Molecular mediators of odontogenesis at ectopic sites

Odontogenesis is a well characterized model of instructive epithelial-mesenchymal interactions. Previous tissue recombinations indicate that mouse mandibular epithelium prior to E12 has odontogenic potential and can elicit tooth formation at variety of ectopic sites, including chick mandible. In the present study, by using heterospecific recombinations between E11 mouse mandibular molar-forming epithelium and stage 23 chick lateral mandibular mesenchyme, the initial steps of odontogenesis at ectopic site were investigated.^ Our results demonstrated the formation of tooth bud-like structures in these recombinations that resemble early tooth buds in vivo in morphology and patterns of genes expression. The tooth bud-like structures in heterospecific recombinations consist of invaginated mouse mandibular epithelium and condensed chick mandibular mesenchyme around the dental lamina-like structures expressing Msx-1 and Bmp-4 but not Msx-2. Moreover, when grafted on chick chorioallantoic membrane, the ectopic dental lamina differentiated into enamel organ-like structures containing stellate reticulum. Our results also suggest the involvement of BMP-4, BMP-7, and EGF as components of the inductive signals from mouse odontogenic epithelium mediating the formation of dental mesenchyme-like tissue in chick mandibular mesenchyme. Similar to mouse odontogenic epithelium, agarose beads soaked in BMP-4 and BMP-7 when placed in contact with chick lateral mandibular mesenchyme and mouse tooth-forming mesenchyme induced the expression Msx-1, Msx-2, and Bmp-4. On the other hand, EGF may be involved in either activating other transcription factors or modulating the effects of BMP-4. Alternatively, EGF may regulate events involved in the formation of dental lamina.^ In addition, in parallel experiments, the possibility that yet other regulatory genes may be involved in mediating the initial inductive events in odontogenesis was examined by comparing the mRNAs from mandibular and hyoid epithelium using the differential display technique. Ninety-seven differentially displayed bands were identified. We were able to successfully sequence eight differentially displayed bands. Out of eight sequenced cDNAs, two corresponded to 28S ribosomal RNAs, one corresponded to ribosomal protein S28, one corresponded to Mus musculus proteasome $\beta$ subunit, and four showed no homology to any sequences in the GenBank database suggesting that they could represent unknown genes.