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sall1a and sall4 are required for fgfr2 expression in the pectoral fin primordia. A section of a 23 hpf embryo shows that fgfr2 is expressed in the pectoral fin primordia mesenchyme but is excluded from the ectoderm (A). Dorsal views highlight fgfr2 expression in a wild-type embryo at 24 hpf (B). At the same time point, fgfr2 expression is absent in the pectoral fin primordia of embryos injected with 4 ng sall1a MO + 4 ng sall4 MO (C), but continues to be expressed in the primordia of embryos injected with 9 ng of fgf24 MO (D). At this time point, fgfr2 is also downregulated in the trunk of sall1a/sall4 double morphant embryos (C and data not shown). The red arrows indicate the pectoral fin primordia in B-D. (E) A schematic representation of the regulatory relationships between genes required for pectoral fin development. Green arrows signify relationships between FGF receptors and ligands, while black arrows represent transcriptional regulatory relationships.
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