In vertebrate embryos, the ophthalmic trigeminal (opV) placode contributes sensory neurons to the trigeminal ganglion during development of the peripheral nervous system. FGFR4 is expressed transiently in the trigeminal placode as cells undergo an epithelial-mesenchymal transition and begin to migrate toward the condensing ganglion. Because of the restricted spatiotemporal expression of FGFR4 in the opV placode, it was hypothesized that FGFR4 is necessary for the process of epithelial-mesenchymal transition in the opV placode. Using electroporation, an FGFR4 inhibitory gene construct was introduced into 6—10 somite stage chick embryos. This secreted inhibitory form of the FGFR4 gene blocked endogenous FGFR4 signaling, which resulted in trigeminal placode cells remaining in the ectoderm that would have normally begun migration into the mesenchyme. These results show that FGFR4 is involved in trigeminal placode cell delamination.

This dissertation is divided into three chapters, each discussing the study of different regulatory molecules involved in sensory neurogenesis occurring in the trigeminal placode. Chapter one is a spatiotemporal description of Notch pathway genes in chick opV placode by stage-specific expression analysis, showing expression of many Notch pathway genes and effectors in the opV placode. Notch pathway gene expression is primarily confined to the ectoderm with highest expression of these genes at the beginning stages of peak neuronal differentiation. This information preceded studies of the functional roles that Notch signaling has in the opV placode and how it may affect the transcription factor, Pax3. Chapter two is a study of the transcription factor Pax3 and its role in opV placode development and sensory neuron differentiation. Pax3 is known to activate or repress gene transcription, and its activity may be dependent on the splice variant or isoform present. We show through RT-PCR that alternative splice forms of Pax3 are present at stages of chick development corresponding to cellular competence, cellular differentiation and ingression, and cellular aggregation. We have named these splice forms, Pax3V1 and Pax3V2. Using quantitative RT-PCR we show that Pax3V2 is consistently expressed at lower levels compared to Pax3 during cellular competence and differentiation. In order to determine the function of the three splice forms, we misexpressed them in the opV placode and analyzed the effect on neurogenesis. We looked at markers for neuronal differentiation of targeted cells after in ovo electroporation of Pax3, Pax3V1, and Pax3V2, which showed a significant difference between the control and each construct, but not between the groups of constructs. To enhance the process of neurogenesis we exposed the electroporated embryos to DAPT, a Notch signaling inhibitor that enhances sensory neurogenesis. Using this method we found that misexpression of Pax3 and Pax3V1 resulted in cells failing to differentiate, while Pax3V2 misexpression more closely resembles the neuronal differentiation seen in controls. These results show that the Pax3V2 isoform allows for neuronal differentiation of opV placodal cells after misexpression, while the Pax3 isoform and the Pax3V1 isoform block neuronal differentiation. Chapter three is a study of the necessity of Wnt signaling originating from the neural tube to induce Pax3 expression in the opV placode. A double knockout of Wnt1 and Wnt3a was produced to determine the necessity of these genes in opV placode development. Pax3 expression in the opV placode at E8.5 and E9.5 was markedly reduced in the double mutants when compared to wild type mice. This study shows that Wnt1 and Wnt3a genes are necessary for normal Pax3 expression, but that other signals may contribute to its induction.