Fig. 6. Premigratory and migratory cardiac and pharyngeal neural crest do not express Fgf8. Wnt1-Cre was used to drive expression of the alkaline phosphatase (AP) reporter gene from the Fgf8^H allele in premigratory and migrating neural crest; Cre-mediated recombination of the Fgf8^H allele results in expression of the AP gene regulated by the Fgf8 promoter and is detected as blue staining. (A) E8.5 control embryo bearing the Fgf8^H allele. (B) E8.5 embryo bearing the Fgf8^H and the Wnt1-Cre transgene; no staining is detected, indicating that Fgf8 is not expressed in any cells of the Wnt1-Cre lineage at this stage. (C) E9.5 control embryo with the Fgf8^H allele. (D) E9.5 embryo bearing the Fgf8^H allele and the Wnt1-Cre transgene; blue staining reveals overlapping domains of Wnt1 and Fgf8 expression in the isthmus and frontonasal prominences (arrows). No activity in neural crest of rhombomeres 6-8 (R6, R7 arrowheads), which are the origin of premigratory third and fourth pharyngeal and cardiac neural crest.

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