Abnormal anteroposterior and dorsoventral patterning of the limb bud in the absence of retinoids

We describe here how the early limb bud of the quail embryo develops in the absence of retinoids, including retinoic acid. Retinoid-deficient embryos develop to about stage 20/21, thus allowing patterns of early gene activity in the limb bud to be readily examined. Genes representing different aspects of limb polarity were analysed. Concerning the anteroposterior axis, Hoxb-8 was up-regulated and its border was shifted anteriorly whereas shh and the mesodermal expression of bmp-2 were down-regulated in the absence of retinoids. Concerning the apical ectodermal genes, fgf-4 was down-regulated whereas fgf-8 and the ectodermal domain of bmp-2 were unaffected. Genes involved in dorsoventral polarity were all disrupted. Wnt-7a, normally confined to the dorsal ectoderm, was ectopically expressed in the ventral ectoderm and the corresponding dorsal mesodermal gene Lmx-1 spread into the ventral mesoderm. En-1 was partially or completely absent from the ventral ectoderm. These dorsoventral patterns of expression resemble those seen in En-1 knockout mouse limb buds. Overall, the patterns of gene expression are also similar to the Japanese limbless mutant. These experiments demonstrate that the retinoid-deficient embryo is a valuable tool for dissecting pathways of gene activity in the limb bud and reveal for the first time a role for retinoic acid in the organisation of the dorsoventral axis.