Тіс тамырының қабықшасы - цемент

Specialized calcified substance covering the root of a tooth

Cementum is a specialized calcified substance covering the root of a tooth. The cementum is the part of the periodontium that attaches the teeth to the alveolar bone by anchoring the periodontal ligament.

The cells of cementum are the entrapped cementoblasts, the cementocytes. Each cementocyte lies in its lacuna, similar to the pattern noted in bone. These lacunae also have canaliculi or canals. Unlike those in bone, however, these canals in cementum do not contain nerves, nor do they radiate outward. Instead, the canals are oriented toward the periodontal ligament and contain cementocytic processes that exist to diffuse nutrients from the ligament because it is vascularized. After the apposition of cementum in layers, the cementoblasts that do not become entrapped in cementum line up along the cemental surface along the length of the outer covering of the periodontal ligament. These cementoblasts can form subsequent layers of cementum if the tooth is injured. Sharpey fibers are part of the principal collagenous fibers of the periodontal ligament embedded in the cementum and alveolar bone to attach the tooth to the alveolus. This is often due to gingival recession and may be an indication of periodontal disease.

The cementum joins the enamel to form the cementoenamel junction (CEJ), which is referred to as the cervical line. Three possible types of transitional interfaces may be present at the CEJ. The traditional view was that certain interfaces dominated in certain oral cavities. The CEJ may exhibit all of these interfaces in an individual's oral cavity, and there is even considerable variation when one tooth is traced circumferentially. This interface is not as defined, either clinically or histologically, as that of the dentinoenamel junction (DEJ), given that cementum and dentin are of common embryological background, unlike that of enamel and dentin.

The different categories of cementum are based on the presence or absence of cementocytes, as well as whether the collagen fibres are extrinsic or intrinsic. It is thought that fibroblasts, and some cementoblasts, secrete extrinsic fibres, but only cementoblasts secrete intrinsic fibres. The extrinsic fibres within acellular extrinsic fibre cementum, travel perpendicular to the surface of the root and allow the tooth to attach to the alveolar bone by the periodontal ligament (PDL), continuous with the cementodentinal junction (CDJ). Acellular cementum only contains extrinsic collagen fibres. Whereas, cellular cementum is quite thick and contains both extrinsic and intrinsic collagen fibres. The acellular layer of cementum is living tissue that does not incorporate cells into its structure and usually predominates on the coronal half of the root; cellular cementum occurs more frequently on the apical half. Cellular cementum contains cells and is the medium of attachment of collagen fibres to the alveolar bone. It is also responsible for minor repair of any resorption by continued deposition to keep the attachment apparatus intact. Acellular cementum does not contain cells and has a main purpose of adaptive function.

Cementum is slightly softer than dentin and consists of about 45% to 50% inorganic material (hydroxylapatite) by weight and 50% to 55% organic matter and water by weight. The organic portion is composed primarily of collagen and proteoglycans. Cementum is avascular, receiving its nutrition through its own imbedded cells from the surrounding vascular periodontal ligament.

Some root resorption of the apical portion of the root may occur, however, if orthodontic pressures are excessive and movement is too fast. Some experts also agree on a third type of cementum, afibrillar cementum, which sometimes extends onto the enamel of the tooth. The excessive buildup of cementum on the roots of a tooth is a pathological condition known as hypercementosis. Cementum thickness can increase on the root end to compensate for attritional wear of the occlusal/incisal surface and passive eruption of the tooth. When cementum is exposed through gingival recession, it quickly undergoes abrasion by mechanical friction because of its low mineral content and thinness. The exposure of the deeper dentin can lead to problems such as extrinsic staining and dentinal hypersensitivity. Cemental spurs can be found at or near the CEJ. These are symmetrical spheres of cementum attached to the cemental root surface, similar to enamel pearls. Cemental spurs result from irregular deposition of cementum on the root. They can present some clinical problems in differentiation from calculus and may be noted on radiographs; yet, because they are hard dental tissue, they are not easily removed, and thus may also interfere with periodontal treatment. Teeth are increasingly utilized as a source of nuclear DNA to aid identification of human remains. DNA extraction and the results of genetic analysis from the tissue are extremely variable and to some extent unpredictable. However, the quantity of DNA available in dentin is affected by age and dental disease, whereas that in cementum is not.