Fig. 5. BMPR-IA is required for apical ectodermal ridge formation. (A-D) Fgf8 expression was detected in the pre-AER region before overt hindlimb bud development in normal embryos (A; 10.0 dpc), and is a molecular marker for the apical ectodermal ridge at later stages of hindlimb formation (C; 11.5 dpc). Fgf8 expression was detected in very few cells at 10.0 dpc in most mutant embryos examined (B). Presumably incomplete penetrance of the Bmpr conditional knockout allows the expression of Fgf8 in a small number of cells (arrow). Expression was detected later in hindlimb embryogenesis, but the levels of expression are variable (D-F). (C-F) Hindlimbs (11.5 dpc) double labeled for Fgf8 expression in the AER (arrowhead) and Shh expression in the ZPA (arrow). (C) The normal pattern of Fgf8 and Shh expression. Expression of Fgf8 and Shh varies in the mutant from no expression detected (D; Category ‘0’ in Table 1) to a majority of the pattern detected in normal embryos (F; Category ‘3’ in Table 1). A focal patch of Fgf8 expression is depicted in F (small arrow). Focal patches were detected that do not lie at the distal tip of the hindlimb, but they were not consistently deflected in either a dorsal or ventral direction. Interestingly, we did not see a ventral extension of AER gene expression, as seen with the En1 knockouts (Loomis et al., 1998). (G) At 10.5 dpc, Bmp4 expression is detected in both the ZPA (arrow) and the AER (arrowhead) of normal embryos. (H) Bmp4 gene expression is variable in the AER of mutant embryos and absent in the embryo depicted. Bmp4 expression is consistently detected in the ZPA of 10.5 dpc mutants.
