[P1.48]: Wnt and Shh cooperate in postcommissural axon guidance

the diencephalon (bordering this turn site) throughout the period of optic tract development, suggesting that FGF signalling in the neuroepithelium couldmaintain the pattern of guidance factors for axons in the developing optic tract. Furthermore, we found that exposing the developing optic tract and neuroepithelium to an FGFR inhibitor (SU5402) just prior to when RGC axons reach the turn site in the diencephalon has two effects: (1) RGC axons stall at the turn in the mid-diencephalon, and (2) the patterning in the diencephalon, adjacent to the turn site, is specifically and rapidly disrupted (as evidenced through in situ hybridization of postmitotic transcription factors, xLhx9, xLhx2, xLhx1, xLhx5, and xDll3). This suggests that FGF signalling is required both for RGC axons to navigate this guidance decision and to maintain the pattern of expression in the surrounding tissue. Moreover, the rapidity of this effect argues that the patterned cues required to guide axons in the developing optic tract are not stably expressed in that they must be maintained with the continued presence of signalling molecules.