Figure 7

Model depicting the cooperation of vHnf1 and FGF signals in the induction of MafB and Krox20 in the caudal hindbrain. (a) vHnf1 is very early expressed in the caudal neural plate with a sharp boundary laying in the prospective r4/r5 boundary. This expression may be initiated very early in response to RA from the axial and paraxial mesoderm during mid/late stages of gastrulation [12, 64, 65]. Anterior limit of expression of vHnf1 may be established by mutual repression with an irx gene [61, 65], probably irx3 (unpublished results).vHnf1 rapidly induces Fgf3 in the caudal hindbrain [1], which activates the Ras-ERK1/2 pathway. vHnf1 and Fgf3-ERK1/2 co-operate for the induction of MafB expression in r5 and r6 at 4-5ss and Krox20 in r5 at 6-7ss. Krox20 induction is probably also dependent on MafB expression as suggested in mouse and zebrafish [8, 11, 66, 67]. Krox20 is initially expressed in a narrow domain caudal to r4 and subsequently expands its expression area due to non-cell autonomous induction. Coinciding with the onset of Krox20 in r5, vHnf1 progressively regresses to r6 between 7 and 10ss. Mutual repression between vHnf1 and Krox20 may prevent expansion of Krox20 to r6. Anterior is to the left. (b) Putative regulation of MafB and Krox20 expression by vHnf1 modulating FGF signaling through MKP3.