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Fig. 4. BMP signaling and expression in Grem1-deficient limb buds. (A) Expression of the BMP target Msx1 during early limb bud development (E9.75). (B) Expression of the BMP target Msx2 during limb bud development (E10.25). Note ectopic Msx2 expression in the distal to anterior limb bud mesenchyme of Grem1 (Gre{Delta}ORF/{Delta}ORF)-deficient embryos, which is indicative of enhanced BMP signaling. (C) Expression of Bmp2, Bmp4 and Bmp7 in the limb bud mesenchyme (E10.75). Mesenchymal Bmp2 expression is significantly reduced at this stage in Grem1-deficient limb buds, while Bmp4 and Bmp7 are maintained at normal levels. (D) Bmp2, Bmp4 and Bmp7 expression in the AER of wild-type and Grem1 mutant forelimb buds (E10.75). Expression of all three (Bmp2, Bmp4 and Bmp7) is lacking from the AER of mutant limb buds. (E) Scanning electronic microscopy analysis of wild-type and Grem1-deficient forelimb buds (E11.5). Note that the AER of mutant limb buds is poorly differentiated and the anteroposterior limb bud axis is shortened in comparison with the wild type. In D,E, posterior is towards the bottom and dorsal is towards the left. (F) High-power SEM revealing the morphology of AER-type ectodermal cells (outlined in red).