The tissues of the dental follicle in the developing root are comprised of three layers:
• Adjacent to the epithelial root sheath is the inner investing layer of the dental follicle, which is said to be derived from the neural crest.
• Adjacent to the developing alveolar bone is the outer layer of the dental follicle.
• The outer layer is separated from the inner layer by an intermediate layer.
The outer and intermediate layers are mesodermal in origin. Cells of the inner layer of the dental follicle differentiate into the cementoblasts. Once cementogenesis has begun, cells of the remaining dental follicle become obliquely oriented along the root surface and become the fibroblasts of the periodontal ligament.
Cementogenesis is here considered in terms of the formation of primary (acellular) cementum and then of secondary (cellular) cementum. As for the crown, the hard tissues that comprise the root (i.e. cementum and dentine) develop under the control of epithelial/mesenchymal interactions. Unlike the crown, the epithelial component involved in root formation retains a simpler morphology, rapidly loses its continuity with adjacent cells, and is not evident as a conspicuous layer during initial cementum formation.
Primary (acellular) cementum
Once the crown has fully formed, the internal and external enamel epithelia proliferate downwards as a double-layered sheet of somewhat flattened epithelial cells, the epithelial root sheath (of Hertwig) that maps out the shape of the root(s). The process of cementogenesis is initiated at the cervical margin and extends apically as the root grows downwards. The epithelial root sheath is separated by a basal lamina on both of its surfaces from the adjacent connective tissues of the dental follicle and dental papilla. The epithelial root sheath induces the adjacent cells of the dental papilla to differentiate into odontoblasts. As these odontoblasts initially retreat inwards, they synthesize and secrete the organic matrix of the first-formed root predentine. As the odontoblasts do not leave behind an odontoblast process in this initial few microns of tissue, its structureless (and later glass-like) appearance is responsible for the term hyaline layer that is given to this (approximately 10 μm) layer once it is mineralized. The epithelial root sheath is in contact with the initial predentine layer for only a short distance before the continuity of its cells is lost (i.e. the sheath ‘fenestrates’). There is evidence that the epithelial root sheath cells secrete enamel-related protein(s) into the collagenous matrix of the hyaline layer at the cement–dentine boundary. Thus, the hyaline layer is formed by contributions from both the odontoblast and epithelial root sheath layers. The enamel-related protein(s) has been identified as amelogenin, although there is some dispute as to whether another enamel-related protein, ameloblastin, is also present. The function of such enamel-related proteins is unclear but may concern epithelial/mesenchymal interactions involving the induction of odontoblasts and cementoblasts, and/or the process of mineralization. During the subsequent mineralization of cementum and the hyaline layer, the enamel-related protein(s) is lost, although remnants may be retained in the granular layer of the root dentine. Mineralization of the first-formed dentine does not initially occur at the outermost surface of the hyaline layer, but a few microns within it. From this initial centre, mineralization spreads both inwards towards the pulp and outwards towards the periodontal ligament (centrifugally). Thus, the outermost part of the hyaline layer undergoes delayed mineralization.
The cause of ‘fenestration’ of the epithelial root sheath is not known, but may be due to programmed cell death (apoptosis). Fibroblast-like cells of the adjacent dental follicle pass through the fenestrations and come to lie close to the surface of the hyaline layer. These cells become cementoblasts associated with the formation of primary cementum but they do not form a conspicuous layer on the forming root surface and may retreat and mingle with adjacent fibroblasts of the periodontal ligament. The precise origin of the fibroblast-like cells is not clear. They might appear to be derived from the cells of the investing layer of the dental follicle. However, there is also evidence suggesting that they may be derived from epithelial root sheath cells as a result of epithelial/mesenchymal transformation. Whatever their origin, these cells are responsible for producing a ‘fibrous fringe’ on the surface of the dentine. During the next phase of development in the formation of acellular cementum, the delayed mineralization front in the hyaline layer gradually spreads outwards (centripetally) until this layer is fully mineralized and then continues on into the first few microns of the ‘fibrous fringe’. In this manner, the first few microns of primary cementum are firmly attached to the root dentine. At this stage, the collagen fibres in the adjacent periodontal ligament are oriented to be more parallel to the root surface and have not yet gained an attachment to the ‘fibrous fringe’.
As with bone, the early stage of acellular cementum formation results in the secretion by the associated cementoblasts of various non-collagenous proteins (e.g. osteopontin, cementum-attachment protein, bone sialoprotein), cytokines and growth hormones. The precise roles of such molecules await clarification but it has been suggested that they may play a role in bonding the cementum to the outer surface of the root dentine. The subsequent development of acellular cementum involves:
• its slow increase in thickness
• the establishment of continuity between the principal collagen fibres of the periodontal ligament with those of the ‘fibrous fringe’ at the surface of the root dentine
• continued slow mineralization of the collagen.
It is only with the establishment of continuity between periodontal ligament fibres and those of the initial ‘fibrous fringe’ that the tooth can be properly supported within the socket. Once periodontal ligament fibres become attached to the surface of the cementum layer, the cementum may be classified as acellular extrinsic fibre cementum (see page 195). It increases slowly and evenly in thickness throughout life at a rate of about 2 μm per year. Although the cementoblasts may not form a distinctive and recognizable layer of cells that can be distinguished from adjacent cells of the periodontal ligament, some cells lying between the perpendicularly oriented periodontal fibre bundles may become more cuboidal and contain small amounts of the intracellular organelles associated with protein synthesis and secretion. Such secretion is polarized at the surface of the cells adjacent to the cementum surface and, together with the slow rate of formation, ensures that the cells are not entombed by their own secretion.
Mineralization of the cementum matrix does not appear to be controlled by its cells and initiation of mineralization probably occurs from the dentine. Indeed, when mineralization of initial root dentine is interfered with, there is inhibition of cementogenesis. The adjacent periodontal ligament fibroblasts are rich in alkaline phosphatase and may also play a role in mineralization. Mineralization proceeds very slowly in a linear fashion. Owing to the slow progress of mineralization, there is usually no evidence of a layer of precementum associated with acellular cementum.
Cementogenesis occurs rhythmically, periods of activity alternating with periods of quiescence. Structural lines may be visible within the tissue, indicating the incremental nature of its formation. The periods of decreased activity are associated with these incremental lines, which are believed to have a higher content of ground substance and mineral and a lower content of collagen than the adjacent cementum. These lines may also reflect changes in crystallite orientation. The periodicity of the incremental lines might be annual and can be used to age individuals. As acellular cementum is formed very slowly, the incremental lines are closer together than corresponding lines seen in cellular cementum that is deposited more rapidly.
Secondary (cellular) cementum
Following the formation of primary cementum in the cervical portion of the root, secondary cementum appears in the apical region of the root at about the time the tooth erupts. Secondary cementum is also formed in the furcation area of the cheek teeth. This type of cementum is associated with an increase in the rate of formation of the tissue. The early inductive changes associated with the development of odontoblasts and dentine appear to be similar to those described for primary cementum. However, following the loss of continuity of the epithelial root sheath, large basophilic cells are seen to differentiate from the adjacent cells of the dental follicle against the surface of the root dentine. These cells form a more distinct cuboidal layer of cementoblasts adjacent to the root surface. They generally possess more cytoplasm and more cytoplasmic processes than the cells associated with the formation of acellular cementum. The basophilia at the light microscope level corresponds to roughened endoplasmic reticulum at the ultrastructural level and indicates that the cementoblasts secrete the collagen (together with ground substance) that forms the intrinsic fibres of the secondary, cellular cementum. These fibres are oriented parallel to the root surface and do not extend into the periodontal ligament. Associated with the increased rate of formation, a thin unmineralized precementum layer (about 5 μm thick) will be present on the surface of cellular cementum. Mineralization in the deeper layer of the precementum occurs in a linear manner but, overall, this type of cementum is less mineralized than primary cementum. As in bone, the multipolar mode of matrix secretion by the cementoblasts and its increased rate of formation result in cells becoming incorporated into the forming matrix, and these are converted into cementocytes. Thus, this is a cellular cementum and, since it usually presents as the intrinsic fibre type, this type of cementum does not act in a supportive role, there being no Sharpey fibres from the periodontal ligament inserted into it. Incremental lines will be present in secondary (cellular) cementum but, due to the increased rate of formation, are more widely spaced than in acellular cementum.
As the chemical composition of primary and secondary cementum differs, it is assumed that this reflects differences in the secretory activity of the cells involved. Thus, dentine sialoprotein, fibronectin and tenascin, as well as a number of proteoglycans (e.g. versican, decorin and biglycan), are present in cellular cementum but not in acellular cementum. This may be related to the presence of cementocytes, as many of the proteoglycans are located at the periphery of the lacunae and canaliculi. The precise origin of the cells in the dental follicle associated with the formation of cellular cementum awaits clarification. The possibility exists that different cell populations are responsible for the formation of primary (acellular) and secondary (cellular) cementum. Due to the similarity between osteoblasts and cementoblasts, it has been suggested that stem/progenitor cells primarily associated with alveolar bone could migrate into the periodontal ligament and provide a source of new cementoblasts.
The development of the periodontal ligament is described on page 210