Early Tooth Formation

Teeth form as a result of a series of interactions between oral epithelium and neural crest-derived ectomes-enchyme [Thesleff and Sharpe, 1997; Cobourne and Sharpe, 2003; Tucker and Sharpe, 2004]. Early tooth development advances through initiation and bud stages. Genetic disturbances in early odontogenesis can arrest tooth development causing familial tooth agenesis [for a review please see D'Souza and Klein, pp 60-69, this issue]. During the cap stage, the primary enamel knot serves as a signaling center [Thesleff and Jernvall, 1997]. In multi-cusped teeth, secondary enamel knots guide the differentiation of the enamel epithelium at each cusp tip during the bell and crown formation stages [Thesleff et al., 2001; Matalova et al., 2005]. During the crown formation stage, cellular differentiation is completed and fully differentiated odontoblasts and ameloblasts deposit dentin and enamel matrices. Odontoblasts differentiate first and secrete a collagen-rich predentin matrix directly beneath the epithelial-derived basal lamina, but dentin mineralization does not occur until the basement membrane material is removed. Differentiating ameloblasts (pre- and presecretory ameloblasts) start expressing small amounts of enamel proteins even before the basal lamina disintegrates. As the basal lamina fragments, ameloblasts send cytoplasmic projections through the gaps. Dentin starts to mineralize with the disappearance of the basal lamina, as the apical surfaces of ameloblasts associate with the superficial collagen fibrils of the mantle dentin. Patches of enamel matrix proteins appear in quantity on the irregular dentin surface and enamel mineralization begins. As the (now) secretory ameloblasts recede, the patches grow larger and merge until a continuous and uniform layer of initial enamel is deposited. This initial enamel layer is aprismatic, i.e. not separated into rod and interrod enamel. Secretory ameloblasts form a novel cell extension called a Tomes' process at their secretory ends. This extension has secretory and nonsecretory regions and provides the architectural basis for organizing enamel crystals into rod and interrod enamel, which differ from each other in the orientation of their crystals [Meckel et al., 1965; Cevc et al., 1980; Fejerskov and Thyl-strup, 1986].

Fig. 1. Ameloblast changes during enamel formation. The epithelial cells of the inner enamel epithelium (1) rest on a basement membrane containing laminin. These cells increase in length as differentiating ameloblasts above the predentin matrix (2). Pre-secretory ameloblasts send processes through the degenerating basement membrane as they initiate the secretion of enamel proteins on the villous surface of mineralizing dentin (3). After establishing the dentinoenamel junction and mineralizing a thin layer of aprismatic enamel, secretory ameloblasts develop a secretory specialization, or Tomes' process. Along the secretory face of the Tomes' process, in place of the absent basement membrane, secretory ameloblasts secrete proteins at a mineralization front where the enamel crystals grow in length (4). Each enamel rod follows a retreating Tomes' process from a single ameloblast. At

Fig. 1. Ameloblast changes during enamel formation. The epithelial cells of the inner enamel epithelium (1) rest on a basement membrane containing laminin. These cells increase in length as differentiating ameloblasts above the predentin matrix (2). Pre-secretory ameloblasts send processes through the degenerating basement membrane as they initiate the secretion of enamel proteins on the villous surface of mineralizing dentin (3). After establishing the dentinoenamel junction and mineralizing a thin layer of aprismatic enamel, secretory ameloblasts develop a secretory specialization, or Tomes' process. Along the secretory face of the Tomes' process, in place of the absent basement membrane, secretory ameloblasts secrete proteins at a mineralization front where the enamel crystals grow in length (4). Each enamel rod follows a retreating Tomes' process from a single ameloblast. At the end of the secretory stage, ameloblasts lose their Tomes' process and produce a thin layer of aprismatic enamel (5). At this point the enamel has achieved its final thickness. During the transition stage, the ameloblasts undergo a major restructuring that diminishes their secretory activity and changes the types of proteins secreted (6). KLK4 is secreted, which degrades the accumulated protein matrix and amelotin (AMTN) is secreted as part of the new basement membrane. During the maturation stage am-eloblasts modulate between ruffled and smooth-ended phases (7). Their activities harden the enamel layer by promoting the deposition of mineral on the sides of enamel crystals laid down during the secretory stage. The histology of the developing tooth is adapted from Uchida et al. [1991].

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