Читать книгу Biological Mechanisms of Tooth Movement - Группа авторов - Страница 41

The PDL, the root cementum, and the alveolar bone consist, like all connective tissues, of cells and ECM of which the principal component is formed by fibers, embedded in a gel‐like ground substance (Kerrigan et al., 2000; Nanci and Bosshardt, 2006).

The most predominant type of fiber in the alveolar bone, the PDL, and the root cementum is collagen type I, which is mainly present as strong extracellular fibers (Figure 3.1A). This protein is synthetized by fibroblasts starting with the intracellular synthesis of triple helices called procollagen, containing two type‐α1 and one type‐α2 “pre‐procollagen” peptide chains. Procollagen is secreted by exocytosis. Outside the cell, procollagen is assembled into collagen fibrils and subsequently, by crosslinking, into collagen fibers. These fibers have a very well organized and complex internal structure that resembles a hawser. This structure results in flexible fibers with a great tensile strength (Kerrigan et al., 2000; Wenger et al., 2007; Dean, 2017).

Figure 3.1 Photomicrographs of the normal PDL in a dog, showing the main orientation of the collagen fibers (A, H & E staining) and the oxytalan fibers (B, Oxone‐Halmi Aldehyde Fuchsin staining).

(Source: Jaap Maltha.)

A second type of fiber in the PDL is the oxytalan fiber (Figure 3.1B). This type of fiber belongs to the elastic fiber family, which consists of elastic, elaunin, and oxytalan fibers. The elastic and elaunin fibers mainly contain elastin and fibrillins, while the oxytalan fibers lack elastin and only contain fibrillin‐1 and fibrillin‐2. These glycoproteins are synthetized in fibroblasts and polymerize after exocytosis, and, through lateral association and the incorporation of other components, they form microfibrils. Individual microfibrils again associate with one another to form microfibril bundles, the oxytalan fibers (Marson et al., 2005; Hubmacher et al., 2006; Kielty, 2006; Strydom et al., 2012).

The ground substance is primarily composed of water and large organic molecules, such as glycosaminoglycans (GAGs), including hyaluronic acid, heparan sulfate, dermatan sulfate, and chondroitin sulfate. Most of the GAGs are bound to proteins and then called proteoglycans. They are able to bind a considerable amount of water, giving the ground substance a gel‐like texture (Nanci and Bosshardt, 2006; Bergomi et al., 2010; Ortun‐Terrazas et al., 2018).

In the PDL, but also in all other connective tissues, the fibrous components are embedded in the “ground substance”, a network of proteoglycans, such as heparan sulfate, dermatan sulfate, and chondroitin sulfate, consisting of a core protein covalently bound to GAG chains. These GAGs can form large complexes when hundreds of GAG molecules become noncovalently attached to a single long polysaccharide molecule, such as hyaluronic acid. Under physiological conditions, GAGs have a strong water‐binding capacity as the GAG chains are negatively charged due to the presence of sulfate and uronic acid groups (Nanci and Bosshardt, 2006; Bergomi et al., 2010; Ortun‐Terrazas et al., 2018). They form an amorphous gel‐like structure, of which the stiffness depends on the amount of bound water. Apart from the bound water, also free water is present in the ground substance. The viscoelastic characteristics recorded for the PDL essentially result from interactions between unbound fluid and the compressible visco‐elastic porous matrix that makes up the bulk of the ground substance. Proteoglycans of the ground substance also bind to fibrous matrix proteins, such as collagen and oxytalan (Svensson et al., 2001). Together, these components form the ECM, which acts as a substrate for PDL cells and allows them to migrate and to communicate with each other (Kerrigan et al., 2000; Waddington and Embery, 2001; Nanci and Bosshardt, 2006; Dean, 2017; Listik et al., 2019).

Finally, the PDL contains extensive vascular and neural systems. The blood vessels originate from three sources: apical vessels, which branch from vessels that supply the pulp; perforating vessels, which originate from the lamina and perforate the cribriform plate in the socket wall; and the gingival vessels, which come from the gingival tissue. Blood vessels in PDL may help in mechanical suspension and support of the tooth and supply surrounding PDL. These vessels transport blood cells, nutrients, and oxygen to the tissues of the PDL and remove waste and carbon dioxide (Lee et al., 1991; Selliseth and Selvig, 1994; Dean, 2017)

The neural system in the PDL contains free and specialized nerve endings. Free nerve endings are nociceptive and are present along the whole length of the tooth. The specialized nerve endings are divided in Ruffini‐like endings, coiled nerve endings, spindle‐shaped nerve endings, and expanded nerve endings. The Ruffini‐like endings are mainly present near the root apex and secrete various neuropeptides, such as calcitonin gene‐related peptide (CGRP) and substance P. The coiled type endings are mainly located in the mid‐region of the PDL. Both act as mechanoreceptors and as fast acting nociceptors (Maeda et al., 1990; Davidovitch, 1991; Maeda et al., 1999; Krishnan and Davidovitch, 2006; Yamaguchi et al., 2012 Dean, 2017)