Understanding how instructed determination of phenotypes through reciprocal epithelial-mesenchymal interactions remains a significant problem area in developmental craniofacial biology. An excellent model for examining instructive epithelial-ectomesenchymal interactions is the developing tooth organ. In the developing tooth, ectomesenchymal signals determine epithelium to express de novo amelogenin gene transcript and translation products. We have identified five aims to characterize differential gene expression during instructive or permissive epithelial-mesenchymal interactions: i) determine when and where nascent amelogenin gene transcription and translation products appear during in vivo mouse mandibular first molar development; ii) determine when and where amelogenin gene transcript and translation products appear during the in vitro development of mouse mandibular first molar tooth organs, in the presence and absence of serum; iii) determine when and where amelogenin gene transcript and translation products are expressed during cap stage dental ectomesenchyme-instructed mouse foot pad epithelial differentiation to ameloblasts; iv) in tooth like heterotypic recombinants composed of tooth mesenchyme and foot pad epithelium determine if keratin gene expression is either arrested prior to, or is concurrent with, amelogenin gene expression; and v) to characterize the molecular basis of dental ectomesenchyme-derived signals for the instruction of epithelial differentiation to ameloblasts. The methods for analysis are nucleic acid hybridization to a cDNA probe to the 26 kilodalton amelogenin gene product and immunodetection of amelogenin polypeptides using a rabbit anti-mouse amelogenin antibody. The successful completion of these specific aims will produce significant insight into the mechanisms operant during epithelial-mesenchymal interactions in craniofacial morphogenesis. A more complete understanding of this problem area will enhance the clinical management and perhaps modes of intervention in abnormal craniofacial development.