Hh signaling has an early negative role in the induction of the fin field. Expression analysis of fgf24 and prdm1 in a wild type embryo shows that these genes are restricted to the fin field at 26 hpf (A,C). ptc1;ptc2 mutants do not express these markers confirming that aberrant activation inhibits fin bud formation (B,D). To determine at which level Hh is inhibiting fin formation, the expression of tbx5, one of the earliest markers expressed in the finbud, was analyzed. At 20 hpf (E,F) tbx5 expression is lost in the presumptive finbud region (scale bar 100 μm). At 40 hpf (G,H), the fin bud has established and tbx5 expression is restricted to the pectoral fin in a wild type situation. However, in the ptc1;ptc2 double mutant a scattered low expression can be observed, showing that the pectoral fin bud is not formed. At the 10 ss, however, the initial expression domain of tbx5, encompassing heart and fin primordia is established. hand2, acting upstream of tbx5 is not expressed in the future pectoral fin area (I-L: white arrow), suggesting a very early negative role for Hh signaling in fin bud induction.
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