During the morphogenesis of any organ, numerous dialogues are occurring between the interacting tissues. In epithelial-mesenchymal interactions, the mesenchyme influences the epithelium; the epithelial tissue, once changed by the mesenchyme, can secrete factors that change the mesenchyme. Such interactions continue until an organ is formed with organ-specific mesenchyme cells and organ-specific epithelia. Some of the most extensively studied interactions are those that form the mammalian tooth. Here, the neural crest-derived mesenchyme cells become the dentin-secreting odontoblasts, while the jaw epithelium differentiates into the enamel-secreting ameloblasts. A summary of recent research that correlates mesenchyme induction and differentiation in the mammalian tooth is shown in Figure 13.9.

Tooth development begins when the mandibular (jaw) epithelium causes neural crest-derived ectomesenchyme (i.e., mesenchyme produced from the ectoderm) to aggregate at specific sites. The polarity of the mandibular epithelium is determined by interactions between BMP4, which is located distally, and FGF8, which is located proximally (closest to the skull). Those teeth formed in the FGF8 regions will become molars, while those teeth that develop in the BMP4 regions will become incisors (Tucker et al. 1998). Soon afterward, the expression pattern of BMP4 and FGF8 changes, and the sites of the tooth primordia are determined by the interactions between these same molecules in the epithelium. FGF8 induces Pax9 expression in the underlying ectomesenchyme, while BMP4 inhibits Pax9 expression. Pax9 is a transcription factor whose expression in the ectomesenchyme is critical for the initiation of tooth morphogenesis, and in Pax9-deficient mice, tooth development ceases early. The only places where ectomesenchyme condense and teeth develop are where FGF8 is present and BMPs are absent (Vainio et al. 1993; Neubüser et al. 1997). Thus, spaces develop between the teeth.

At this time, the epithelium possesses the potential to generate tooth structures out of several types of mesenchyme cells (Mina and Kollar 1987; Lumsden 1988b). However, this tooth-forming potential soon becomes transferred to the ectomesenchyme that has aggregated beneath it. These ectomesenchymal cells form the dental papilla and are now able to induce tooth morphogenesis in other epithelia (Kollar and Baird 1970). At this stage, the jaw epithelium has lost its ability to instruct tooth formation in other mesenchymes. Thus, the “odontogenic potential” has shifted from the epithelium to the mesenchyme. This shift in the odontogenic potential coincides with a shift in the synthesis of BMP4 from the epithelium to the ectomesenchyme.

As the dental mesenchyme cells condense, they are induced to synthesize the membrane protein syndecan and the extracellular matrix protein tenascin. These proteins (which can bind each other) appear at the time the epithelium induces mesenchymal aggregation, and Thesleff and her colleagues (1990) have proposed that these two molecules may interact to bring about this condensation. Moreover, after the ectomesenchyme has aggregated, it begins to secrete BMP4 as well as other growth and differentiation factors (FGF3, BMP3, HGF, and activin) (Wilkinson et al. 1989; Thesleff and Sahlberg 1996). These proteins from the ectomesenchyme induce a critical structure in the epithelium. This structure is called the enamel knot, and it functions as the major signaling center for tooth development (Jernvall et al. 1994). This group of cells appears as a nondividing population of cells in the center of the growing cusps. Moreover, in situ hybridization has demonstrated that the enamel knot is the source of Sonic hedgehog, FGF4, BMP7, BMP4, and BMP2 secretion (Figure 13.9B; Koyama et al. 1996; Vaahtokari et al. 1996a). As a nondividing population secreting growth factors capable of being received by both the epithelium and the ectomesenchyme, the enamel knot is thought to direct the cusp morphogenesis of the tooth and to be critical in directing the evolutionary changes of tooth structure in mammals (Jernvall 1995).

The mesenchyme cells begin to differentiate into odontoblasts, and tenascin expression is induced at much higher levels and at the same sites as alkaline phosphatase expression. Both of these proteins have been associated with bone and cartilage differentiation, and they may promote the mineralization of the extracellular matrix (Mackie et al. 1987). Finally, as the odontoblast phenotype emerges, osteonectin and type I collagen are secreted as components of the extracellular matrix. The enamel knot disappears through apoptosis, responding to its own BMP4 (Vaahtokari et al. 1996b; Jernvall et al. 1998). By this steplike process, the cranial neural crest cells of the jaw are transformed into the dentin-secreting odontoblasts.