Duplication of the msh-like homeobox gene of Drosophila may be related to the evolution of the vertebrate head. The murine homologues of this gene, msx 1 and msx 2 are expressed in the developing craniofacial complex including the branchial arches, especially in regions of epithelial-mesenchymal organogenesis including the developing tooth.

By performing in vitro recombination experiments using homochronic dental and non-dental epithelial and mesenchymal tissues from E10 to E18 mouse embryos, we have found that the maintenance of homeobox gene expression in the tooth is dependent upon tissue interactions. In homotypic recombinants, dental-type tissue interactions occur, leading to expression of both genes in a manner similar to that seen during in vivo development. msx 1 is expressed exclusively in mesenchyme, both in the dental papilla and follicle. msx 2 is expressed in the dental epithelium and only in the mesenchyme of the dental papilla. In heterotypic recombinants, the dental epithelium is able to induce msx 1 expression in non-dental mesenchyme, this potential being lost at the bell stage. In these recombinants msx 2 was induced by presumptive dental epithelium prior to the bud stage but not thereafter. The expression of msx 1 and msx 2 in dental mesenchyme requires the presence of epithelium until the early bell stage. However, whereas non-dental, oral epithelium is capable of maintaining expression of msx 1 in dental mesenchyme throughout tooth development, induction of msx 2 was temporally restricted suggesting regulation by a specific epithelial-mesenchymal interaction related to the inductive events of tooth formation.

msx 1 and msx 2, as putative transcription factors, may play a role in regulating the expression of other genes during tooth formation. We conclude that expression of msx 1 in jaw mesenchyme requires a non-specific epithelial signal, whereas msx 2 expression in either epithelium or mesenchyme requires reciprocal interactions between specialized dental cell populations.