The periodontium is defined as those tissues supporting and investing the tooth and consists of cementum, periodontal ligament (PDL), bone lining the alveolus (socket), and that part of the gingiva facing the tooth. Proper functioning of the periodontium is achieved only through structural integrity and interaction between these various tissues. Together, these tissues form a specialized fibrous joint, a gomphosis, the components of which are of ectomesenchymal origin. The widespread occurrence of periodontal diseases and the realization that periodontal tissues lost to disease can be repaired has resulted in considerable effort to understand the factors and cells regulating the formation, maintenance, and regeneration of the periodontium. This chapter describes the histologic events leading to the formation of supporting tissues (Figure 9-1), except for the dentogingival junction, which is covered under oral mucosa (see Chapter 12).
Cementum is a hard, avascular connective tissue that covers the roots of teeth (Figure 9-2). Cementum is classified according to the presence or absence of cells within its matrix and the origin of the collagen fibers of the matrix. The development of cementum has been subdivided into a prefunctional stage, which occurs throughout root formation, and a functional stage, which starts when the tooth is in occlusion and continues throughout life. Several varieties of cementum exist. The beginning student, however, needs only to think of the two main forms of cementum that have different structural and functional characteristics: acellular cementum, which provides attachment for the tooth, and cellular cementum, which has an adaptive role in response to tooth wear and movement and is associated with repair of periodontal tissues.
Four mineralized tissues are found in the oral cavity, and three of these—enamel, dentin, and cementum—are components of the tooth. Their characteristics and biochemical composition are summarized in Table 1-1. The composition of cementum is similar to that of bone. Cementum is approximately 45% to 50% hydroxyapatite by weight and the remaining portion is collagen and noncollagenous matrix proteins. Type I collagen is the predominant collagen of cementum (constitutes up to 90% of the organic components in cellular cementum); in cellular intrinsic fiber cementum, just as in bone, it accommodates mineral deposition. Type I collagen is also the major collagen within the PDL region, and its main function is to structure the fiber bundles that anchor the tooth to the bone and distribute masticatory forces. Other collagens associated with cementum include type III, a less cross-linked collagen found in high concentrations during development and during repair and regeneration of mineralized tissues but is reduced with maturation of this tissue, and type XII collagen, a fibril-associated collagen with interrupted triple helices that binds to type I collagen and to noncollagenous proteins. Type XII collagen is found in high concentrations in ligamentous tissues, including the PDL, with lower levels noted in cementum. This nonfibrillar collagen interacts with type I collagen and may assist in maintaining a functional and mature PDL that can withstand the forces of occlusion. Trace amounts of other collagens, including types V, VI, and XIV, also are found in extracts of mature cementum; however, these may be contaminants from the PDL region, produced by PDL fibroblasts associated with collagen fibers inserted into cementum. Noncollagenous proteins identified in cementum also are associated with bone and include the following: alkaline phosphatase, bone sialoprotein, dentin matrix protein 1, dentin sialoprotein, fibronectin, osteocalcin, osteonectin, osteopontin, proteoglycans, proteolipids, tenascin, and several growth factors. Enamel proteins also have been suggested to be present in cementum but not yet convincingly demonstrated. Two apparently unique cementum molecules, an adhesion molecule (cementum attachment protein) and an insulin-like growth factor have been identified, but further studies are warranted to confirm the existence and function of these molecules. The recent finding that cementoblasts can express the osteoblast-specific membrane protein bone restricted ifitm-like protein (Bril, a member of interferon inducible transmembrane protein family) adds additional support for similarity between these two cells.
Although cementum formation takes place along the entire root, its initiation is limited to the advancing root edge (Figure 9-3). At this site, Hertwig’s epithelial root sheath (HERS), which derives from the coronoapical extension of the inner and outer enamel epithelium (see Chapter 5), is believed to send an inductive message, possibly by secreting some enamel proteins or other epithelial product, to the facing ectomesenchymal pulp cells. These cells differentiate into odontoblasts and produce a layer of predentin (Figures 9-4 and 9-5). The next series of events results in formation of cementum on the root surface; however, the specific cells and trigger factors responsible for promoting its formation still are unresolved. Current theories include the following: (1) Soon after, HERS becomes interrupted, and ectomesenchymal cells from the inner portion of the dental follicle then can come in contact with the predentin; (2) infiltrating dental follicle cells receive a reciprocal inductive signal from the dentin and/or the surrounding HERS cells and differentiate into cementoblasts; or (3) HERS cells transform into cementoblasts (a process discussed subsequently). During these processes, some cells from the fragmented root sheath form discrete masses surrounded by a basal lamina, known as epithelial cell rests of Malassez, which persist in the mature PDL (Figure 9-6; see also Figure 9-5). Evidence is increasing that these rests are not simply residual cells but instead may participate in maintenance and regeneration of periodontal tissues. If some HERS cells remain attached to the forming root surface, they can produce focal deposits of enamel-like material called enamel pearls (Figure 9-7), most commonly found in the area of furcation of roots.
The long-standing view is that precursor cells for cementoblasts and PDL fibroblasts reside in the dental follicle and that factors within the local environment regulate their ability to function as cementoblasts that form root cementum or as fibroblasts of the PDL. Cells involved in regenerating periodontal tissues include stem cells migrating from the vascular region, as well as local progenitor cells. The precise location of the progenitor cells and whether there exists a common progenitor or distinct progenitors for each cell type remain to be defined. In addition, there is now increasing evidence that epithelial cells from HERS may undergo epithelial-mesenchymal transformation into cementoblasts during development. Such transformation is a fundamental process in developmental biology that occurs, among other processes, as we have seen during neural crest cell migration and medial edge fusion of the palatal shelves. Structural and immunocytochemical data support the possibility that at least in part cementoblasts are transformed from epithelial cells of HERS. In rodents, initial formation of acellular cementum takes place in the presence of epithelial cells, and some studies have shown that enamel organ–derived cells are capable of producing mesenchymal products, such as type I collagen, bone sialoprotein, and osteopontin.
Uncertainty still exists as to whether acellular (primary) and cellular (secondary) cementum are produced by distinct populations of cells expressing spatiotemporal behaviors that result in the characteristic histologic differences between these tissues. This potential cellular and formative distinctiveness is highlighted in mice null for tissue nonspecific alkaline phosphatase gene or rats treated with bisphosphonates. In these animals, acellular cementum formation is affected significantly, whereas cellular cementum appears to develop normally. In hypophosphatemic mice, formation of acellular cementum can be rescued by enzyme replacement therapy for tissue nonspecific alkaline phosphatase. This suggests differences in cell types or factors controlling development of these two varieties of cementum. In the human counterpart, hypophosphatasia, characterized by low levels of alkaline phosphatase, cementum formation appears to be limited or nonexistent, not exclusive to acellular versus cellular. In contrast, in mice with mutations in genes that maintain extracellular pyrophosphate levels, such as ank and PC-1, resulting in limited levels of pyrophosphate, one sees formation of cellular cementum even at early stages of root development. These findings suggest an important role for phosphate in controlling the rate of cementum formation.
To understand the specific role of phosphate and other molecules, additional studies that focus on defining the cells and factors controlling development, maintenance, and regeneration of periodontal tissues are required. Some of the factors known to be involved in controlling these events are discussed next and are summarized in Table 9-1.
|SUGGESTED FUNCTION RELATED TO CEMENTOGENESIS|
|Transforming growth factor β superfamily (including bone morphogenetic proteins)||Reported to promote cell differentiation and subsequently cementogenesis during development and regeneration.|
|Platelet-derived growth factor and insulin-like growth factor||Existing data suggest that platelet-derived growth factor alone or in combination with insulin-like growth factor promotes cementum formation by altering cell cycle activities.|
|Fibroblast growth factors||Suggested roles for these factors are promoting cell proliferation and migration and also vasculogenesis—all key events for formation and regeneration of periodontal tissues.|
|These molecules may promote adhesion of selected cells to the newly forming root. Bone sialoprotein may be involved in promoting mineralization, whereas osteopontin may regulate the extent of crystal growth.|
|Epithelial/Enamel Proteins||Epithelial-mesenchymal interactions may be involved in promoting follicle cells along a cementoblast pathway.
Some epithelial molecules may promote periodontal repair directly or indirectly.
|Collagens||Collagens, especially types I and III, play key roles in regulating periodontal tissues during development and regeneration.
In addition, type XII may assist in maintaining the periodontal ligament space versus continuous formation of cementum.
|Matrix Gla protein/Bone Gla protein (osteocalcin)||These proteins contain γ-carboxyglutamic acid, hence the name Gla proteins. Osteocalcin is a marker for cells associated with mineralization—that is, osteoblasts, cementoblasts, and odontoblasts—and is considered to be a regulator of crystal growth. It has also been proposed to act as a hormone regulating energy metabolism through several synergistic functions favoring pancreatic β-cell proliferation, increasing insulin secretion (in pancreas) and sensitivity in peripheral tissues, promoting energy expenditure (in brown adipose tissue) and testosterone production by Leydig cells in testis. Matrix Gla protein appears to play a significant role in preventing abnormal ectopic calcification.|
|Runt-related transcription factor 2 (Runx-2)
|As for osteoblasts, these may be involved in cementoblast differentiation.|
|Osteoprotegerin||These molecules mediate bone and root resorption by osteoclasts.|
|Receptor-activated NF-κB ligand|
|Cementum protein 1 (Cementum-derived protein 23)||May play a role as a local regulator of cell differentiation and extracellular matrix mineralization.|
Bone morphogenetic proteins (BMPs) are members of the transforming growth factor β superfamily that act through transmembrane serine/threonine protein kinase receptors. These signaling molecules have a variety of functions during morphogenesis and cell differentiation, and in teeth they are considered to be part of the network of epithelial-mesenchymal signaling molecules regulating initiation of crown formation. The roles for BMPs in root development, including whether they are implicated in epithelial-mesenchymal signaling, and the signal pathways and transcription factors involved in modulating their behavior remain to be defined. However, several of the BMPs, including BMP-2, BMP-4, and BMP-7, are known to promote differentiation of preosteoblasts and putative cementoblast precursor cells. In addition, BMPs have been used successfully to induce periodontal regeneration in a number of experimental models and in certain clinical situations.
Epithelial-mesenchymal interactions are required for formation of the tooth crown, and epithelial factors are implicated. The same two populations of cells involved in crown morphogenesis—that is, epithelial and ectomesenchymal cells—also take part in root formation. The possibility that such interactions also are required for development of periodontal tissues and that some of the same signaling molecules are involved is thus a logical assumption. Prospective candidates include enamel proteins, parathyroid hormone-related protein, and basal lamina constituents. In the case of enamel proteins, the debate centers around the fact that enamel proteins have not been detected consistently along forming roots. However, this does not rule out a transient expression at early stages of root formation where they could influence odontoblast and/or cementoblast differentiation. Along this line, an enamel matrix derivative, consisting predominantly of amelogenin molecules, is used clinically to stimulate repair and regeneration, but its mechanism of action remains to be determined (see Chapter 15).
Bone sialoprotein and osteopontin are multifunctional molecules associated with cementum formation during development and in repair and regeneration of periodontal tissues. They contain the cell adhesion motif arginine–glycine–aspartic acid and thereby are believed to promote adhesion of selected cells onto the newly forming root. Present data further suggest that both proteins may be implicated in regulating mineral formation on the root surface. The balance between the activities of these two molecules may contribute to establishing and maintaining an unmineralized PDL between cementum and alveolar bone. However, no developmental root anomalies have been reported in knockout mice models, suggesting that other proteins are involved or that there exist compensatory mechanisms.
Proteins enriched in γ-carboxyglutamic acid (Gla), a calcium-binding amino acid, are known as Gla proteins. Bone Gla protein (osteocalcin) is a marker for maturation of osteoblasts, odontoblasts, and cementoblasts, and is considered to regulate the extent of mineralization. Also, this osteoblast-derived hormone may regulate insulin secretion, insulin sensitivity, and energy expenditure. Matrix Gla protein (MGP) has been identified in periodontal tissues and, based on its suggested role as an inhibitor of mineralization, may act to preserve the PDL width. Mice null for MGP exhibit substantial ectopic calcification. However, periodontal development and tooth formation appear to be normal in MGP-null mice; thus additional studies are required to define the role of MGP within periodontal tissues.
As shown in Chapter 6, Runx-2 (runt-related transcription factor 2), also known as Cbfa1 (core binding factor alpha 1), and osterix, downstream from Runx-2, have been identified as master switches for differentiation of osteoblasts. Runx-2 now has been found to be expressed in dental follicle cells, PDL cells, and cementoblasts. Based on similarities between cementoblasts (at least in cellular cementum) and osteoblasts, it is likely that both factors may be involved in cementoblast differentiation. The exact factors triggering expression or activation of these key transcription factors currently are being investigated; BMPs already have been identified as factors promoting expression of Runx-2.
Other molecules that are found within the developing and mature periodontal tissues include alkaline phosphatase, several growth factors (e.g., insulin-like growth factor, transforming growth factor β, and platelet-derived growth factor), metalloproteinases, and proteoglycans. The significance of alkaline phosphatase to cementum formation has long been appreciated and is discussed in a previous section. Proteoglycans accumulate at the dentin-cementum junction, and it has been proposed that, together with noncollagenous matrix proteins such as bone sialoprotein and osteopontin, they may mediate initial mineralization and fiber attachment.
Mineralized tissues such as bone are turning over continually and require a delicate balance between formative and resorptive cells. Two key factors that have emerged as critical to this balance are osteoprotegerin and receptor-activated NF-κB ligand (RANKL). Both are produced by osteoblasts and PDL fibroblasts. As discussed in more detail in Chapter 6, RANKL activates osteoclasts by binding to specific cell surface receptors (RANK), whereas osteoprotegerin acts as a decoy interfering with the binding of RANKL to RANK. Growth factors and cytokines in the local region of the periodontium have been shown to modulate expression of osteoprotegerin and RANKL and thus may be important for controlling osteoclastic-mediated bone and root resorption; thus they may be attractive factors in designing therapeutic agents to regulate the behavior of this cell.
Equally important to the process of cell maturation and function are the timed expression of specific cell surface receptors and the ability of certain factors to regulate their expression and, subsequently, the signaling pathways mediated by ligand-receptor interactions.
Table 9-2 lists the various types of cementum along with the origin, location, and function of each.
|TYPE||ORIGIN OF FIBERS||LOCATION||FUNCTION|
|Acellular (primary)||Extrinsic (some intrinsic fibers initially)||From cervical margin to the apical third||Anchorage|
|Cellular (secondary)||Intrinsic||Middle to apical third and furcations||Adaptation and repair|
|Mixed (alternating layers of acellular and cellular)||Intrinsic and extrinsic||Apical portion and furcations||Adaptation|
|Acellular afibrillar||—||Spurs and patches over enamel and dentin||No known function along the cementoenamel junction|
Cementoblasts that produce acellular extrinsic fiber cementum differentiate in proximity to the advancing root edge. During root development in human teeth, the first cementoblasts align along the newly formed but not yet mineralized mantle dentin (predentin) surface following disintegration of HERS (Figure 9-8, A and B). The cementoblasts exhibit fibroblastic characteristics, extend cell processes into the unmineralized dentin, and initially deposit collagen fibrils within it so that the dentin and cementum fibrils intermingle. Mineralization of the mantle dentin starts internally and does not reach the surface until mingling has occurred. Mineralization then spreads across into cementum under the regulatory influence of noncollagenous matrix proteins, thereby establishing the cementodentinal junction. In rodents, initial cementum deposition occurs onto the already mineralized dentin surface, preventing the intermingling of fibers (see Figure 9-3, A).