Fig. 2. Dorsal pancreatic development is impaired in Flk1^-/- embryos after the induction of an initial field of Pdx1-positive cells. Immunostaining of transverse sections of control (upper row) and Flk1^-/- (lower row) embryos at E9.0-10.0 (somite stages are indicated on the top of each panel). (A,B) Pecam staining to identify blood vessels; no vessels develop in Flk1^-/- embryos. (C-I) Pdx1 staining. Although at 15S, Flk1^-/- embryos (D) develop nearly the same size domain of Pdx1-positive cells in the dorsal endoderm as control embryos (C), these cells diminish over time and fail to form a bud (compare G and H). By 31S, very few Pdx1-positive cells can be found in the dorsal endoderm (I). (J) RT-PCR analysis of dorsal midgut RNAs at the designated somite stages (S). The normalizing cycle steps for Pecam and Tal1 were based on prior cycle step analysis of actin. Embryonic aorta served as positive controls. PCR cycle steps: Pecam, 30, 33; Tal1, 30, 33; actin, 26, 29.