Engrailed-1 misexpression in chick embryos prevents apical ridge formationbut preserves segregation of dorsal and ventral ectodermal compartments

Using lineage tracers, we recently showed dorsal and ventral ectodermal com partments along the sides of the body in chick embryos. The compartments ar e formed both in presumptive limb-forming regions where they position the a pical ridge and also in presumptive interlimb (flank). Here we show, using a novel technique combining fate mapping and in situ hybridisation, that th e ventral compartment coincides with the Engrailed-1 (En-1) domain of expre ssion. This coincidence suggests that En-l could maintain the ventral compa rtment and be necessary for apical ridge formation. To test this hypothesis , we ectopically expressed En-1 via retroviral transfer and then examined l imb development and cell lineage restriction in the ectoderm. En-1 misexpre ssion can completely prevent formation of both normal limbs and ectopic lim bs induced in the flank by application of FGF-2. In both cases, there are n o morphological signs of apical ectodermal ridge formation and expression o f ridge-associated genes is undetectable. In striking contrast, the lineage restriction between dorsal and ventral ectoderm is not altered. Therefore, En-1 is involved in the regulation of ridge formation but not compartment maintenance. (C) 2000 Academic Press.