Development of the dentition

4 Development of the dentition

Humans have two dentitions, the deciduous (primary) and permanent (secondary). Each dentition is heterodont, meaning that it consists of teeth with different shapes and functions. The classes of human teeth are:

Deciduous teeth are progressively replaced by permanent teeth, with the addition of a molar dentition in the posterior region of the jaws.

Prenatal development of the dentition

Teeth form on the frontonasal process and on the paired maxillary and mandibular processes of the first pharyngeal arch. They are derived from two embryonic cell types:

The anatomy of tooth development

In the human embryo, development of the deciduous dentition begins at around 6 weeks with the formation of a continuous horseshoe-shaped band of thickened epithelium around the lateral margins of the primitive oral cavity. The free margin of this band gives rise to two processes, which invaginate into the underlying mesenchyme:

Discreet swellings of the dental lamina form the enamel organs of the future developing teeth. Epithelial cells of the enamel organ proliferate and progress through characteristic bud, cap and bell stages. Simultaneously, the dental papilla is formed by localized condensation of neural crest-derived ectomesenchymal cells around the epithelial invaginations. More peripherally, ectomesenchymal cells extend around the enamel organ to form the dental follicle. Together, these tissues constitute the tooth germ and will give rise to all structures that make up the mature tooth (Fig. 4.1).


Figure 4.1 Early tooth development.

Localized proliferation of the oral epithelium gives rise to a thickening, which invaginates into the underlying jaw mesenchyme to form the tooth bud. Simultaneously, neural crest cells condense around the bud and these two tissues form the tooth germ. At the cap stage, the tooth bud folds to demarcate the early morphology of the crown, which is modified by further folding at the bell stage. During the bell stage, the innermost layer of cells within the epithelial component of the tooth germ, the inner enamel epithelium, induce adjacent cells of the dental papilla to differentiate into odontoblasts, responsible for the formation and mineralization of dentine. Dentine formation is preceded by the formation of predentine. The first layer of predentine acts as a signal to the overlying inner enamel epithelial cells to differentiate into ameloblasts and begin secreting the enamel matrix. At the margins of the enamel organ, cells of the inner enamel epithelium are confluent with the outer enamel epithelial cells at the cervical loop. Growth of these cells in an apical direction forms a skirt-like sheet called Hertwig’s epithelial root sheath, which maps out the future root morphology of the developing tooth and induces the further differentiation of root odontoblasts. Degeneration of this root sheath leads to exposure of the cells of the dental follicle to the newly formed root dentine and differentiation into cementoblasts, which begin to deposit cementum onto the root surface. Surrounding the enamel organ, the cells of the dental follicle produce the alveolar bone and collagen fibres of the periodontium. The developing tooth remains housed in this cavity of alveolar bone until the process of eruption begins. cl, cervical loop; dp, dental papilla; df, dental follicle; dl, dental lamina; eee, external enamel epithelium; iee, internal enamel epithelium; oe, oral epithelium; sr, stellate reticulum.

The permanent dentition replaces the deciduous dentition and is composed of both successional and accessional teeth (Fig. 4.2):

The molecular control of tooth development

The histological basis of tooth development has been understood for some time, but in recent years progress has been made in understanding molecular mechanisms that underlie the process of odontogenesis using mouse models (Box 4.1). The generation of a tooth requires coordinated molecular signalling between epithelium of the early jaws and the underlying neural crest cells that migrate into these regions (Cobourne & Sharpe, 2003; Tucker & Sharpe, 2004).

Patterning the dentition: a molecular code for tooth shape

The mouse jaw is demarcated into future incisor and molar-forming regions on a molecular basis before any morphological evidence of tooth development has occurred. Fgf8 is a signalling molecule belonging to the fibroblast growth factor (Fgf) family, which localizes to the future molar regions of the jaw epithelium. In contrast, Bmp4, a signalling molecule of the bone morphogenetic protein (Bmp) family, localizes to the early incisor epithelium. These signalling molecules induce the expression of a number of homeobox-containing genes that encode transcription factor proteins in the tooth-forming neural crest-derived ectomesenchyme of the early jaws.

Initially, signalling from the epithelium to neural crest cells that migrate into the jaws can induce expression of a range of genes. However, expression domains very rapidly become established and then independent of epithelial signalling. It has been suggested that the differing combinations of gene expression patterns act to specify tooth shape (Sharpe, 1995). The ‘odontogenic homeobox code’ predicts that for each tooth-forming region of the early maxilla and mandible, the morphology of the developing tooth is dictated by a specific combination of homeobox genes within the ectomesenchyme (Box 4.2). Thus, for the molar region of mouse jaws, an overlapping code of Barx1 and Dlx2 exists (Fig. 4.3). There are several important points to note with regard to the homeobox model (Sharpe, 2001):

This final point is important because the peripheral regions of overlap between teeth of different classes appear to be particularly vulnerable with regard to human hypodontia. In these cases, teeth at the end of a series (upper lateral incisors, lower second premolars, third molars) are those most commonly congenitally absent.

Initiation of tooth development

Once the ectomesenchyme within each mouse jaw has been regionalized into presumptive incisor and molar domains, tooth development is initiated within the jaw epithelium. A key player in this process is sonic hedgehog (Shh), a protein produced in localized regions of jaw epithelium where the teeth are going to form. Shh drives proliferation of the dental lamina within these regions, resulting in formation of the tooth buds; if Shh signalling is lost in the early dental lamina, teeth fail to develop. Restriction of Shh production is therefore important in ensuring that teeth develop in the correct regions of the jaws and this is orchestrated by molecular compartmentalization of the jaw epithelium into tooth-forming and non-tooth-forming regions. Specifically, expression of the Shh gene is restricted to the tooth-forming regions because it is repressed throughout the non-dental epithelium by another signalling molecule called Wnt7b. Therefore, tooth formation in the correct regions of the jaws is established via reciprocal expression domains between two different signalling molecules within the jaw epithelium (Fig. 4.4).

Once the tooth bud has formed, a number of homeobox-encoding genes subsequently localize to the condensing dental papilla, including Msx1 and Pax9. These genes play an important role in mediating later signalling between the underlying ectomesenchyme and the epithelial bud as tooth development progresses to the cap stage (Fig. 4.5). A loss of either gene leads to the arrest of tooth development at the bud stage in the mouse. Mutations associated with human MSX1 and PAX9 have also been implicated in hypodontia.

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Jan 1, 2015 | Posted by in Orthodontics | Comments Off on Development of the dentition

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