It is possible to recognize different types of cementum based on the presence or absence of cells and fibers. Afibrillar (no fibers) cementum is uncommon but may be seen overlapping the enamel for a short distance at the CEJ. Most cementum contains fibers from two sources. Intrinsic fibers are thin and sparse and laid down as part of the ground substance. Extrinsic fibers come from the periodontal ligament and are trapped in the cementum as it forms. These extrinsic fibers provide an anchor of attachment between the periodontal fibers and the root of the tooth. The cells which form cementum are not evenly distributed but are more common toward the apex of the root surface, where there is active formation of new cementum (▶ Fig. 3.11).
Cementum shows incremental lines which correspond to periods of inactivity. Mostly, there is apposition of cementum which continues throughout life; in fact, cementum thickness is a useful indication of the age of a tooth (see Chapter 11 Ageing). Cementum rarely seems to resorb under natural conditions but may do so in response to excessive forces used during orthodontic tooth movement. Sometimes, cementum accumulates in unusually thick deposits (hypercementosis), and this may make extraction of teeth difficult as the bulbous apex locks the tooth into the bony socket.
3.6.2 Origins of Cementum
Cementum is formed by cementoblasts which may be derived from the dental epithelium, but there is also a view that their origins are from the dental follicle. There is a close relationship between mature enamel and cementum. In the teeth of herbivores, such as elephants and cows, cementum is deposited against enamel between the cusps of the tooth and contributes to the composite surface of the tooth when wear has exposed dentin. This association of cementum may be found in its origins. There is evidence that cementoblasts are derived from remnants of the enamel epithelium under the inductive influence of cells from dental mesenchyme.3 On the cell membranes of cementoblasts and osteoblasts, there are specific receptors (integrins) for an adhesion molecule in the matrix of mineralizing tissue, called bone sialoprotein (BSP). Just before the first layer of cementum is formed on the developing root surface, the dental follicle cells produce BSP which is subsequently found in mineralizing cementum. This suggest an important role played by BSP in the differentiation of cementoblasts prior to mineralization. While the participation of dental follicle cells is necessary, the progenitor cell of the cementoblasts appears to be derived from the dental epithelium. The epithelial cell rests described by Malassez have, in tissue cultures, been found to be essential for cementum formation. They synthesize a protein, amylin, which is localized to the area of cementum formation. The epithelial cell rests also appear to induce the formation of the fibrous attachment between root surface and adjacent bone. Between the cementum and dentin there is an intermediate, highly calcified layer. It has been called intermediate cementum, but there is evidence that it is not produced by cementoblasts or odontoblasts. This layer contains enamel proteins and may be a very thin layer of enamel. The conclusions from many studies is that the epithelial cell rests retain their potential to differentiate into mesenchyme stem–like cells capable of secreting both bone, collagen, and cementum and thus may be the progenitor cells of cementoblasts.4 It has also been argued that cementoblasts are not derived from either Hertwig's root sheath or ECRs but from mesenchymal cells of the dental follicle under the influence of ECRs.5 The epithelial cell rests may play an important role in periodontal regeneration.
Fig. 3.11 A undecalcified section through the root of a molar tooth (magnification × 500). The image has been selected from an area halfway toward the root apex showing the transition between acellular (ac) and cellular (c) cementum. The periodontal ligament (pl) has been removed during processing D, dentin.
3.6.3 Changes in Cementum with Aging
We have noted that continued eruption of the tooth, to accommodate for occlusal wear, may occur due to the deposition of new cementum to the root apex. This thickening of apical cementum appears to occur even if there is little tooth wear. It is a feature of aging and is one of the more reliable features used by forensic pathologists in estimating the age of a tooth or the body in which it is found.
3.6.4 Cementum Formation in Healing
The specific morphology of tooth support requires that fibers, embedded in the bone of the tooth socket, are also embedded in cementum covering the tooth root. Periodontal ligament fibers cannot adhere to the tooth root if the surface is covered by epithelium. Downgrowth of the junctional epithelium onto the surface of the cementum thus prevents fiber attachment to the root surface. If the tooth root is cleaned and the epithelium removed, it grows back during healing, faster than new cementum can form. Surgical techniques have been developed which prevent this downgrowth of epithelium. An artificial membrane is placed over the bone and root surface with the intention of preventing downgrowth of epithelium while healing occurs. This technique, called guided tissue regeneration, may promote the deposition of new cementum onto the root surface. The principle of guided tissue regeneration has been successfully applied in encouraging new bone formation around a dental implant, although the evidence for its success in regenerating periodontal ligament reattachment is less secure.
The new cementum is dependent on the differentiation of cementoblasts from cells dormant in the periodontal ligament. Only after new cementum is laid down against the root surface are fibers able to become incorporated in the new cementum and reattach the root to the alveolar bone. New bone then forms in the tooth socket, which traps the periodontal fibers, and a structural ligament is once more created.
The stimulus for the differentiation of new cementoblasts is provided by enamel matrix protein produced by epithelial cell rests. A derivative of these proteins from pigs has been shown to induce the differentiation of cementoblasts, fibroblasts, and osteoblasts. Clinical trials have shown that enamel matrix proteins are able to increase the success rate of reattachment of the tooth to the bony socket.6
Tooth displacement by an orthodontic appliance is followed by resorption of bone lining the tooth socket. This resorption is effected by osteoclasts. These are large multinucleate bone cells which appear to remove a lacuna (Latin lake) of bone around them. Once the tooth has repositioned, healing occurs first by migration of fibroblast-like cells into the resorption lacunae. After 3 weeks, new bone appears in the resorption defects. Associated with new bone formation after physiological tooth drift in rats are the noncollagenous bone proteins, osteonectin, osteopontin, and osteocalcin. It is likely that all three proteins are influential in controlling the balances of resorption and deposition of mineral in bone remodeling and healing (see Chapter 7.7.4 Tooth Repositioning).
Key Notes
Following periodontal disease, the reattachment of collagen fibers to the root surface is an essential step in repair of the entire periodontium. It is impossible without the regeneration of cellular cementum in which new fibers may be embedded. This regeneration cannot take place while the root surface is covered by a downgrowth of the epithelial attachment.
3.7 Junctional Epithelium
The junctional epithelium is an extension of the epithelium of the gingival sulcus but has some important differences. Firstly, unlike the epithelium of the gingival sulcus, it is