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Fig. 7. A model of AER function in development of the mouse forelimb autopod. (A,B) AER maintenance is essential for AER-FGF signaling and generation of autopod progenitors. Schematic diagrams of early limb buds in transverse views (top row in A,B) and whole-mount dorsal views (bottom row in A,B) to illustrate development of the AER (green) and autopod progenitors, as marked by Hoxa13 expression (red). Red dots indicate dying cells in the ventral ectoderm and AER. (A) In control forelimb buds, the AER is maintained and AER-FGF production (orange) is normal. As a result, FGF signaling in the distal limb bud mesenchyme (LBM) is sustained and autopod progenitors are generated at the 31-32 s stage. The progenitor pool expands subsequently and a sufficient number of skeletal progenitors are available to form a normal autopod when condensation starts at around the 46 s stage. (B) In Fgfr2^AER-KO forelimb buds, the AER is not maintained because of increased cell death and AER-FGF production progressively decreases. As a result, FGF signaling in the distal mesenchyme is reduced and generation of autopod progenitors is delayed by 2-somite stages until the 33-34 s stage. Although the progenitor pool expands grossly normally at later stages, it fails to produce a sufficient number of skeletal progenitors to form a normal autopod at the onset of autopod condensation.

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