The external enamel epithelium forms the outer layer of cuboidal cells which limits the enamel organ. It is separated from the surrounding mesenchymal tissue by a basement membrane. The external enamel epithelial cells contain large, centrally placed nuclei and have relatively small amounts of the intracellular organelles associated with protein synthesis. The cells contact each other via desmosomes and gap junctions. The external enamel epithelium is thought to be involved in the maintenance of the shape of the enamel organ and in the exchange of substances between the enamel organ and the environment. The cervical loop, at which there is considerable mitotic activity, lies at the growing margin of the enamel organ where the external enamel epithelium is continuous with the internal enamel epithelium.

The stellate reticulum is most fully developed at the bell stage. The intercellular spaces become fluid-filled, presumably related to osmotic effects arising from the high concentration of glycosaminoglycans. The cells are star-shaped with bodies containing conspicuous nuclei and many branching processes. The mesenchyme-like features of the stellate reticulum include the synthesis of collagens in the tissue. The cells of this layer possess little endoplasmic reticulum and few mitochondria. However, there is a relatively well-developed Golgi complex, which, together with the presence of microvilli on the cell surface, has been interpreted as indicating that the cells contribute to the secretion of the extracellular material. Numerous tonofilaments are present within the cytoplasm, and there are desmosomes and gap junctions between the cells.

The main function of the stellate reticulum is ‘mechanical’, protecting the underlying dental tissues against physical disturbance and maintaining tooth shape. It has been suggested that the hydrostatic pressure generated within the stellate reticulum is in equilibrium with that of the dental papilla, allowing the proliferative pattern of the intervening internal enamel epithelium to determine crown morphogenesis. The stellate reticulum also produces colony-stimulating factor (CSF-1), transforming growth factor beta-1 (TGF-β ₁) and parathyroid hormone-related protein (PTHrP). These molecules may be released into the dental follicle and help recruit and activate the osteoclasts necessary to resorb the adjacent alveolar bone as the developing tooth enlarges and erupts.

The stratum intermedium first appears at the bell stage and consists of two or three layers of flattened cells lying over the internal enamel epithelium (and its derivatives). The cells of the stratum intermedium resemble the cells of the stellate reticulum, although their intercellular spaces are smaller and the cells contain much alkaline phosphatase. It has been suggested that the stratum intermedium is concerned with:

The cells of the internal enamel epithelium are columnar at the bell stage but, beginning at the regions associated with the future cusp tips (i.e. the sites of initial enamel formation), the cells become elongated. The internal enamel epithelial cells are rich in RNA but, unlike the stratum intermedium and stellate reticulum, do not contain alkaline phosphatase. Desmosomes connect the internal enamel epithelial cells and link this layer to the stratum intermedium. The internal enamel epithelium is separated from the peripheral cells of the dental papilla by a basement membrane and a cell-free zone.

The differentiation of the dental papilla at the early bell stage is less striking than that of the enamel organ. Until the late bell stage, the dental papilla consists of closely packed mesenchymal cells with only a few delicate extracellular fibrils. Histochemically, the dental papilla becomes rich in glycosaminoglycans.

At the early bell stage, downgrowths on the lingual aspect of the enamel organs indicate the early development of the successional (permanent) teeth.

Experiments have shown that the epithelium lining the first pharyngeal (branchial) arch has ‘odontogenic potential’. This potential only exists in the very early stages of odontogenesis because, after initiation, the ‘control’ of tooth development passes to the mesenchyme. At the cap stage of tooth development, the principal organizer is the dental papilla, in terms of both morphogenesis and histogenesis. The result of culturing dental papilla mesenchyme with epithelium from the developing foot pad is normal tooth development, illustrating the importance of the dental papilla. On the other hand, if the enamel organ of a tooth is cultured with mesenchyme from the developing foot pad, tooth development does not occur. Furthermore, should an incisor enamel organ be combined with a molar papilla, the resulting tooth is molariform and, if a molar enamel organ is combined with an incisor papilla, the resulting tooth is incisiform.

It has been established that the presumptive incisor and molar regions contain some difference in their homeobox gene arrays (in the incisor region Msx-1 (but not Barx-1) is expressed; in the molar region Barx-1 (but not Msx-1) is expressed). The odontogenic homeobox code can be considered in the light of a ‘field model hypothesis’, where each tooth germ starts the same but different concentrations of morphogens (e.g. growth factors) in the local environment are responsible for producing different tooth types. In contrast to the ‘field model hypothesis’, a ‘clonal model hypothesis’ has been forwarded to explain tooth form. Accordingly, the tooth type is prespecified and is not dependent on the environment within the jaws. Consequently, if a molar tooth germ is cultured in a site well away from the jaws the complete series of molars can form by budding off from this single precursor.

During tooth development, ‘messages’ pass between the epithelium and mesenchyme to produce changes of increasing complexity (i.e. differentiation) within the cell layers. It has been clearly shown that bioactive signalling molecules in the form of small proteins pass between the epithelium and mesenchyme, and usually have important interactions with the receptors on the cell membrane. Experimental evidence suggests that induction is due to the presence of the initial extracellular matrix, a thin layer situated between the epithelium and mesenchyme and comprising the basal lamina and adjacent region.

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:

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.

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.

Tooth eruption is the process whereby a tooth moves from its developmental position in the jaw into its functional position in the mouth. However, there is no evidence to suggest that eruption entirely ceases once a tooth meets its antagonist in the mouth. Prior to the formation of the root of the tooth, there is concentric growth of the tooth within its follicle without any active bodily movement in a direction indicating eruption towards the oral cavity. Once the root starts to form, the active phase of eruption commences.

As a tooth approaches the oral cavity, the overlying bone is resorbed and there are marked changes in the overlying soft tissues. The enamel surface is covered by the reduced enamel epithelium, which is a vestige of the enamel organ. As the tooth erupts, the outer cells of the reduced enamel epithelium proliferate into the connective tissue between the cusp tip and the oral epithelium. It has been suggested that these proliferating epithelial cells secrete enzymes that degrade collagen. Reduced enamel epithelial cells may also remove breakdown products resulting from resorption of connective tissue. Depolymerization of the non-fibrous components of the extracellular matrix has been detected in the connective tissue overlying erupting teeth. Although a relationship between the degeneration of the connective tissue and the pressure exerted by the underlying erupting tooth has not been established, ischaemia is thought to be a contributory factor. Many of the fibroblasts in the connective tissue overlying an erupting tooth cease fibrillogenesis, actively take up extracellular material (as evidenced by intracellular collagen profiles) and synthesize acid hydrolases. Eventually, the cells degenerate.

The development of the dentogingival junction occurs as the tooth emerges into the oral cavity. As the tooth approaches the oral epithelium, the cells of the outer layer of the reduced enamel epithelium and the basal layer of the oral epithelium actively proliferate and eventually unite. The epithelium covering the tip of the tooth then degenerates at its centre, enabling the crown to emerge through an epithelial-lined pathway into the oral cavity. Further emergence of the tooth results from active eruptive movements and passive separation of the oral epithelium from the crown surface. When the tooth first erupts into the mouth, the reduced enamel epithelium is attached to the unerupted part of the crown, thus forming an epithelial seal — the junctional epithelium. It is generally believed that the reduced epithelial component of the junctional epithelium is eventually replaced by oral epithelium. With continued eruption, as more of the crown is exposed, a gingival crevice is formed.