Fig. 1. Fgfr2 removal causes loss of the apical ectodermal ridge (AER), reduction of AER-FGF and loss of distal limb skeletal elements. (A-F) Skeletal preparations of E18.5 embryos, with cartilage stained blue and bone stained red. In Fgfr2^AER-KO embryos, the hindlimb was missing except for the pelvic bones (arrow in D) and the distal autopod (hand-plate) was absent in the forelimb (asterisk in E,F; n=18). Individual digits in control embryos are numbered I-V, from anterior to posterior (C). Scale bars: 1 mm. (G-N) AER histology as examined by CD44 immunofluorescence (green) on vibratome sections. Samples were counterstained with a nuclear dye To-Pro3 (red). Scale bars: 50 µm. (O-V) Levels of AER-Fgf8 expression as detected by whole-mount RNA in situ hybridization assays at the stages indicated. Fgf8-expressing cells were sparser in the mutant AER (R,R') than in the control AER (Q,Q') at the 33 s stage. Black arrowheads indicate gaps in the AER that lack Fgf8 expression in the mutant limb buds (T,T'). Arrow indicates a small patch of the limb bud edge that was still expressing Fgf8 (V). Scale bars: 200 µm. Abbreviations: an, anterior; di, distal; do, dorsal; pr, proximal; po, posterior; ve, ventral; s, somite.