes that is present in an approximately constant density among individuals and races. Unlike keratinocytes, melanocytes are attached neither to the subjacent basal lamina nor to adjacent cells of the stratum basale. The only known function of melanocytes is to produce melanin, a pigment that serves to protect against the ionizing effects of electromagnetic radiation, and to distrib- ute it to keratinocytes. The transfer process occurs via phagocytosis by the keratinocyte of the tip of the melanocytic process of the melanocyte (54). The intensity of skin color and presumably of gingival color as well is determined by the number and aggre- gation pattern of melanosomes present within keratinocytes (121). Yet another resident of the gingival epithelium is the Langerhans cell, which derives from a progenitor cell in bone marrow. As a resident of the gingival epithelium, however, it maintains its ability to undergo mitosis; it has recently been demonstrated that Langerhans cells harbor surface antigens resembling those of host defense cells (such as lymphocytes and macrophages) and that they also possess receptors for immunoglobulin and comple- ment. Thus, it has been proposed that these cells play a pivotal early role in the host response to microbiological insult at the gingival margin (78). Clearly, then, the cells of the basal layer of gingival epithelium are responsible for the important functions of protecting underlying structures and producing new epithelial cells; in addition, these cells synthesize and secrete the macromolecules that constitute the basal lamina that separates the stratum basale from underlying connective tissue. Nu- tritive supply for and disposal of metabolic by-products of gingival epithelial cells are provided via capillary loops within connective tissue rete that project into the basal surface of the epithelial layer (55) (see also below).

Immediately suprajacent to the stratum basale is the stratum spinosum, composed of cells that no

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Fig. 6. Light photomicrograph of the epithelium covering a normal, healthy human interdental papilla. The 4 classical epithelial strata are easily discernible. Intercellular bridges are clearly visible in the stratum spinosum (magnification ~ 4 3 ) .

longer synthesize basal lamina constituents and that contain an elevated quantity of cytoplasmic filaments. This epithelial layer is characterized at the light microscopic level by apparent “bridges” between adjacent cells (Fig. 6). Upon the spinous layer one sees the stratum granulosum with its flattened cellular elements containing keratohyaline granules and enzyme-containing Odland bodies. Desmosom- al junctions among cells of the granular layer are more frequently observed than in subjacent layers, and elongated gap junctions are more predominant also. As cells approach the outermost layer of oral epithelium, the stratum corneurri, the intracellular process of keratinization nears completion as the cells are transformed into corneocytes, which appear compressed flatly parallel to the surface of the gingiva and lack any nuclei at all (97). In addition, most of the intracellular synthetic and respiratory machinery (such as Golgi bodies, mitochondria and endoplasmic reticulum) is lost, resulting in a keratin-filled superficial oral epithelial cell that does not correspond morphologicallv to its counterpart in the

Fig. 7. Electron photomicrograph depicting the basement membrane of normal human gingiva. Note the tonofilaments that converge (or emerge?) at the attachment plaques of two adjacent hemidesmosomes. Collagenous filaments of the subepithelial lamina propria are revealed in cross-section and longitudinally (magnification ~64 ,000) .

stratum corneum of, for example, skin (102). How- ever, despite the rather abrupt and distinct intracellular and extracellular alterations in cellular contour and configuration that would appear to reflect cellular degradation, the cell-cell attachments remain for the most part intact in the stratum corneum.

Schroeder & Page (103) have summarized the events of continuous differentiation that occur as a new basal cell derives from a mitotic event in the stratum basale and makes its way toward the intra- oral epithelial surface:

cells lose the ability to multiply by mitotic division; cells produce elevated amounts of protein, and accumulate keratohyalin granules, keratin filaments and rriacromolecular matrix in their cytoplasm; cells lose the cytoplasmic organelles responsible for protein synthesis and energy production; cells eventually degenerate into a cornified layer due to the process of intracellular keratinization, but without loss of cell-cell attachment; and

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Tissues and cells of the periodontium

0 cells are finally sloughed away from the epithelial surface and into the oral cavity as the cell-cell attachment mechanisms (that is, hemidesmosomes and gap junctions) ultimately disintegrate.

As the tissue whose function it is to provide protection for underlying periodontal structures, the gingival epithelium rests upon and is intimately associated with a basement membrane (Fig. 7), which separates the epithelium from the immediately subjacent connective tissue compartment of the gingiva (lamina propria). The electron microscope reveals this basement membrane as a basal lamina consisting of two distinct layers: the lamina lucida and the lamina densa. The former is in immediate contact with the epithelial cells of the stratum basale, to which it is connected via hemidesmosomes, whereas the latter (lamina densa) contacts the connective tissue compartment and is attached firmly to it via anchoring fibrils (80) that have been described as originating from the hemidesmosomes of the epithelial basal cells, traversing the basement membrane and terminating in the most superficial layer of the connective tissue (122). Thus, the basement membrane separates the oral epithelium from the gingival connective tissue compartment and provides a confluent film-like layer that connects the epithelium to the connective tissue. Gaseous or nutritive exchanges must therefore occur across the intact basal lamina. Similarly, because intercellular communication normally occurs by means of effec- tor molecules (such as activation factors or growth factors; see Hefti and Walters in this volume), the basal lamina assumes another important function since it has been demonstrated that signals emanating from connective tissue directly influence epithelial maturation and differentiation. Basal lamina is comprised primarily of proteoglycans and water- insoluble collagenous macromolecules (28, see also Mariotti in this volume).

Oral sulcular epithelium

The oral sulcular epithelium (see no. 2 in Fig. 8) is the extension of the oral gingival epithelium into the gingival sulcus. Its coronalmost boundary is therefore the height of the free marginal gingiva and its apicalmost boundary is the sloughing surface of the junctional epithelium (see below); thus, the oral sulcular epithelium forms one lateral wall of the gingival sulcus. The oral sulcular epithelium exhibits the same 4 epithelial strata as oral gingival epithelium, but a definitive and continuous cornified layer is ab-

sent. Although the surface of the oral sulcular epithelium may sometimes exhibit a degree of paraker- atinization, a primary distinguishing feature between it and the oral gingival epithelium is that the oral sulcular epithelium does not keratinize. In histological cross-section (Fig. 8 and 91, the oral sulcular epithelium can be sharply demarcated visually from the adjacent junctional epithelium because the cells of the oral sulcular epithelium appear much darker due to elevated basophilicity of these cells (104). Transmigrating leukocytes are only seldom observed in the oral sulcular epithelium owing to its lack of permeability (vis-A-vis the junctional epithelium). It has been speculated that the lack of keratinization of the oral sulcular epithelium may play a role in rendering the gingival sulcus more susceptible to attack by pathogenic periodontal microorganisms, and some reports (23, 33) have demonstrated keratinization of the oral sulcular epithelium in response to physical stimulation (such as sulcular brushing), but a keratinized inner surface of the oral sulcular epithelium should be viewed as a pathological change without favorable consequences in terms of gingival health.

Junctional epithelium

The third (and most intriguing) component of the epithelial integument of the periodontium, in addition to the oral gingival epithelium and the oral sulcular epithelium, is the junctional epithelium (Fig. 8-10). It is the junctional epithelium that provides the attachment mechanism of the epithelium to the surface of tooth hard substance (enamel or cementum, or dentin).

The junctional epithelium also provides a protective function relative to the subjacent periodontal ligament. One of the initial events in inflammatory gingival and periodontal diseases is the destruction of the normal relationships among the components of the attachment apparatus. There is an intimate association of both the periodontal ligament (see below) and the epithelial attachment to the tooth surface. Both of these structures are dynamic in terms of alterations in attachment location and quality with changes in age, health status and local environment. Thus, the discussion that follows emphasizes function more than purely anatomic relationships.

The history of the accumulation of knowledge of the development, structure and dynamics of the epithelial attachment, although fascinating, has been reviewed and retold since 1971 and need not be re- capitulated here. The now classic monograph by

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Fig. 8. Gingival sulcus, oral sulcular and junctional epithelia, viewed in histological cross-section. The flat and vertically elongated cells of the junctional epithelium (1) are oriented parallel to the tooth surface and are sharply demarcated (dashed line) from the more deeply staining cells of the oral sulcular epithelium (15). All cells of the junctional epithelium originate from mitotic activity in the stratum basale; these cells migrate (arrow) in an apic- ocoronal direction toward the bottom of the gingival sulcus (4) into which they are sloughed. The stratum basale of the junctional epithelium is normally 1.5-2 mm in axial expanse, whereas the sulcus bottom may be only 100-150 pm across. Note the polymorphonuclear leukocytes (circled); such host defense cells emigrate from the subepithelial Venus plexus in the lamina propria (3), then - - transmigrate the junctional epithelium to be shed into the gingival sulcus.

is located at the cementoenamel junction. The cuboidally shaped cells of the stratum basale (B) are in constant mi-

Fig. 9. This schematic depiction of the structures of the marginal periodontium as viewed in buccolingual section reveals the 3 major components of normal gingiva: oral gingival epithelium, junctional epithelium and the connective tissue compartment. The arrows indicate the mi- gratory pathway of cells originating from the basal layer of the junctional epithelium. The normal junctional epithelium is about 1.5-2 mm in the coronoapical dimension, and the width at the bottom of the gingival sulcus is 0.15 mm. The histologic depth of the sulcus is 0.5 mm, but clinical probing usually gives values of 1-3 mm, as the

gration (arrows) coronally, toward the gingival sulcus. Junctional epithelial cells that come into contact with hard tooth structure (enamel, cementum) establish an attachment through formation of hemidesmosomes (see Fig. 1 l) , enabling them to attach to the internal basal lamina (IBL), thus to the tooth. Note that the internal basal lamina courses around the apicalmost cell of the junctional epithelium to become continuous with the external basal lamina (EBL), which demarcates the junctional epithelium from the subjacent connective tissue compartment.

probe tip penetrates the junctional epithelium to greater

picted in greater detail in Fig. 8, 1 1 and 12, respectively.

Fig. l l . Expanded schematic depiction of the detail of

the lamina lucida (LL) and the lamina densa (LD). Note

Or lesser extent* The circled segments and are de- the internal basal lamina (IBL). It consists of two layers:

Fig. 10. Schematic depiction of the apicalmost segment of the junctional epithelium of a healthy young patient, where the base of the junctional epithelium (black wedge)

that hemidesmosomes (HD) originate from the lamina lucida, and tonofilaments splay out from each hemides- mosome.

Schroeder & Listgarten (100) described the structure that is the junctional epithelium; it is in intimate of the epithelial attachment in terms that are univer- contact with the exterior surface of the tooth ce- sally accepted today. mentum or enamel. Functionally, the epithelial

In anatomic terms, the epithelial attachment attachment is best conceived as a biological mech- mechanism consists of a basal lamina and hemides- anism by which specialized epithelial cells adhere mosomes, both of which are produced by the spe- organically to calcified dental tissues, thus protect- cialized band of longitudinally oriented epithelium ing the underlying structures (such as the peri-

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odontal ligament) from invasion by foreign substances. As a dynamic mechanism, the junctional epithelium renews itself continuously throughout life. New cells are produced at its base, migrate coronally and slough at its surface, all the while maintaining biological attachment to the tooth.

The attachment of gingival soft to dental hard tissue develops as a sequence of events that occur as the tooth erupts through the oral mucosa. Prior to eruption of the tooth into the oral cavity, the surface of the enamel is covered by the reduced enamel epithelium, which is actually the remains of the enamel organ and thus comprises the reduced (post-) ameloblasts and the former stratum intermedium. These cells produce a basal lamina called the epithelial attachment lamina, which is in contact with the hard tissue structure in the absence of any intervening layers. It is to this lamina that the epithelial cells attach, via hemidesmosomes, to form the primary epithelial attachment in unerupted teeth.

As tooth eruption is then initiated (by forces even today not clearly understood), the reduced ameloblasts and some cells of the stratum intermedium undergo a dramatic transition into synthetically active junctional epithelial cells, exhibiting extensive Golgi complexes, endoplasmic reticulum and cytoplasmic filaments. Mitosis occurs in this cell layer and in the basal layer of the oral epithelium. As these events proceed and the tooth erupts through the oral mucosa, the attachment of soft to hard tissue is termed the secondary epithelial attachment, which may consist simply of a basal lamina (the epithelial attachment lamina) and hemidesmosomes. How- ever, as eruption continues, the structural relationships become more complex. If enamel is denuded of its epithelial covering (such as at the cementoenamel junction during eruption or possibly other areas of the crown), cells of the neighboring connective tissue elaborate a matrix over the exposed enamel that may calcify as afibrillar cementum; an epithelial attachment can also form on such calcified matrices. Furthermore, in some cases a dental cuticle, presumed to be a secretory product of epithelial cells, may intervene between the tooth surface and the secondary epithelial attachment, but its appearance is often irregular.

Thus, the junctional epithelium, the collar of epithelium constituting the epithelial attachment mechanism of soft to hard tissue, develops from cells of the stratum intermedium and the post- ameloblastic layer. These cells produce the attachment mechanism - a basal lamina and hemidesmosomes (Fig. 11) - which is structurally identical re-

gardless of whether the attachment is directly to calcified tooth structure or a dental cuticle or afibrillar cementum intervene.

When tooth eruption is complete, the junctional epithelium extends from the base of the gingival sulcus to an area at the cervix of the tooth near the cementoenamel junction. The junctional epithelium is widest (20-30 cell layers) at its most coronal aspect and tapers to only a few cells at the cementoenamel junction (Fig. 10). Processes of adjacent junctional epithelial cells interdigitate less frequently than cells of the oral epithelium or oral sulcular epithelium. Even in health, the junctional epithelium always exhibits transmigrating neutrophils, which make their way coronally to be shed into the gingival sulcus at the sulcus bottom.

The junctional epithelium is bordered on one aspect by the tooth surface with the intervening internal basal lamina (Fig. 10). On the other side, where the junctional epithelium interfaces with the connective tissue compartment, the external basal lamina intervenes. These 2 laminae, which are probably identical in origin and composition, are continuous around the tip of the most apically positioned junctional epithelial cell. Cells adjacent to the internal and the external basal laminae, at least at the apical termination of the junctional epithelium, are essentially basal cells in both appearance and capacity for self-replication. However, the other 3 normal epithelial strata (spinosum, granulosum and corneum) are absent from the junctional epithelium; rather, all of the intervening junctional epithelial cells may be considered simply as a suprabasal cell layer. The suprabasal layer consists of cells that are more flattened and elongated, with their long axes oriented in a coronoapical plane, which would appear to facilitate cell migration in a coronal direction. The suprabasal cells and all cells along the internal basal lamina are in constant coronal migration toward the base of the gingival sulcus, where they are sloughed at the coronalmost surface of the junctional epithelium. The junctional epithelium is, therefore, a highly specialized structure. It is a fully mature, functional, highly dynamic epithelial tissue composed of only 2 cell layers. The turnover rate of the cuboidal cells in the basal layer is extremely rapid, as demonstrated by investigators using auto- radiographic techniques in experimental animal models.

That cells other than the embryologically reduced ameloblasts and stratum intermedium cells can differentiate into junctional epithelial cells has been demonstrated by complete surgical removal of all

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marginal gingival tissue, including the entire junctional epithelium, in animals. A new junctional epithelium forms during the wound-healing process and its appearance is identical to a post-eruptive junctional epithelium. The cells of such newly formed junctional epithelia take their origins from basal cells of the remaining oral epithelium or the oral sulcular epithelium. In periodontal therapy, post-surgical regeneration of an elongated junctional epithelium is now recognized as an undesir- able sequela, because it appears to prevent the formation of new cementum and the subsequent new attachment of connective tissue fibers to the denuded tooth surface. A fair amount of research is currently directed toward preventing such post-surgical overextended reformation of the junctional epithelium, through the use of chemical root treat- ments and the placement of various physical membrane barriers.

Current knowledge of the structure and function of the junctional epithelium permits definition of the anatomic boundaries of the gingival sulcus. The base of the gingival sulcus comprises the coronalmost aspect of the junctional epithelium. This is the sloughing surface of the junctional epithelium, where spent epithelial cells and transmigrating neutrophils are shed into the oral cavity. One wall of the gingival sulcus is comprised of toothhard structure. In health, this wall is enamel, but with age, apical migration of the gingival margin may occur, and the sloughing surface of the junctional epithelium may come to be located apical to the cementoenamel junction. In such a case, the cementum (or exposed dentin) will form the lateral wall of the gingival sulcus. The other wall is represented by soft tissue, the oral sulcular epithelium (see above). The demarcation of the oral sulcular epithelium with the junctional epithelium appears dramatically in histological sections (Fig. 8). The demarcation is not gradual: there does not appear to be any blending of the junctional epithelial cells and the oral sulcular epithelium in health; yet, there is no definitive laminar structure intervening between the two epithelia. However, the oral sulcular epithelial cells do have the capacity to differentiate into junctional epithelial cells if the latter are severely compromised by disease or lost entirely, such as after surgery.

The importance of maintaining normalcy (homeostasis) in the gingival sulcular region cannot be overemphasized. This is the site at which in- flammaton gingival and, ultimately, periodontal diseases originate. This is the site where microbial ac- cumulations (dental plaque) accrue, eliciting a

response from the body’s immune system. Such a response appears designed evolutionarily to ward off the noxious substances deriving from plaque bacteria, but in many instances it seems also to influence negatively the normal functions of the epithelial and connective tissue cells (immuno- pathology). The gingival sulcus is the area in which dentists determine whether disease is present in a patient, and this is the locus where severity of disease is assessed. It is in the sulcular area that any requisite periodontal therapy will be instituted. It is toward the sulcus that the patient must target rigor- ous home care measures.

Microstructural anatomy of gingival connective tissue

Fiber groups of the lamina propria

Beneath the epithelial integument resides the gingival connective tissue compartment, also referred to as the lamina propria. Connective tissue accounts for the major proportion of the free marginal gingiva, the interdental gingiva and the attached gingiva, including both facial and oral aspects of these structures. The lamina propria exhibits an exquisitely complex architecture, which anticipates its functions. Approximately 60-65% of the connective tissue compartment of healthy gingiva is occupied by collagen, with the individual fibrils highly organized into discrete and easily discernible fiber bundles; this arrangement was well described early on by Arnim & Hargerman (3) and more recently in much deeper detail, both histomorphometrically and biochemically (99). The other, less ubiquitous elements comprising the lamina propria of non-inflamed human gingiva include fibroblast cells (about 5%), other cells (such as various leukocytes, mast cells, tissue macrophages, etc., 3%), with about 35% of the remaining volume accounted for by vascular elements (blood and lymph), nerves and ground substance (99). The ground substance (see Mariotti in this volume) is composed mainly of various glycoproteins and proteoglycans (75).

Even in a relatively low-power light photomicrograph, the delicate discreteness of the architecture of the gingival collagen fiber bundles can be readily appreciated. Collectively, these fiber bundles have been referred to as the gingival ligament, which is contiguous with the periodontal ligament (see below). The fiber groups constituting the gingival ligament vary in spatial orientation and in size. There are 5 principal fiber bundle groups as well as 6 mi-

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nor groups, the latter having been described rather more recently (84). The principal groups include dentogingival fibers, alveologingival fibers, circular fibers, dentoperiosteal fibers and transseptal fibers (Table 1). The secondary or minor collagen fiber bundles are the periostogingival, interpapillary, transgingival, intercircular, intergingival and semicircular fibers (Table 1). Although the named fiber bundle groups are discrete histological entities, it is commonly observed that individual fibrils and groups of fibrils branch off from the major trunk and intertwine with adjacent major fiber bundles; thus, the gingival fiber architecture is not only composed of strong major elements, but these elements provide support for each other as well. The main functions of the gingival ligament complex are to provide tone and resistance for the free gingival margin as well as contour and support in the attached gingiva. In addition, these fiber bundles provide the most coronally positioned connective tissue attachment to the tooth surface.

Fig. 12 and 13 schematically depict the connective tissue compartment of gingiva and its anatomic re-

lationship to the other structures that comprise the periodontium. The dentogingival fiber bundles originate from the cementum just apical to the cementoenamel junction and then splay out into the free marginal and attached gingiva as 3 relatively well- demarcated subgroups (sometimes referred to as A, B and C subgroups). One subgroup turns coronally and obliquely into the free gingival margin, where it appears to provide support for the junctional epithelium and the interdental papilla. A second subgroup courses laterally into the attached gingiva and the apicalmost segment of the free gingival margin. The third subgroup turns apically from the cementum near the cementoenamel junction, appear- ing to sweep down and across the crest of the alveolar bone and into the attached gingiva. Some elements of this subgroup may actually insert into the osseous alveolar crest, co-mingled with the dentoperiosteal fiber bundle.

The alveologingival fiber bundles emanate from the periosteum covering the height of the alveolar crest and splay coronally into the substance of the attached gingiva, terminating in the free gingival

Table 1. Structure and function of collagen fiber groups in gingiva Name of fiber group Origin and orientation Supposed function Principal groups

Dentogingival

Alveologingival

Dentoperiosteal

Circular

Transeptal

Secondary groups Periostogingival

Interpapillary

Transgingival

Intercircular

Intergingival

Semicircular

From cementum, splay laterally into lamina propria From periosteum of the alveolar crest, splay coronally into lamina propria From cementum near the cementoenamel junction, into periosteum of the alveolar crest Within free marginal and attached gingiva coronal to alveolar crest, encircle each tooth (“purse string”) From interproximal cementum coronal to alveolar crest, course mesially and distally in interdental area into cementum of adjacent tooth

From periosteum of the lateral aspect of alveolar process, splay into attached gingiva Within interdental gingiva (gingival papilla), orofacial course Within attached gingiva, intertwining along the dental arch between and around the teeth From cementum on distal surface of a tooth, splaying buccally and lingually around adjacent tooth and inserting on mesial cementum of next tooth Within attached gingiva, immediately subjacent to epithelial basement membrane, course mesiodistally From cementum on mesial surface of tooth, course distally, insert on cementum of distal surface of same tooth

Provide gingival support

Attach gingiva to bone

Anchor tooth to bone; protect periodontal ligament Maintain contour and position of free marginal gingiva

Maintain relationships of adjacent teeth; protect interproximal bone

Attach gingiva to bone

Provide support for interdental gingiva

Secure alignment of teeth in arch

Stabilize teeth in arch

Provide support and contour of attached gingiva

None intuitively obvious

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Fig. 13

Fig. 12. Architecture of the healthy periodontium. Colla- gen fiber bundles represent the major components of the connective tissue compartment (A) of both free marginal and attached gingiva. The principal fiber groups (see text and Table 1) originate from root cementum and splay out into the connective tissue compartment, forming a highly organized construct. Secondary (minor) fiber groups course more or less horizontally within the gingiva, and between and around individual teeth (see also Fig. 13). The periodontal ligament space (B) also contains discrete collagen fiber groups (see text and Table 3). Note that many collagen fibers are embedded on the external aspect of the alveolar bone (C), splaying into the attached gingiva. Collagen fibers representing the periodontal ligament (13) are attached on the internal surface of the alveolus (cribriform plate). Numerical legend for Fig. 12 and 13:

1 . Dentogingival fibers

margin facially and lingually and in the interdental papillae mesially and distally.

The circular fiber bundles do not attach to any calcified structure; they encircle each tooth within the substance of the gingiva near the cervix, in a manner that has been described as a purse string. Circular fiber bundles appear to be smaller in diameter than the other principal components of the gingival ligament.

Dentoperiosteal fiber bundles are anchored in the cementum near the cervix of each tooth, but apical to the dentogingival fibers. They traverse a relatively short distance to insert into the crest of the alveolar process and onto the lateral aspect of the cortical plate. Some of the dentoperiosteal fibers may also insert into the muscles of the oral vestibule or the sublingual sulcus.

Transeptal fiber bundles are located exclusively in the interdental tissue coronal to the crest of the interseptal bone. These fibers span the interdental

2. Alveologingival fibers 3. Interpapillary fibers 4. Transgingival fibers 5. Circular and semicircular fibers 6. Dentoperiosteal fibers 7. Transeptal fibers 8. Periostogingival fibers 9. Intercircular fibers

10. Intergingival fibers

Fig. 13. Schematic depiction of a horizontal section cut mesiodistally at about the level of the cementoenamel junctions of two molars and a bicuspid. The courses of the various supracrestal gingival fiber bundles are depicted. This schematic clearly reveals the purposeful architecture, with some fibers connecting tooth-to-tooth, others connecting tooth-to-gingiva, and others free within the gingiva providing support and contour. The numbered fiber groups refer to the legend of Fig. 12.

space, with their ends inserted into the cervical cementum of the mesial or distal neighbor tooth. The transeptal fibers are best viewed in histological sections cut along the mesiodistal plane, although they can also be discerned in transverse sections. These fibers provide support for the interdental gingiva and probably also aid in securing the positions of adjacent teeth and maintaining the integrity of the dentition within the dental arch.

Of the secondary gingival fiber groups, the periostogingival fibers are the most ubiquitous. These fibers originate in the periosteum on the lateral aspect (facial and oral) of the alveolar bone and splay laterally, coronally and apically to provide support and tone within the attached gingiva that is apical to the underlying alveolar crestal bone. Interpapillary fibers are located within the substance of the interdental papilla coronal to the transeptal fiber group and course in an orofacial direction, providing support for the interdental

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gingiva. The transgingival fibers describe a slalom- like course around the dental arch, winding in a serpentine fashion in-and-out between the teeth coronal to the cementoenamel junction. These fibers probably serve to maintain tissue consist- ency, enhance arch alignment and provide additional support for the marginal gingiva.

Intertwining of transgingival fibers with fibers of the circular group is common. The intercircular fibers originate from cementum near the distal line angles on the oral and vestibular aspects of each tooth, leapfrog distally around the adjacent tooth and insert into the mesial cementum of the next distal tooth. In the posterior segment of the arch, these intercircular fibers appear to fuse distal to the last tooth in the arch. These fibers likely aid in maintaining arch integrity. The semicircular fibers also originate in mesial cementum, then course distally to insert into cementum on the distal surface of the same tooth, thus forming a half-ring about each tooth on both oral and facial aspects. These fibers are at approximately the same height within the gingiva as the circular fibers, with which some branches intertwine, thus helping with support of the free marginal gingiva. The intergingival fibers follow a path just subjacent to the epithelial basement membrane, coursing in a mesiodistal direction within the attached gingiva, giving it form and contour; these fibers, like the circular group, do not insert into any calcified structure.

It should be clear from these descriptions that the various fiber groups of the gingival ligament are highly interdependent. Damage to or loss of one or several groups reconciles other fibers to assume additional functions. Inflammatory disease processes within the gingiva lead rapidly to loss of collagen in discrete portions of affected tissue and to collapse of the support provided by fiber bundles. It is clear also how the loss of a tooth or several teeth can have serious consequences throughout the dental arch, as continuity among the fiber bundle groups is severed. It is the fiber bundles that protect and support the delicate junctional epithelium and maintain turgor within the interdental gingiva. Perhaps most import- antly, the complex of interdependent fiber bundles that characterize healthy gingiva also serves to shield the periodontal ligament from insult.

Cells of the connective tissue compartment

The periodontium is a dynamic structure that must respond continually to the stresses and insults brought to bear upon its component tissues. The

cells normally present in the soft connective tissues of the periodontium (gingiva and periodontal ligament) reflect this dynamism. Individual cells comprise approximately 8% by volume of the connective tissue compartment (99).

All types of blood cells are present, the great majority of them within the periodontal vasculature. In histological sections cut through normal human gingiva, blood cells can be observed in the connective tissue compartment, mostly inside the lumen of the periodontal vasculature, but also within the lamina propria among the collagenous elements, usually perivascularly; the full range of leukocytic cells is also observed. Although neutrophils (Fig. 14) are only rarely observed within the substance of healthy gingival connective tissue, they do frequently emigrate from the subepithelial vascular plexus to enter and transmigrate the junctional epithelium, probably directed by substances deriving from plaque microorganisms, substances that have been shown to be chemotactic for polymorphonuclear leukocytes (48, 96). The neutrophils thus represent the host defense mechanism that is deployed precisely at the front line in the periodontium. These cells are capable of moving toward offensive microbes and engulfing them (phagocytosis), with subsequent kill- ing and by intracellular enzymes including myelo- peroxidase, lysozyme and alkaline phosphatase, among others. Polymorphonuclear leukocytes can also destroy toxic substances in the gingiva, such as necrotic tissue or immune complexes. Tempering these talents of the polymorphonuclear leukocyte, however, is the fact that these cells also can participate in destroying host tissue, through the action of the collagenase, acid hydrolases and neutral proteases they synthesize (24). These substances can break down collagen as well as other connective tissue components (such as proteoglycan ground substance). Because of the omnipresence of neutrophils in gingival tissues, it is likely that - even in health - these cells play an important role in periodontal homeostasis.

Plasma cells as well as small and medium-size lymphocytes (Fig. 15) are also commonly observed in the lamina propria of gingiva (111) excised from sites that are devoid of clinical signs of inflammation. Such cells are usually observed either perivascularly or close to the boundary between the connective tissue compartment and the junctional epithelium; there, these cells (mostly T cells) are believed to participate in the host response to minor insults (such as microbiological and chemical) borne by the periodontal tissues on a continuous basis

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Fig. 14. Neutrophil within normal human gingival connective tissue. Notice the dense peripheral heterochromatin and lobulation of the nucleus. Cytoplasmic granules of different sizes and densities are numerous. These granules are probably specific and azurophilic, but exact identification requires histochemical techniques. Colla- gen fibers cut longitudinally and in cross section are evident (uranyl acetate and lead citrate staining, magnification X25,900).

Fig. 15. Lymphocytes within normal, healthy human gingival connective tissue. A. Small lymphocyte (magnification X25,900). B. Medium-sized lymphocyte (magnification X 18,500) (uranyl acetate and lead citrate preparations).

Fig. 16. Mast cells from normal human gingival connective tissue. A. One mast cell from an infiltrated area of the connective tissue. Numerous cytoplasmic processes are evident. Abundant cytoplasmic granules occupy virtually the entire cytoplasm, obscuring all other organelles (magnification X7500). B. Higher magnification of a mast cell. Here the cytoplasmic granules can be seen to contain la- mellar or scroll-like structures. Mitochondria are also evident (magnification X25,920).

throughout life. Thus, they also contribute to homeostasis; in addition, they would seem to be primed to participate in an inflammatory response should one arise. The possible role of plasma cells in healthy connective tissues remains obscure. The abundance of endoplasmic reticulum exhibited by these cells is testament to their ability to synthesize immunoglobulin; it is likely that they are primed to respond to an appropriate antigenic signal.

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Mast cells (Fig. 16) are also commonly observed in histological sections of normal, healthy gingiva. These cells, which are often located near vascular elements, exhibit a characteristic histological picture consisting of large cytoplasmic granules. The granules are electron-dense, and they contain substances (such as histamine and heparin) known to elicit inflammation. Mast cells can release their potent vaso- active mediators by the process of degranulation; this is triggered when an allergenic substance causes the production of IgE antibody, which binds to the mast cell surface and perturbs the membrane (61).

Tissue macrophages (Fig. 171, which derive from blood monocytes, are a common finding even in non-inflamed connective tissues of the periodontium. These cells are capable of synthesizing and se- creting powerful hydrolytic enzymes, and it is rather universally accepted that the primary role of such cells in diseased tissue is one of scavenger, dispens- ing with bacteria, debris and toxic substances (29). The role of macrophages in healthy tissue is less well defined, although it is in an excellent position to direct or orchestrate homeostasis within the periodontium (51, 64). It has been demonstrated that, if macrophages are eliminated as functioning cells, the consequences for wound healing and tissue homeostasis are dramatic and negative (63, 110, 120). Macrophages that have been activated, for example, by bacterial products or lymphokines secreted by other cells (42, 86) synthesize and release a broad array of potent substances that can regulate connective tissue turnover. These substances may stimulate fibroblast proliferation (371, which enhances collagen production (53) and fibroplasia (25). Activated macrophages also release neutrophil chemotactic factor, prostaglandins, interleukin- 1 and factors that regulate the immune response (31, 59, 60). These and other properties of macrophages have caused them to be considered both competence and pro- gression factors (11). On the other hand, macrophages also participate in the breakdown of host tissues, a fact that has come to light only recently. For example, macrophages that encounter various lymphokines may respond by producing and secret- ing various neutral proteases (such as collagenase) that rapidly destroy connective tissues. Other macrophage products are known to inhibit collagen production by fibroblasts (62, 123). Bertolami & Bronson (11) have proposed that the diverse range of anabolic and catabolic functions ascribed to various macrophage-derived substances may reside in an inherent heterogeneity of macrophage (and monocyte) subpopulations. This concept, which

would support the notion of a major role of macrophages as regulators of tissue homeostasis in health, has found substantiation in studies that demonstrated functional heterogeneity among macrophage subpopulations from several species (58).

Fibroblasts

By far the most common cell in the periodontal connective tissue, and the most important functionally, is the fibroblast (Fig. 18, 19). Sixty-five percent of the cells in gingival connective tissue are fibroblasts (99). The role of this cell is to produce the structural connective tissue proteins, collagen and elastin, as well as the glycoproteins and glycosaminoglycans that comprise the periodontal ligament ground substance. These secretory products are described in detail by Mariotti in this volume. Periodontal fibroblasts also secrete an active collagenase (13) as well as a family of enzymes known collectively as matrix metalloproteinases (Table 2) (13-15, 89). These enzymes have the capacity to degrade the extracellular matrix (85). All of the metalloproteinases are secreted by fibroblasts in an inactive, precursor form. Also, even when activated, the metalloproteinase family of enzymes is effectively inhibited by a variety of tissue inhibitors (35). In addition to fibroblasts (70), metalloproteinases and other enzymes that destroy periodontal tissues are also produced by keratinocytes and tissue macrophages. Under normal conditions, periodontal fibroblasts produce and maintain the extracellular matrix, maintaining homeostasis by also playing a role in modifying parts of the matrix they created (85). Thus, this cell is in an ideal position to regulate the constitution and condition of the gingiva and to maintain tissue integrity. Especially noteworthy in this regard are reports that fibroblasts are capable of phagocytosing foreign objects and ingesting cross-linked collagen, increasing further this cell’s ability to control homeostasis in the periodontium (34). The large, pale-staining nuclei of fibroblasts are readily seen in histological sections of the principal fiber groups, even at low power. The cell bodies appear elongated, oriented parallel to the long axis of the fiber bundle, with a basophilic cytoplasm that is often difficult to discern. When viewed with the electron microscope, fibroblasts exhibit all the features characteristic of actively synthesizing cells: abundant rough endoplasmic reticulum, numerous mitochondria and extensive Golgi complexes. Some fibroblasts in the gingiva of patients manifesting clinical signs of inflammation have been reported to appear greatly

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Fig. 17. Macrophages observed within normal, healthy gingival connective tissue of a young adult male. A. Macrophage containing numerous lysosomes with easily discernible membranes. B. Macrophage with a large par- ticle of phagocytosed debris. Lysosomes can be seen ap- proaching the phagosome (arrows). Note the circular densities in some of the lysosomes, which is often characteristic of these structures (magnification X 18,500).

Fig. 18. Electron photomicrograph of a portion of a gingival fibroblast from non-inflamed human gingival tissue. On the right, part of the nucleus is visible, with clumped chromatin (nuclear zone, NZ). Note the abundant lamel- lae of rough endoplasmic reticulum, mitochondria, Golgi apparatus (Golgi zone, GZ) and dense bodies. The frontal zone (FZ), located most distal to the nuclear zone, contains predominantly rER cisternae and mitochondria. Blebing of the plasma membrane is evident at several sites. The pericellular area contains immature collagenous elements as well as structured fibrils seen cut in cross-section (magnification X 18,500).

Fig. 19. Fibroblast profiles from a non-inflamed portion of normal human gingiva. A. Features of metabolic quiescence. The cell is relatively small, with most of its volume occupied by the nucleus. The organelle comple- ment is reduced, consisting of only a few short segments of rough endoplasmic reticulum, an occasional mitochon- drion and infrequent ribosomes. (magnification x 18,500). B. Features of a synthetically active gingival fibroblast. The characteristic features of a well developed Golgi apparatus, numerous lengthy segments of rER and abundant mitochondria, are in evidence (magnification X 13,000).

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Table 2. Matrix metalloproteinase familv Enzyme Molecular weights MMP Substrates

Gelatinases Denatures collagens (kDa) number

Gelatinase A 72 MMP-2 Native collagens JY, V, VII and X Gelatinase B 92 MMP-9 Elastin and fibronectin

Fibroblast-type CL 52 MMP- 1 Collagen I, 11, 111, VII, VIII and X Collagenases

PMN-type CL 75 MMP-8 Stromelysins

Stromelysin- 1 55 MMP-3 PG core protein, fibronectin and laminin S tromelysin-2 55 MMP-10 Collagen IV, V, IX, X and elastin

Metalloelastase 54 MMP-12? Elastin Stromelysin-3 61 MMP- 11

Matrilysin 28 MMP-7 Fibronectin, laminin and collagen IV

Adapted from: Birkedal-Hansen H. Host mediated collagen destruction, metalloproteinases. In: Genco R, ed. Mol- ecular basis for pathogenesis and molecular targeting in periodontal diseases. Washington, DC: ASM Publishers (in press), with permission.

PG core protein

enlarged and cytopathically altered (1111, as evidenced by dispersion of the chromatin, formation of surface blebs, enlargement and dilation of mitochondria and plasma membrane breaks.

Experiments over the past decade have provided new knowledge regarding the fibroblasts of the periodontium (38, 46, 52, 75). There is ample evidence from in vitro investigations that phenotypically distinct and functionally different subpopulations of fibroblasts exist in the adult periodontium, even though all such cells appear identical in both light and electron microscopy. This is not without biological precedent (for example, the exclusive production of a single antibody by subpopulations of plasma cells that appear identical to sister subpopulations producing quite a different antibody). The implications of this concept for better understanding of normalcy and disease are great, not only for the periodontium but for connective tissues throughout the body. For example, the historical concept of connective tissue diseases has been that pathological changes result from cell injury, as cells injured by noxious endogenous or exogenous substances might be expected to behave functionally in an abnormal manner. Such abnormal functions were believed to result in the clinical manifestations known as disease. Data are accumulating, however, that support the existence of various subpopulations of fibroblasts characterized by quite different functions. For example, one subpopulation of cells may secrete collagen at a high rate, and another synthesizes less collagen but also produces considerable amounts of collagenase. Different cell subpopulations probably are responsible for the production of

differing quantities of the various collagen types as well. The theory is that clinical normalcy is the result and sum of the particular mixture of cell subpopulations within the healthy tissue. In the face of insult (for example, microbially elicited inflammation) and over the span of several weeks, months or years, the subpopulation mixture representing normalcy may shift in its composition such that specific connective tissue cell subpopulations that formerly comprised an insignificant proportion of the total tissue now comprise a much greater proportion. If the new mixture is dominated by cells that produce structural proteins at high rates, connective tissue fibrosis ensues (44, 46, 76); if collagenase-active cells predominate, a lesion characterized by connective tissue loss (such as chronic periodontal disease) results. It is important to realize that disease as interpreted ac- cording to this theory is not the result of cellular injury; the cells present in the pathologically altered tissues are functioning as prescribed by genetic direction. However, since the percentage of these cells is now higher, a clinical picture other than normalcy is manifested. Phenotypically stable and functionally different fibroblast subpopulations have been demonstrated in the gingiva, periodontal ligament, skin and other tissues (26, 45, 52, 56, 65, 68, 73, 91, 115).

Microstructural anatomy of the periodontal ligament

Function and development

Unlike reptiles and lower species, teeth are not attached rigidly to bone in humans and other mam-

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mals. Rather, there is a 0.15- to 0.4-mm space intervening between the cementum covering the root and the bone of the alveolar process. This space is occupied by a soft connective tissue complex that supports and maintains each tooth within an osseous crypt. This structure, referred to as the periodontal ligament, completely fills the circumferen- tially intervening space. In health, the primary component of the periodontal ligament is a system- atic arrangement of coarse bundles of dense fibrous tissue, closely interlaced and supple yet pliant and flexible. Disease of or damage to the periodontal ligament and the other components of the periodontium accounts for a great deal of tooth loss in the adult population of industrialized countries today. The periodontal ligament, and more specifically the cells of the periodontal ligament, probably play the pivotal role in maintaining homeostasis in the alveolar segment of the periodontium. These cells have the capacity to synthesize and resorb the macromolecules of connective tissues, and therefore probably contribute to the regulation of synthesis and resorption of the extracellular substance of periodontal ligament as well as alveolar bone and cementum (71). To understand fully how the activities of the cells of the periodontal ligament are regulated, it is necessary to understand the origins of the cells involved, particularly the fibroblasts, osteoblasts and cementoblasts (see Hefti and Walters in this volume). Clearly, a thorough understanding of the periodontal ligament is essential for all oral health care personnel.

The functions attributed to the periodontal ligament are varied and may be broadly classified as tooth anchorage, fibrous tissue development and maintenance, calcified tissue development and maintenance, nutritive and metabolite transport and innervation.

Anchorage is achieved by means of tough bundles of collagenous fibers that attach the root cementum to the alveolar bone. The development and maintenance functions of the periodontal tissues for both soft and calcified tissues are initiated with the development of the tooth root and continue at a rapid pace until the tooth acquires functional compet- ency. Thereafter, maintenance function continues for as long as the root is retained in its alveolus. Bor- dering the periodontal ligament on one side is the osteogenic layer of the alveolar process, which en- gages in bone formation; on the other side is the cementogenic layer of the root surface, which partici- pates in the formation of new cementum.

The vascular and lymphatic networks supply nu-

trients to and remove metabolic by-products from the soft tissues of the periodontal ligament and, in part, the gingiva. The nervous elements (see below) generally follow the paths of the vascular and the lymphatic channels, providing stimulation for the muscular components of the vessel walls and afford- ing sensory perception as well as reflex loops for the periodontium.

The formation of the periodontal ligament is of interest and has been studied elegantly by many authors (36). Before the tooth erupts into the oral cavity, the connective tissue surrounding the developing root is loose and unstructured. As the crown approaches the suprajacent oral mucosa, the mesenchymal cells previously quiescent within the loose stroma become highly active. This is evidenced by the dramatic increases in the cytoplasmic organelle populations of these cells. The active fibroblasts produce collagen fibrils that accumulate in the periodontal space, but without any detectable orientation. Immediately before tooth eruption, and for some time thereafter, active fibroblasts adjacent to the cementum of the coronal third of the root appear to become aligned in an oblique orientation to the long axis of the tooth. Soon thereafter the first collagen fiber bundles of the periodontal ligament become discernible; these are the precursors of the alveolar crest fiber bundle group (see below). Upon eruption of the tooth into the oral cavity, only the alveolar crest fibers of the periodontal ligament are histologically discernible. Further apically, organized fiber groups cannot be resolved. However, examination of the root surface at magnifications between X150 and ~ 1 4 0 0 reveal fine, brush-like fibers extending from the cementum. Somewhat later, similar fibers may be observed on the adjacent osseous surface of the developing alveolar process. Both sets of fibers, cemental and alveolar, continue to elong- ate toward each other, ultimately to meet, intertwine and fuse as covalent bonding and cross-linking of individual collagen molecular units occur. By the time of first occlusal contact of the tooth with its antagonist, the principal fibers around the coronal third of the root, the horizontal group, are almost completely developed. The oblique fibers in the middle third of the root, however, are still being formed. As eruption continues and definitive occlusion is established, there is a progressive apical maturation of the oblique fiber bundles. With the formation of the apical fiber group, the definitive periodontal ligament architecture is established. It is important to note that the formation of fiber bundles destined to become the principal fiber

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groups of the gingival ligament antecede develop- mentally any of the fiber groups of the periodontal ligament.

For many decades, the theory was espoused that, when new fibers emanating from the cementum meet their counterparts extending from bone, a physically functional intertwining of the two groups occurs (108). It was believed that this physical phen- omenon made possible the kind of periodontal re- orientation necessitated by function, by mesial drift of the teeth and by orthodontic movement. This area, referred to as the intermediate plexus, was believed to be an area of high metabolic activity in which expeditious splicing and unsplicing of fibers might occur. Research over the past decades has demonstrated, however, that once the cemental fibers meet and fuse with the osseous fibers, no such plexus remains. The use of radioactive labels has shown that the entire periodontal ligament is highly active metabolically, not just the middle or intermediate zone (27). With tooth movement, the areas of highest activity have been found to be at the fiber terminals near cementum and bone, not at the middle. Also, investigators have traced single collagen fibers from cementum to bone and found no interruption by a plexus of any sort. Nevertheless, the concept of an intermediate plexus in the periodontal ligament persists, and contemporary research continues (8). Beertsen & Everts (6) have provided electron microscopic evidence for the existence of two distinct compartments in the periodontal ligament, containing cells with the characteristic of adhering to each other but not to cells of the other compartment. Perhaps unfortunately, much of the research into the question of the existence of an intermediate plexus has been performed using rodent and rabbit incisors, continuously erupting teeth that do not provide a good parallel for the human situation.

Principal fiber bundles

The mature periodontal ligament is composed predominantly of principal fiber bundles (109) (Table 3) exhibiting orientational architecture in definite planes. Between and among these are found islands of loose connective tissue known as the interstitial spaces, where periodontal ligament cells, secondary fibers, vessels, lymphatic channels and nervous elements are found. The periodontal ligament fiber complex is located immediately subjacent to its gingival counterpart. The boundary between the two is always indistinct. As in the gingiva, most of the colla-

gen of the periodontal ligament proper is aggregated into bundles known as the principal fiber groups. These dense bundles of fibers are attached to the cementum, span the space of the periodontal ligament in various planes and insert as Sharpey’s fibers into the cribriform plate of the alveolus. In the normal, at-rest and unstressed situations, the path of the principal periodontal fibers is not straight from cementum to bone; rather, the fibers appear wavy or undulating. As the collagen fibers are inelastic, such undulation provides sufficient slack to accom- modate the minute movements of the tooth within its socket that occur constantly during such activities as chewing, speech and swallowing. Four principal fiber groups are distinguished anatomically in all teeth (Fig. 20): alveolar crest, horizontal, oblique and apical groups. In multi-rooted teeth, a fifth group is noted, the interradicular group.

The fibers of the alveolar crest group are attached to the cervical cementum and follow an apically directed path across the periodontal space to become inserted in the crest of the alveolar process. They can be visualized readily in both vestibular and mesiodistal sections. The functions that have been suggested for this fiber group include securing teeth in their sockets and opposing lateral forces. Confusion often arises concerning anatomic differentiation of the periodontal alveolar crest group from an immediately suprajacent gingival fiber group, the dentoperiosteal fibers. The collagenous elements of these two anatomic groups intertwine along their re- spective courses. For clarity, one may imagine a line joining the height of each interdental bony septum; any collagenous elements located apical to this line may be conveniently termed periodontal, and those coronal to the line are considered to belong to the gingival architecture.

The horizontal fibers are found immediately apical to the alveolar crest fiber group. They are oriented, however, roughly parallel to the occlusal plane of the arch. The fiber bundles of this group pass from their cemental attachment directly across the periodontal ligament space to become inserted in the alveolar process as Sharpey’s fibers. They are limited mostly to the coronal one-fourth of the periodontal ligament space and may be visualized in the vestibulo-oral, mesiodistal or transverse planes of a histological section. One function of the horizontal fibers appears to be to prevent lateral tooth movements.

The oblique group of collagen fibers (see detail, Fig. 21) is inserted into the alveolar bone at a position coronal to their attachment to cementum,

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Table 3. Structure and function of collagen fiber groups in the periodontal ligament Name of fiber group Origin and orientation Supposed function Alveolar crest From cementum near cementoenamel junc-

tion, apicalwards into bone of alveolar crest

From cementum of coronalmost 10-15% of root surface, direct laterally into wall of alveolus From cementum of middle 80-85% of root surface, coronally and obliquely into bone of alveolar wall From cementum of root apex, splaying apic- Prevent tooth tipping; resist luxation; pro- ally and laterally into bone of the alveolar fun- tect blood, lymph and nerve supplies to dus tooth From cementum of bi- and trifurcation Aid in resisting tipping and torquing; re- areas, splaying apicalwards into furcal bone sist luxation

Retain tooth in socket; oppose lateral forces; protect deeper periodontal ligament structures Restrain lateral tooth movement Horizontal

Oblique Resist axially directed forces

Apical

Interradicular

thereby resulting in their oblique orientation within the periodontal space. These fibers span the greater area of the root and the alveolus, occupy- ing the middle two-thirds of each. Thus, these fibers are the most widespread of the periodontal ligament. The unique orientation of these fibers would appear to serve the function of resisting apically directed chewing forces.

From the cementum at the root tip, fibers of the apical bundles radiate through the periodontal space to become anchored into the fundus of the bony socket. These fibers may be seen in the vestib- do-oral, mesiodistal and transverse sectional planes. The apical fiber bundles resist the forces of luxation, may prevent tooth tipping and probably also protect the delicate blood and lymph vessels and nerves traversing the periodontal ligament space at the root apex.

From their cementa1 attachment at the furcations of multi-rooted teeth, the principal fibers of the interradicular group pass through the periodontal space to become inserted in the bony crest of the interradicular septum. These fiber groups are visualized in mesiodistal sectional planes, and their function is believed to be resisting tooth tipping, torquing and luxation. Some of the interradicular collagen fiber bundles may be lost if age-related gingival recession proceeds to the extent that the furcation area is exposed. Total loss of these fibers often occurs in chronic inflammatory periodontal disease, when a through-and-through furcation invasion may result from the apical migration of the gingival margin accompanied by osseous resorption at the crestal bone height.

Secondary fibers

Located between and among the principal fiber groups are the secondary fibers. These are relatively nondirectional and randomly oriented collagen bundles of unknown function. They may represent newly formed collagenous elements that have not yet been incorporated into the principal fiber bundles. The secondary fibers often appear to traverse the periodontal ligament space coronoapically, and they are often associated with the paths of vascular and nerve elements. This is also true of reticu- lar fibers, which are fine, immature collagen fibers with an argyrophilic staining property. These often appear to form a lattice-like arrangement. Similarly, the periodontal ligament contains some elastic fibers composed primarily of the protein elastin. Elastic fibers are generally observed only in the walls of afferent blood vessels, where they constitute the elastic laminae of larger arterioles and of arteries of greater caliber.

Blood supply

Considering the fiber density of the periodontal ligament, its vascular supply is abundant, as is the blood supply to the periodontium in general (Fig. 22). Else- where in the mammalian body, dense fibrous tissue generally exhibits fewer vascular elements. The periodontal ligament’s departure from this norm of vascularity may be explained by the presence of the developmental and regenerative layers for cementum and bone, which flank the periodontal ligament; the cells of these layers manifest elevated metabolic re-

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Fig. 20. Principal collagen fiber groups of the periodontal ligament. This schematic depiction of a sagittal section through 2 mandibular molars and a bicuspid shows that the anchoring of the teeth in the alveolar bone is accomplished via dentoalveolar fibers in the periodontal ligament space. All of the fibers originate in root cementum, traverse the periodontal ligament space (0.2-0.4 mm) and insert as Sharpey’s fibers into the wall of the alveolus. The oblique fiber group (3) is the most ubiquitous and re- solves axially applied forces. The alveolar crest fibers (l), horizontal fibers (2), interradicular fibers (4) and apical fibers (5) all serve to absorb and resolve tipping and rota- tory forces, as well as forces that would tend to dislodge the tooth from its alveolus. Fig. 21. Schematic concept of a detail of the periodontal ligament in a section through the oblique fiber group. Note that the collagen fiber bundles (13) are intertwined and pass uninterruptedly across the periodontal ligament space. Numerous fibroblasts (FIB) are present in the ligament as well as vessels and nerves that are interspersed among the fibers. The cementum (C) of the root surface is lined by cementoblasts (CB), and osteoblasts (OBI line the surface of the alveolar bone (A). D=root dentin. Fig. 22. Diagram of the major components of the blood vascular system of the periodontium. The periodontal ligament (l), the alveolar process (2) and the gingiva (3) are supplied primarily by 3 vascular sources. The vessels exhibit frequent arborization and anastomoses. Within the periodontal ligament the vascular network is especially dense, taking on the appearance and character of a thickly woven net. Adjacent to the junctional epithel-

ium, the vessels splay into a very dense plexus (A) with numerous venules. The connective tissue rete that occupy invaginations in the basal epithelial surface contain abundant capillary loops. The oral gingiva (B) is also supplied by capillary loops within rete pegs. Fig. 23. Schematic diagram of the osseous periodontal support structure in the mandible. The alveolar process comprises the alveolar bone proper (I) , trabecular bone (2) and compact bone (3). Synonyms for the alveolar bone proper include cribriform plate, alveolar wall and lamina dura (as viewed in radiographs). The alveolar crest (arrow) is formed where the alveolar wall is connected to the compact cortical bone, usually 2-3 mm apical to the cementoenamel junction. Note the absence of trabecular bone near the alveolar crest; the alveolar bone is often extremely thin in this region. The various periodontal ligament fiber bundles insert as Sharpey’s fibers into the alveolar wall, the alveolar crest and root cementum. A histological section of the area denoted by the box is pre- sented in Fig. 24. Fig. 24. Low-power (X 10) photomicrograph of cementum (left), periodontal ligament (center) and the wall of the alveolus (right). Sharpey’s fibers are faintly visible within the cementum and the wall of the alveolus. The periodontal ligament fibers are obvious, as are the numerous nuclei of periodontal ligament fibroblasts, and vessels. The cementum surface is lined by a layer of cementoblasts. Osteoblasts line the alveolar wall. Note the layers of incremental bone (bundle bone) that have been laid down, with entrapment of osteocytes. Also visible are osteons and a Haversian canal.

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quirements. Although Schweizer (105) provided a general description of the course of vessels in the periodontal ligament, Hayashi (47) was the first to identify and describe the complex architecture of the periodontal blood supply. Subsequent investigations added only minor details to the original descriptions.

The afferent blood supply to the periodontal ligament is derived from 3 primary branches from the alveolar arteries: the dental, the interradicular and the interdental. The dental artery emerges from the bony fundus of the socket, on the way to vasculariz- ing the dental pulp. Prior to entering the apical foramen of the tooth, the dental artery gives off afferent branches that further arborize to provide a basket- like network of vessels in the apical third of the periodontal ligament.

In its course through the substance of the alveolar process, the principal trunk of the interradicular artery branches into vessels of lesser caliber, which course laterally to emerge from the cribriform plate as perforating arteries. These supply the periodontal ligament along most of its coronoapical extent, including the bi- and trifurcation areas. The interdental artery also travels through the alveolar process and distributes lateral branches, exiting the bone as perforating arteries to supply the middle three fifths of the periodontal ligament. As the interdental artery emerges from the crest of the alveolar process, it branches immediately to provide arterial blood supply for most of the coronal aspect of the periodontal ligament as well as for the gingiva. Irrespective of their origins, all vessels within the periodontal ligament intercommunicate, forming an arborizing plexus throughout the periodontal space.

The diameter of the arterioles of the periodontal ligament ranges between 15 and 50 pm. The larger ones course more or less parallel to the long axis of the tooth from interstitial space to interstitial space, traversing through and between the principal fiber groups. The apical and coronal thirds of the periodontal ligament are generally more highly vascular- ized than the middle third. It has been additionally suggested that the perforating channels are more abundant in the periodontal tissue of teeth of the upper arch than in the lower arch, and more in the posterior than in the anterior teeth. Furthermore, there are palatolabial discrepancies in the vascularity of the periodontal ligament. For example, Frohlich (32) suggested that there is a diagonal sym- metry of the periodontal vessels; that is, if there is abundant vascularity palatally at the apical area, there will be a similar abundance labially in the cor-

onal area of the periodontal ligament. This causes minute (0.5 pm) labiopalatal pulsation of teeth with each heartbeat (43, 50).

Venous channels accompany their arterial counterparts. They are somewhat larger in diameter (mean = 28 pm). These channels receive blood via the capillary network in the periodontal ligament. Early reports (90) that specialized shunts (glomera) are interposed between the arterial and venous systems have not been widely accepted. Such shunts were said to provide an arteriovenous anastomosis and drainage, bypassing the capillary networks, but evidence to support this idea has not been forth- coming.

Lymph channels appear to originate as culs-de- sac in the gingival and palatal mucosa, spongy bone and the tissue of the periodontal ligament. The larger lymphatic channels follow the paths of the blood vessels. They may course apically within the substance of the periodontal tissue to arrive at and pass through the fundus of the socket or they may pass through the cribriform plate to empty into larger channels pursuing intraosseous paths. Thus, lymph from the free and interproximal gingiva en- ters the interdental vessels to be joined with that from the periodontal ligament. On the way to the lymph nodes, the flow is via the alveolar lymph channels, which are joined by the dental and interradicular lymph channels.

The adequacy of the vascular and lymphatic architecture is manifested in the efficiency of the various developmental, repair and healing processes that occur continually within the various connective tissues surrounding the teeth. Routinely rapid healing, which occurs following the biologically harsh process of tooth extraction, may be offered as a prime example.

Innervation

The sensory innervation of the periodontal ligament subserves touch, pressure and pain as well as pro- prioceptive function (41). In unerupted teeth, the developing periodontal ligament harbors fine, un- myelinated nerve fibers that are believed to provide autonomic function to the developing vasculature of the periodontal ligament. Whether this situation persists after complete eruption is not known. All periodontal ligament innervation is mediated by the dental branches of the alveolar nerves by way of the apical perforations of the tooth socket or from the perforating branches of the interalveolar nerves traversing through the bone. The endings of some

28


nerves form a delicate arborization among the stromal cells of the periodontal ligament. Byers et al. (19-22) have been instrumental in significantly ex- panding knowledge of periodontal innervation by developing a novel method of investigation whereby axons are radioactively labelled and then visualized by means of autoradiography. Various animal models have been used to demonstrate that the mature, healthy periodontal ligament is richly inner- vated by mechanoreceptors; the bodies of these cells are located in either the trigeminal ganglion or in the mesencephalic trigeminal nucleus. The receptors are activated primarily by touch and pressure and by the natural micromovement of teeth during mastication, speech etc. Other mechanoreceptors serve the proprioception that is involved in the unconscious de- tection of tooth contact for reflex control of mandibular movements. The periodontal ligament also exhibits nociceptors, supplied by neurons in the trigeminal ganglion (107). Sensory receptors are most numerous in the apical segment of the periodontal ligament. The majority of nerve axons in the periodontal ligament terminate in unencapsulated branched Ruffini-like endings that exhibit a broad range of size and complexity; these receptors are always observed in intimate association with periodontal ligament fibers. There are exquisite types of specialization of nervous elements that vary from tooth to tooth, with inter-species differences. Gener- ally, receptors of the trigeminal ganglion seem to be located in the periodontal ligament at the sites that would be most easily stretched during tooth contact (21). It appears that the different sites and intensity of the stretch forces that result as different tooth groups are in function (incising, grinding, etc.) may determine the observed variation in size and localiz- ation of mechanoreceptors. Clearly, the sensory innervation of the periodontal ligament is of importance for oral reflexes and chewing and for displacement sensitivity (proprioception) of the teeth; sensory receptors are more numerous in the apical third of the periodontal ligament.

Physiological drift

In any discussion of the periodontal ligament, the concept of normal physiological drifting of teeth needs to be addressed. Tooth movement does not cease when active tooth eruption is complete and the teeth have achieved functional occlusion. Throughout life, gradual movement continues in two directions: occlusally and mesially. Both types of movements are extremely gradual, with move-

ment of less than 1 cm in a lifetime. Additionally, both types also result from progressive attrition of hard substance from the clinical tooth crown: attrition from the occlusal surface due to abrasion during mastication, clenching and bruxism and attrition from the mesial and distal surfaces resulting from tooth-tooth abrasion at the proximal surfaces. Further, both types are also accompanied by appropriate responses in the osteogenic and cementogenic cells bordering the periodontal ligament. Physio- logical tooth movement may be thought of as a compensatory movement, the organism’s attempt to maintain the integrity of the dental arch in the face of the loss of tissue substance.

The horizontal movement of teeth toward the midline throughout life, referred to as mesial drift, can be demonstrated in histological sections through the alveolar process. On its mesial aspect, the periodontal ligament appears somewhat compressed, the principal fiber bundles are wavy, the interstitial spaces are rounded and the surface of the alveolus exhibits concavities (Howship’s lacunae) that frequently contain active osteoclasts. The distal aspect of the periodontal ligament, on the other hand, appears stretched, with obvious ovoid spaces among the fiber bundles. The adjacent surface of the alveolar bone is covered with a periosteum composed of actively synthesizing osteoblasts, and the vertical striations characteristic of secondarily formed bundle bone are evident. Functional trac- tional forces on the alveolar bone effect mobilization of osteoblastic activity to the end that the osseous matrix is elaborated; the calcified matrix is known as bundle bone (see below). The latter is characterized by parallel layers of lamella with a conspicuous absence of Haversian systems. Comparable histological manifestations are often observed in sections through the furcation area of multi-rooted teeth, in- dicating that gradual occlusal movement has also occurred. Finally, the features described represent the normal, healthy physiological situation. Numer- ous factors may drastically alter histological appearance. Among these factors are orthodontic treatment, chronic inflammatory gingival and periodontal diseases, primary and secondary occlusal trauma, chronic oral habits such as bruxism or clenching and the loss of individual teeth. In the latter instance physiological mesial drift will continue, but the tooth distal to the one lost will tend to tip mesially into the gap rather than moving forward bodily. Severe occlusal dysharmony almost always ensues.

Physiological tooth movement is a foremost ex-

29

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ample of the vital importance of the dynamic response capacity of the cells of the periodontium (116). Two cell populations that border the periodontal ligament are cementoblasts and osteoblasts (see below), which are intimately related to cementum and bone dynamics. The chemical and/or physical triggers that stimulate activity by quiescent cementoblasts (to deposit new cementum) and by osteoblasts (to produce matrix for new bone apposition) likely derive from the periodontal ligament, although factors from bone and from cementum have also been implicated recently. The nature, complexity and interaction of such triggers remains unclear for the most part, but recent research has been targeted toward the elucidation of the roles of such factors (77, 85, 117; see Hefti and Walters in this volume).

The most ubiquitous cell type in the periodontal ligament is the fibroblast. Although it is virtually identical at the histological and ultrastructural levels to fibroblasts in other tissues (such as gingival fibroblasts; see above), contemporary research (67, 118, 119) is demonstrating that the periodontal ligament fibroblast is a phenotypically distinct cell population. For example, periodontal ligament fibroblasts in culture synthesize and secrete elevated quantities of chondroitin sulfates A and C but lesser amounts of hyaluronic acid and heparin than fibroblasts derived from marginal gingiva. The rate of periodontal ligament fibroblast proliferation is slower than that of gingival cells, and periodontal ligament fibroblasts exhibit higher alkaline phosphatase activity than gingival cells. Peri- odontal ligament fibroblasts become senescent in vitro more rapidly than their gingival counterparts, when proliferation rates are compared. In addition, periodontal ligament fibroblasts and gingival fibroblasts exhibit clearly different attachment properties iii vitro in response to various factors such as fibronectin or extracts from various mineralized oral tissues (1 181, with enhancement of gingival cell attachment far outweighing that of periodontal ligament-derived cells. Such pheno- typic differences between fibroblasts of periodontal ligament and gingiva may have important implications for the current wave of activity in efforts to stimulate regeneration of periodontal tissues and structures following surgical intervention in case of destructive periodontal disease.

Another cell, the undifferentiated mesenchymal cell or stem cell, map also exist within the periodontal ligament. The periodontal ligament develops from such cells embryologically, and there

is evidence that such pleuripotent cells remain after the periodontal ligament is fully formed and mature. It has been proposed that the progeny of such stem cells give rise to osteoblasts, cementoblasts and periodontal fibroblasts (5). On the other hand, it is possible that these cells are merely quiescent (Go phase) fibroblasts, quiescent cementoblasts or quiescent osteoblasts anticipating the appropriate trigger factor to stimulate their re- entry into the cell cycle to execute their genetically preprogrammed functions, i.e., that each of these cell populations descended from a different population of ancestral cells (69, 92). This question is being addressed in many laboratories today. The putative mediators that might act upon any pleuripotent stem cells (such as to initiate cell division or protein synthesis) remain to be definitively elucidated, although the candidates are numerous (85). Of note is that some research has provided evidence that fibroblasts and osteoblasts may derive from a common ancestral cell (4, 5, 71). Even in pure in uitro cultures of fibroblasts, only 45-50% are active at any given time; the remainder are quiescent, non-synthesizing and non-dividing cells, some of which can be triggered by the addition of appropriate factors to cell culture medium. This concept of quiescent cells resident within the periodontal ligament is in accordance with the fibroblast subpopulation selection theory (see above) (44, 45).

Unlike most other fibrous connective tissues in the body, the normal, healthy periodontal ligament also harbors clusters and strands of epithelial cells. Known as the epithelial rests of Malassez, they are vestiges of the apical extensions of the two primor- dial layers constituting Hertwig’s root sheath. The epithelial rests may appear as duct-like strands or follicular aggregates or also as individual or clumped cell colonies. Owing to their origins, these epithelial cells are almost always located closer to the root cementum than to the wall of the alveolus within the substance of the periodontal ligament. The shape of the individual cells varies from squa- moid to columnar. Cell nuclei appear round to ovoid; the heavily clumped chromatin yields a hyperchromatically staining nucleus. Although the epithelial rests of Malassez have been studied in great detail (791, a possible normal function for them remains speculative. However, their potential clinical importance is to be found in their potential to begin, for reasons wholly obscure, to pro- liferate and form cysts in the periodontal ligament; fortunately, this is rare.

30


Anatomy and microstructure of alveolar bone

Another of the 4 major anatomic component parts of the periodontium are the alveolar processes, which represent extensions of the body of the mandible and the body of the maxilla. The alveolar processes are tooth-dependent structures in the sense that the teeth are housed within the osseous crypts (alveoli and cribriform plate) comprising the alveolar bone proper, which is one of the 3 components of the alveolar process; the other 2 components are the compact bone (the oral and facial cortical plates) and the trabecular bone (cancellous bone and spongiosum). The anatomic relationships among these osseous components are depicted in Fig. 23.

The cortical plates are continuations of the compact bone of the principal mass or body of the maxilla and mandible. The thickness of the cortical plates varies significantly from tooth to tooth throughout the arches; the labial and buccal cortical plate is generally considerably thicker than the lingual plate in the mandible, except in the incisor region, whereas the palatal cortical plate is usually thicker than the facial plate in the maxilla. Tooth position in the arch (such as buccoversion, linguoversion, supereruption, intrusion, etc.) appears to be the major determinant of cortical plate thickness and contour. Reactive resorption of alveolar bone resulting from inflammatory processes within the periodontium is a frequent cause of tooth mobility and tooth loss in adults. Such bone loss appears to be influenced by numerous factors in humans, including host immune capability, genetic susceptibility, home care (oral hygiene) and the virulence of the organisms present in soft microbial deposits at the gingival margin and within the gingival sulcus or periodontal pocket. Preservation of the alveolar processes has long been a goal of preventive periodontology; today, regeneration to normalcy of alveolar bone that has been lost due to disease is a major activity in research and clinical practice.

The structure and morphology of the alveolar process are unique in their lability and dependence on the tooth. The crest of the osseous alveolar margin normally follows the contour of the cementoenamel junction of the individual teeth and is 2-3 mm apical to it in health. In the maxilla, numerous Volkmann canals have been described near the vestibular osseous surface; through these canals course the vascular, nervous and lymphatic elements into the substance of the bone. Of interest is that Volkmann canals of the mandibular alveolar bone are fewer in

number than in the maxilla but larger in diameter. In both jaws, the Volkmann canals serve the additional function of supplying the periodontal ligament, via the hundreds of pores that are present in the alveolar bone proper (cribriform plate).

The dynamic nature of the alveolar processes is evident in the extent to which the remodeling capacity has been demonstrated in response to functional demands. Bone remodeling consists of an or- dered and predictable sequence of bone resorption followed by bone formation. As described by Miller & Tee (721, initiation of the remodeling sequence in- volves the recruitment of osteoclast progenitor cells to the remodeling site; these cells fuse and differentiate into the mature osteoclastic phenotype. Very little is known about the activation of the bone remodeling process, although evidence is accumulating that cells of osteoblastic lineage play a role in the regulation and modulation of osteoclastic functions, including bone resorption as well as osteoclast recruitment and differentiation. Such interactions between osteogenic and osteoclastic cells may provide the physiological and anatomic basis for the coup- ling of bone formation and resorption (72). Excellent comprehensive reviews of osteoblast (39) and osteoclast (40,95) functions have appeared quite recently; these primary sources provide additional information. As discussed above, normal occurrences such as mesial drift and continuous tooth eruption normally elicit remodeling of the alveolar bone proper. On the osseous surfaces subjected to com- pressive forces (such as the mesial wall of an alveolus), one routinely observes osteoclastic activity and bone resorption; such bony surfaces are characterized by a moth-eaten appearance due to the re- sorptive activities of osteoclastic cells that are normally observed within Howship’s lacunae on the surface of the bone undergoing resorption. Osseous surfaces subjected to tensional forces (such as the distal wall of an alveolus) commonly exhibit the formation and accumulation of bundle bone, which exhibits a laminar appearance and contains no trabecular components, no osteons and no fatty marrow spaces. With time, bundle bone may recon- solidate via remodeling, lose its characteristic histological appearance and assume the appearance of normal alveolar bone. Nevertheless, because the collagen fibrils constituting the matrix of bundle bone are fewer in number and are arranged in a plane perpendicular to the embedded principal fiber bundles (Sharpey’s fibers), and because an increase of mineralizing ground substance accompanies the decreased fibril density, the cribriform plate pos-

31

Hassell

sesses an enhanced radiopacity that is clearly evident on radiographs of the jaws, giving rise to the term lamina dura (alveolar bone proper as viewed on radiographs of the jaws).

Between the massive cortical plates and the alveolar bone proper and connecting them to one another is the cancellous trabecular bone, commonly referred to as spongiosa. The trabeculae of the spongiosum appear to be organized to buttress the functional forces to which the alveolar bone proper is normally exposed. The size and orientation of the trabeculae are correlated with the intensity of the functional stimuli. As a general rule, the magnitude of the spongiosum in the mandible is less than in the maxilla. The cancellous bone of the alveolar bone proper blends imperceptibly into that of the body of the maxilla and mandible. The spatial orientation and mass of the spongiosum is also tooth-related, depending largely on the inclination of teeth within the arch; in some locations, spongiosum may be entirely absent, with fusion of the cortical plate and the cribriform plate. Although a cursory inspection may lead to the conclusion that trabeculation is a haphazard maze, more detailed analysis has demonstrated that the pattern of trabecular organization between the cribriform and cortical plates is structurally and functionally related to the specific stresses to which the various segments of the dental arch are subjected (87).

Each root of all multi-rooted teeth has its own alveolus; the alveoli of multi-rooted teeth are separ- ated by osseous interradicular septa, which are composed of only two components: spongiosum and alveolar wall (cribriform plate). The trabeculae within interradicular septa course mainly horizontally vis-a-vis the occlusal plane of the arch.

The tooth-dependent nature of the alveolar processes has been emphasized. This tooth dependence commonly results in two situations that, although normally encountered and non-pathological, may nevertheless have implications in disease pro- gression: dehiscences and fenestrations. If a tooth is tipped labially, it is normal that its labial plate of bone is thin and the lingual plate is more massive. The thin labial osseous plate often harbors no spongiosum at all. The crest of the alveolar bone may resorb over time, exposing the labial root surface. Al- ternatively, while the crestal bone height may be maintained, a window (fenestration) may form apical to the alveolar crest, exposing the root surface to the soft tissues of the periodontium. It is important to point out that both fenestrations and dehiscences are nothing more than variations within the range of

32

periodontal normalcy, resulting for the most part from anomalies of tooth position; nevertheless, fenestrations and/or dehiscences may predispose to gingival recession and, if undiagnosed or unex- pected, may complicate periodontal surgical procedures.

Cementum

The dentin of the roots of the teeth in mammals is covered by a thin (50-200 pm) calcified tissue referred to as cementum. Although cementum formation (cementogenesis) and cementum structure have been well described using light and electron microscopy (66, 82, 100, 1251, comparatively little is known about the physiology of cementum. Scientific investigation in this area has lagged behind research into the other 3 primary tissues of the periodontiurn (gingiva, periodontal ligament and alveolar process). However, recent attempts at achieving true periodontal regeneration after surgical treatment for periodontitis have occasioned renewed interest in root cementum because, in the absence of newly formed cementum, no new Sharpey fiber attachment to the root occurs. It is to be hoped that the research currently in progress in several laboratories (7, 10, 17, 113, 114) will shed new light on cementum and the cells that produce it (cementoblasts). Many of the recent major advances in knowledge about gingiva, alveolar bone and periodontal ligament have derived from in uitro experimentation with fibroblasts and osteoblasts. Unfortunately, the efforts to obtain and propagate cementoblasts in culture have proven fruitless so far. In uivo, cementoblasts are believed to derive from fibroblast-like cells (stem cells?) in the periodontal ligament. Histologi- cal observation of areas of root resorption has shown that cementoblasts can arise wherever viable dentin is exposed to the soft tissue of the periodontal ligament. Induction of cementoblasts from periodontal ligament cells can apparently take place throughout life, as evidenced by physiological areas of cementa1 repair (94). Cellular turnover among cementoblasts is slow compared with that in the osteoblasts that line the alveolus (93). Furthermore, it appears that cementoblasts are capable of altering their rate of cementum deposition.

The principal function of cementum is to provide anchorage of the tooth in its alveolus. This is accomplished via the collagen fiber bundles of the periodontal ligament, whose terminations (Sharpey’s fibers) become firmly embedded in cementum dur-

Tissues and cells of the Deriodontium

Fig. 25. Low-power photomicrograph of a mandibular posterior tooth and its periodontal support structure. This section was cut in the orofacial plane. The alveolar bone proper (alveolus) is clearly visible. Broad marrow spaces are evident subjacent to the massive buccal cortical plate (compact bone, right) and the somewhat thinner lingual compact bone (left). The tooth is in linguoversion, which has led to the lingual alveolar crest being more apically positioned than the buccal alveolar crest. The apical foramen containing the nerve and blood supply for the tooth is clearly visible.

Fig. 26. Photograph of a cut section of mandibular alveolar bone. The cut was made in a mesiodistal plane through the posterior segment of the jaw. Note that in this molar area the alveolar bone is traversed by numerous Volkmann canals. The mandibular canal is clearly visible as a shadowed groove at the inferior border of the photograph. The section was through the alveolus of a mandibular molar; the interradicular septum can be seen between the two alveoli; the septum

ing the process of cementogenesis. Another function of cementum is to assist in maintaining occlusal relationships. As the occlusal and incisal surfaces of teeth are abraded away due to attrition, tooth eruption occurs to compensate for the lost substance, and deposition of new cementum occurs at the apical root area. This process also serves to maintain the width of the periodontal ligament space at the apex of the root. Although cementum deposition continues on and off throughout life, contrary to the situation with the alveolar process, there appears to be no direct or predictable correlation between the thickness of cementum and functional forces borne by the teeth. Cementogenic activity may contribute to the mechanism by which periodontal ligament fiber reattachment and relocation occur as a conse- quence of mesial drifting of teeth. Via the stimulation of cementoblasts to actively synthesize and secrete the matrix for new cementum, other functions are also accomplished, such as the repair of root fractures, walling in filled canals, sealing off necrotic pulps (apical occlusion) and protection of the subjacent dentinal tubules.

Cementum is similar to bone in its organic fibrous framework, ground substance, crystal type, developmental processes, reorganizational capabilities and

also contains broad marrow spaces (spongy bone). The thickness of the alveolar wall can be appreciated in this figure; it normally ranges from 0.2-0.8 mm. The porous nature of the cribriform plate is well depicted. The apical portion of the alveolus contains but few nutrient canals, whereas the middle and coronal areas are rich in nutrient canals. The alveolar crest is relatively flat in the molar area, following the contours of the cementoenamel junction.

chemical composition. There are, however, some important differences between bone and cementum; for example, about 70% of bone is made up of inor- ganic salts, and the corresponding figure for cellular cementum is only 46% (106). Furthermore, only type I collagen is found in bone, whereas cementum contains type I plus about 5% type I11 collagen in its organic matrix (18). Cementum contains a proteoglycan interfibrillar substance that appears to be a gene product unique to cementoblasts (88). Unlike bone and tooth enamel, cementum is relatively permeable. From the point of view of function, cementum differs fundamentally from bone in that cementum does not undergo the extensive remodeling that is characteristically observed in histological sections through the alveolar processes (57). Some remodeling of cementum does occur, however, as evidenced by the presence of resorption lacunae observed on extracted human teeth (49) and by the ability of cementocytes to resorb the organic cemental matrix (9).

The location, morphology and histological appearance of cementum served historically as the basis for classification of apparently different types of cementum. The classification system devised by Owens (81-83) and others in the 1970s is accepted

33

- Hassell

for the most part today, and was summarized recently by Schroeder & Page (103):

I) acellular, afibrillar cementum 11) acellular, fibrillar cementum

111) cellular cementum containing intrinsic fibers Tv) cellular cementum with intrinsic and extrinsic

fibers.

Type I cementum is found almost exclusively on the enamel near the cementoenamel junction. It does not contain collagen fibers nor does it exhibit en- trapped cementocytes. The origin of this type of cementum remains speculative today, although it may accrue in regions where connective tissue has come into contact with tooth enamel during the developmental phase (100). Type I1 cementum, by definition, encases the extrinsic fiber system consisting of the Sharpey fibers of the periodontal ligament but does not harbor cementocytes within lacunae. De- velopmentally, the acellular, fibrillar cementum comes to occupy primarily the coronalmost one-half of the root surface. Type 111 cementum does not en- case any Sharpey fibers at all; instead, its organic matrix consists of intrinsic fibers alone, fibers syn- thesized and secreted by cementoblasts and not by periodontal ligament fibroblasts. This cementum exhibits lacunae with encased cementocytes and is widely assumed to represent repair or secondary cementum, as it is seldom, if ever observed in speci- mens from recently erupted teeth. Cellular cementum that harbors both intrinsic (cementoblast- derived) and extrinsic (fibroblast-derived) fibers within calcified matrix that also houses viable cementocytes is classified as type IV. The intrinsic fibers predominate over Sharpey fibers in type IV cementum. The dynamic nature of root cementum is obvious in sections exhibiting incremental lines, which attest to alternating periods of cementum apposition and periods of cementoblast dormancy.

Cementum is not normally exposed to the intra- oral environment because it is covered by alveolar bone and gingiva. In patients with gingival recession, however, cementum at the cervical area of the tooth may become exposed. Similarly, in patients with periodontal pockets, considerable ex- panses of root cementum can become exposed. Be- cause cementum is a relatively permeable substance, it can absorb toxins generated from plaque bacteria; indeed, viable periodontopathogen- ic microorganisms routinely inhabit the dentin subjacent to pathologically exposed cementum (l), serv- ing as bacterial reservoirs from which recolonization

of mechanically treated (scaling and root planing) root surfaces could occur. It has been demonstrated that connective tissue cells cannot adhere to previously exposed or toxin-containing cementum (12). Thus, therapeutic measures often include the mechanical removal of root cementum by scraping and planing. The dentin that is inevitably exposed by such procedures is often exquisitely sensitive, a clinical problem that is difficult to manage effectively. In the subgingival area, formation of new cementum upon denuded dentin surfaces has been demonstrated, but the process is neither predictable nor does it result in a layer of cementum that resembles normalcy. This remains an area of active investigation in both clinical and basic science.

Finally, very recent and intriguing studies by Slav- kin et al. (113, 114) are demonstrating that the proteins of human (and mouse) cementum represent a distinct class of enamel-related proteins. These results derived from a series of experiments designed to test the hypothesis that Hertwig’s epithelial root sheath synthesizes and secretes enamel-related proteins that participate in the process of cellular cementum formation. Sequential development of the mouse mandibular molars was examined in uiuo and in long-term organ culture in uitro. Using antibodies to several enamel proteins (such as anti-amelogenin and anti-enamelin), enamel-related antigens were localized within intermediate cementum during the period of differentiation of Hertwig’s epithelial root sheath and root formation. It was possible to corro- borate cytodifferentiation and morphogenesis in uitro and to identify initial cementum formation. The cementum polypeptides isolated (one 72 kDa and one 26 kDa) were different from the compositions of species-identical enamel proteins. This novel approach to the study of cementum will likely prove to be a powerful technique for understanding cellular differentiation and the embryological role of the Hertwig sheath as it relates to the apposition of cementum.

Conclusion

This chapter reviewed the structure and function of the 4 major tissues of the periodontium: gingiva, periodontal ligament, alveolar bone and cementum. It also discussed the predominant cell types harbor- ed by these tissues and the functional phenotypes of those cells. The interactions and reciprocal relationships between and among the tissues and cells have been emphasized. The following chapters will pro-

Tissues and cells of the ueriodontium

vide information about the molecular structure and composition of the periodontal tissues as well as the interactive mechanisms by which the various periodontal cell types communicate. Normalcy - health - in the periodontium is the result of a successful interplay among the resident cell populations. Any imbalance in the anabolic-catabolic axis that exists within the healthy periodontium leads to pathological alteration.

Acknowledgements

I gratefully acknowledge the assistance of Shanna Parrott and Melissa Grace in manuscript prepara- tion. Some of the figures in this chapter originate from the 1989 book Color atlas of periodontology, with the consent of my co-authors K. Rateitschak, E. Rateitschak-Pluss and H. Wolf and with the permission of Thieme Medical Publishers, New York and Stuttgart. I thank Professor Hubert Schroeder for permission to reprint the photomicrographs in Fig. 8 and 24. The original transmission electron photomicrographs in this chapter were prepared by Aaron Bernstein under my direction.

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