Premature regression of the leg apical ectodermal ridge in the Japanese chick wingless mutant

The autosomal dominant Japanese wingless mutant has varying degrees of wing and leg truncations. The wing defects range from complete loss to negligible defects, whereas leg abnormalities are usually restricted to loss of the phalanges. Further analyses of the mutant focusing on the leg, which has been relatively uncharacterized, were performed. The expression pattern of Fgf8, a marker gene for the apical ectodermal ridge (AER) that controls outgrowth of the limbs, revealed premature regression at stage 28. Electron microscopy study showed abnormalities in the basement membrane all through the AER in the same stage. In the mutant, cell death was observed in the mesenchyme underlying AER between stages 31 and 32, although in the wild-type leg, AER regression and cell death occurred almost simultaneously at stages 33–34. To know if the cell death and cessation of the outgrowth are common mechanisms of wild-type and the mutant, we removed the AER in wild-type embryos at stage 28 and followed the fate of the limb. This also resulted in premature cell death 48 h after AER removal (equivalent to stage 32) and limb truncations similar to those observed in mutant limbs. To confirm whether either AER or underlying mesenchyme is responsible for the truncation, transplantation of the AER between the wild-type and the mutant was performed. This revealed that AER is the defective tissue in this mutant.