Fig. 6. Inactivation of Bmpr1a rescues hypodactyly in Msx2-Cre; Fgf8^flox/null mouse forelimbs. (A-C) Skeletal preparation of forelimbs of the genotypes indicated. Individual digits are numbered. Note that hypodactyly in Msx2-Cre;Fgf8^flox/null mutants (B) is rescued by also deleting Bmpr1a (C). (D-I) Dorsal views (anterior to left) of forelimbs of the embryonic age and genotype indicated, stained with an Fgf4 riboprobe for a short time period (2 hours, 4°C) to optimally visualize differences in Fgf4 expression levels. Fgf4 expression ceases in normal forelimbs by E11.5 (D), but continues in Msx2-Cre;Fgf8^flox/null mutants through E12.5 (E,H). This enhanced expression is higher in Msx2-Cre; Bmpr1a^flox/null;Fgf8^flox/null mutants (F,I). Note also that Msx2-Cre Fgf8^flox/null limb buds are smaller (E,H) than normal (D,G), prefiguring the hypodactyly phenotype (Lewandoski et al., 2000). This size difference is rescued in the double mutants (F,I). The boxed regions in H and I are shown at high magnification in the insets.