Meeting Abstract

P2.47  Jan. 5  The evolution of hedgehog signaling in vertebrate tooth development JACKMAN, William R*; STOCK, David W; University of Colorado william.jackman@colorado.edu

We are interested in the diversity of tooth developmental mechanisms in vertebrates, especially between teeth that originate in different germ layers such as oral ectoderm or pharyngeal endoderm. The signaling molecule Sonic hedgehog (Shh) is expressed in both mammalian and fish oral tooth germs. In the mouse, Shh is important in promoting tooth epithelial growth, but it remains unclear if it plays a role in tooth initiation or later morphogenesis. In the zebrafish, which has only pharyngeal teeth, we surveyed the dental expression of all of the five known hedgehog genes. We found expression of only the duplicated orthologs of Shh, shha (shh) and shhb (twhh), suggesting that additional hedgehog ligands are not used in pharyngeal tooth development. However, unlike Shh expression in oral tooth germs, we found shha and shhb to be expressed also in adjacent epithelium, which may represent a significant difference in the development of oral and pharyngeal teeth. We have tested the function of hedgehog signaling in zebrafish tooth development by applying the inhibitor cyclopamine. Early application inhibits tooth mineralization, and several genes expressed during tooth development, including dlx2b and fgf4, are also absent. However, expression of pitx2 is retained, suggesting that hedgehogs are not required for pharyngeal tooth initiation. We also found that late application of cyclopamine disrupts tooth morphology, demonstrating a late hedgehog requirement in tooth morphogenesis. Surprisingly, knockdown of zebrafish shha and shhb with morpholino antisense oligonucleotides had no effect on tooth development, unlike the strong effect seen in mice after Shh inhibition. This could be because of limitations of the technique, and we are currently exploring transgenic and transplantation approaches to circumvent this difficulty.