The Monetary Transmission Mechanism in the United Kingdom: Pass-Through and Policy Rules. manuscript

ectoderm, can be divided into two distinct domains (Fig. 4). The dorsal AER is characterized by the presence of AER-specific markers, whereas the ventral AER is distinguished by the additional expression of En-I. Loss of En-1 function appears to allow a ventral expansion of the AER. In this situation, the Wnt7a-negative region at the distal tip of the mutant limbs might be analogous to the dorsal wild-type AER, whereas the ectoderm expressing Wnt-7a in addition to AER-specific markers might represent the ventral AER. Alternatively, the distal Wnt-7anegative ectoderm might demarcate the entire functional domain of the AER, but several pieces of evidence suggest that this is not the case. Both molphological criteria and gene-expression data suggest that the AER extends beyond the ventral boundary of the Wnt-7a-negative domain. Furthermore, preliminary studies suggest a parallel proximoventral expansion of progress zone markers (data not shown). Finally, limb structures that normally develop only distally were duplicated proximoventrally in En-I mutant mice, a phenotype consistent with a functional expansion of the AER. Thus our data indicate that En-1 is required for delineating the ventral AER boundary and for restricting expression of signalling molecules, such as Fgf-8 and Bmp-2, to the distalmost ectoderm, a function reminiscent of engrailed's role in compartment border formation in D r o s ~ p h i l a ~ ~ ~ ~ . 0