Influence of dendritic structure on firing pattern in model neocortical neurons
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