Chapter 2

Development of the dentition

Alastair J. Sloan

School of Dentistry, Cardiff University, UK

The process of tooth development—or odontogenesis—is a complex series of reciprocal cellular interactions, by which teeth form from epithelial and mesenchymal cells in the stomatodeum. Enamel, dentine, cementum and the periodontium must all develop during appropriate stages of embryonic development. Primary teeth begin to form between the sixth and eighth weeks of intrauterine (i.u.) life, and permanent teeth begin to form in the twentieth week. If teeth do not start to develop around those times, it is likely that they will not develop at all and be missing.

2.1 Early tooth development

The stomatodeum is lined by a primitive epithelium which is two or three cells in thickness. Beneath this is embryonic connective tissue, the ectomesenchyme (Figure 2.1). The first sign of tooth development within the stomatodeum is a thickening of the epithelium and this thickening is called the primary epithelial band. It forms at around 6 weeks of i.u. life and indicates the position of the future dental arches. The primary epithelial band rapidly divides into two structures, the dental lamina and the vestibular lamina. The latter ultimately gives rise to the vestibule/sulcus while the former gives rise the to the tooth germs. At 6 weeks there is no vestibule/sulcus between cheek and tooth-bearing area. The vestibule forms from proliferation of vestibular lamina into the ectomesenchyme. The vestibular lamina cells rapidly enlarge, then degenerate leaving a cleft which becomes the vestibule.

Figure 2.1 (a) Stomatodeum with primary epithelial band (arrow). MP, maxillary process; T, tongue; MA, mandibular arch. (b) Primary epithelial band at high magnification

The dental lamina is the structure that gives rise to the tooth germs, and proliferation of the dental lamina at 6–7 weeks i.u. determines the positions of future deciduous teeth with a series of 20 epithelial ingrowths into ectomesenchyme (10 in each development jaw). This first incursion of the epithelial dental lamina into the mesenchyme leads to a bud of cells at the distal aspect of the dental lamina and is called the bud stage of tooth development (Figure 2.2). Each bud is separated from the ectomesenchyme by a basement membrane. There is little change in shape or function of the epithelial cells at this time. The supporting ectomesenchymal cells congregate around the bud, forming a cluster of cells which are closely packed beneath and around the epithelial bud, which is the initiation of the condensation of the ectomesenchyme. The remaining ectomesenchymal cells are arranged with less regular order.

Figure 2.2 Bud stage of tooth development (arrow). The bud is formed from the invading epithelium and condensation of the surrounding ectomesenchymal cells

As tooth development progresses, two key processes become essential to development. The first is morpho-differentiation, which is the determination of the shape of the crown of the tooth through the shape of the amelodentinal junction of the forming tooth. The second process is histo-differentiation, where cells of the developing tooth differentiate (specialise) into morphologically and functionally distinct groups of cells responsible for secretion of various dental tissues. Control and regulation of this differentiation is through specific and reciprocal cellular interactions between the epithelial/mesenchymal compartments.

As the epithelial bud continues to proliferate into the ectomesenchyme, the first signs of an arrangement of cells in the tooth bud appear in the cap stage. A small group of ectomesenchymal cells stops producing extracellular substances and do not separate from each other, which results in an aggregation or condensation of these cells immediately adjacent to the epithelial bud. This is the developing dental papilla. At this point, the tooth bud grows around the ectomesenchymal aggregation, taking on the appearance of a cap, and becomes the enamel (or dental) organ. A condensation of ectomesenchymal cells called the dental follicle surrounds the enamel organ and limits the dental papilla (Figure 2.3). The enamel organ is responsible for the synthesis and secretion of enamel, the dental papilla will lead to the formation of the dentine and pulp, and the dental follicle will produce the supporting structures of a tooth. This explains why enamel is epithelial in origin whereas dentine, pulp and periodontal tissues are mesenchymally derived.

Figure 2.3 Cap stage of tooth development where the three components of the tooth germ can be observed. EO, enamel organ; DP, dental papillae; DF, dental follicle

As tooth development proceeds there is a distinct histo- and morpho-differentation of the enamel organ as it prepares for secretory function, along with an increase in size of the tooth germ. This change signifies the transition to the early bell stage. The enamel organ takes on a bell shape during this stage with continued cell proliferation, and histo-differentiation of four distinct cell layers within the enamel organ can be observed (Figure 2.4).

Figure 2.4 Bell stage of tooth development where the four cell layers of the enamel organ can be observed. SR, stellate reticulum; SI, stratum intermedium; arrow, outer enamel epithelium; arrowhead, inner enamel epithelium

A single layer of cubiodal cells at the periphery of the enamel organ limit its size and are known as the outer enamel epithelium. Conversely, the single cell layer adjacent to the dental papilla is known as inner enamel epithelium and it is these cells that will differentiate into ameloblasts and give rise to enamel synthesis and secretion. Where these cells of the inner and outer enamel epithelium meet is termed the cervical loop. The majority of the cells that are situated between the outer and inner enamel epithelium are termed the stellate reticulum. These cells secrete hydrophilic glycosaminoglycans which increase the extracellular space and the cells interconnect through desmosomes giving them a stellate or star-shaped appearance. A layer two or three cells thick lying next to the inner enamel epithelium, and having a flattened shape, is termed the stratum intermedium. In summary, the layers of the enamel organ in order of innermost to outermost consist of inner enamel epithelium, stratum intermedium, stellate reticulum and outer enamel epithelium.

During this stage of development, as it progresses from cap stage to early bell stage, a localised thickening of cells at the inner enamel epithelium around the cusp tip appears. This is known as the enamel knot and is a signalling centre of the tooth that provides positional information for tooth morphogenesis and regulates the growth of tooth cusps. The enamel knot produces a range of molecular signals from all the major growth factor families, including fibroblast growth factors (FGF), bone morphogenetic proteins (BMP), Hedgehog (Hh) and Wnt signals. These molecular signals direct the growth of the surrounding epithelium and mesenchyme and have putative roles in signalling and regulation of crown development. The enamel knot is transitory and the primary enamel knot is removed by apoptosis. Later, secondary enamel knots may appear that regulate the formation of the future cusps of the teeth.

2.2 Later tooth development

As tooth development progresses from the early bell stage to a late bell stage of development, epithelial/mesenchymal interactions signal further histo-differentiation of the four cell layers of the enamel organ in preparation for amelogenesis. Cell appearance in the enamel organ is directly related to function. The cells of the outer enamel epithelium are cuboidal with a high nuclear:cytoplasm ratio. These cells have a non-secretory protective role and will eventually become part of the dentogingival junction. The stellate reticulum cells sit in a substantial jelly-like extracellular matrix which protects the interior of a tooth germ. The cells of the inner enamel epithelium have a low columnar appearance with a central nucleus and few organelles. These cells are at a preparatory stage of becoming secretory, the ameloblast.

The inner enamel epithelial cells are separated from the ectomesenchymal dental papillae by the dental basement membrane. This structure mediates interactions between the epithelial and mesenchymal compartments of the tooth germ during development and odontoblast differentiation prior to dentine secretion. At this time, the dental papillae contains undifferentiated ectomesenchymal cells with relatively small amounts of extracellular matrix (apart from a few fine collagen fibrils) and these cells are not yet specialised for secretory function.

The late bell stage is also known as the crown stage of tooth development and further cellular changes occur at this time. In all prior stages of tooth development, all of the inner enamel epithelium cells were proliferating to contribute to the increase of the overall size of the tooth germ. However, during the crown stage, cell proliferation stops at the location corresponding to the sites of the future cusps of the teeth. At the same time, the inner enamel epithelial cells change in shape from cuboidal to short columnar cells with nuclei polarised to the end of the cell away from the basement membrane.

The adjacent layer of cells on the periphery of the dental papilla increases in size, the cells become columnar and...