Fig. 6. Proliferation within the developing arches appeared grossly the same between wild type (A) and Fgf8 mutants (B,C). Examination of cell death between E8.0-E9.5 revealed a striking pattern of increased cell death in Fgf8 mutants. In E8.0-8.5 Fgf8 mutants (E), there was increased death along the flank near the cardiac inflow (arrowheads, E,H) as well as along the dorsal axis (stars, E,H) and anterior neural ridge (E, arrow) when compared with control embryos (D,G). At E9.5, there was increased cell death within the caudal arches (arrowheads, K) and cells entering the arches (N) of Fgf8 mutants when compared with controls (J,M). At E9.5 the pattern of cell death was similar to that of lineage traced NCCs in wild-type embryos using P0-Cre and the Cre reporter R26R. (compare M, N with O). Strikingly, the mRNA expression of Fgf8 in wild-type embryos closely matched the areas of cell death (compare F,I,L with E,H,K, respectively). ot, otocyst.

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