Fig. 5. Expression of genes involved in limb bud initiation in wild type (+/+) and Tbx4 mutants (-/-) at 10.5 dpc. (A,B) Fgf8 expression is visible in the AER of the forelimb bud and the presumptive AER of the hindlimb bud in wild-type and Tbx4-mutant embryos. (C,D) Expression of Msx1 is present in the mesenchyme of the forelimbs and hindlimbs of both wild-type and mutant embryos. (E,F) Twist is expressed normally throughout the hindlimb mesenchyme of wild-type and mutant embryos. (G,H) Thick sections through the forelimbs (top) and hindlimbs (bottom) of 10.5 dpc embryos. Anti-dp-ERK marks regions of FGF signaling and is seen in the mesenchyme immediately underlying the AER of forelimbs, and asymmetrically in the mesenchyme of the hindlimbs in the wild-type embryo. Tbx4-mutant embryos also show similar dp-ERK staining in both sets of limbs. (I,J) Fgf10 is expressed in the distal mesenchyme of the forelimb and throughout the mesenchyme of the hindlimb bud of the wild type. In Tbx4 mutants, forelimb expression of Fgf10 is unaltered, but expression in the hindlimb appears only in the proximal mesenchyme and is absent from the distal hindlimb bud. Arrowheads indicate the distal edge of hindlimb bud.

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