Role Of Collagen In The Periodontal Ligament - A Review

摘要

As in most connective tissues the fibers of the periodontal ligament are mainly collagenous in nature. The fibers of the periodontal ligament appear similar to those of other supportive connective tissues in that they are composed of an integrated unit of fibrous components. Nevertheless, the fibers of the periodontium have a particular structural requirement to withstand intensive forces from mastication and to accommodate tooth eruption. Introduction Ligament The connective tissue fibers are produced by fibroblasts and can be divided into a. Collagen fibers b. Reticulin fibers c. Oxytalan fibers d. Elastic fibers At least 18 species of collagen has been isolated from the extracellular matrix of connective tissues (Kielty et al., 1998). The Collagen of the periodontal ligament is largely type I, with lesser amount of type III, IV, V, VI & XII also present (Butler et al.3; Walker et a16 .; Sloan et al14). Collagen type I, III & V are ‘banded’ molecules which have a characteristic periodicity of 67 nm when stained and examined in the electron microscope, owing to the staggered arrangement of the molecules in the fibril. Collagen type III is co-distributed with collagen type I but is found in higher proportion in fetal tissues (Berkovitz,2 ). Collagen type V also co-distributes with type I but it can only be demonstrated immuno-histochemically by disrupting the fibrila. Collagen type IV is found in the basement membrane of junctional epithelium and epithelial rests of the periodontal ligament (Sloan et aI.14). The collagen fibril diameters of the human periodontal ligament are relatively small, with mean diameters of the order 45-55 mm (Berkovitz et al.2) which is much less when compared to other connective tissues, e.g. Tendon fibril diameters may reach up to 250 mm. The small diameter of the fibrils in the periodontal ligament could be the result of either the high rate of collagen turnover or the absence of mature collagen fibrils (Berkovitz et al.2). It has also been suggested that fibril diameter is regulated by the co-polymerization of different collagen molecules and hence the small diameter of the collagen fibrils in the periodontal ligament may be due to specific fibril composition, but there is no comparative evidence to prove it. Plastic Fibers Of The Periodontal Ligament The elastic fibers are associated with collagen fibers as an elastic meshwork and are composed of three fibrous components – Oxytalan, Elaunin, and elastin. Oxytalan And Elaunin Oxytalan and Elaunin fibers can be demonstrated with elastin stains in light microscope (Fullmer,7). Oxytalan fibers form a three dimensional meshwork that extends from the cementum to the peripheral periodontal blood vessels Depending on the site and species, oxytalan fibers measures between 0.2 and 1.5 μm in diameter in the electron microscope and is reported to occupy 3% of the periodontal ligament in humans (Shore, Moxham and Berkovitz, ). In transmission electron microscope, oxytalan fibers appear similar to developing elastin fibers and are composed of groups of microfilaments embedded within amorphous material (Fullmer et al.,7; Shore, Moxham and Berkovitz,). The similarities between the ultrastructure of Oxytalan microfibrils and fibrils of fibronectin have been commented by Frank and Nalbandian, 1989, like fibronectin Oxytalan fibers are important for fibroblast adhesion and migration.Elastin Elastin fibers are present in only some periodontal ligament of some species and are composed of microfibrillar glycoprotein and amorphous elastin (Ross12). These elementary units of elastin are rod like in shape and are aggregated into globular protein masses. Arrangement Of The Periodontal Collagen Fibers The fibers of periodontal ligament are arranged in 5 major groups.The fiber bundles were considered to pass directly from – bone to cementum, supporting the tooth in the manner of a sling. Grant et al., 8 demonstrated using light and transmission, electron microscopy that the