The talpid 2 chick limb has weak polarizing activity and can respond to retinoic acid and polarizing zone signal

The talpid 2 ( ta 2 ) chick mutant has wide, polydactylous wings and legs. Talpid 2 limb cartilages have abnormal morphology and a very subtle anteroposterior polarity. Specifically, posterior ta 2 limb structures are identifiable, while more anterior cartilages are less distinctive. Here, we investigate the development of anteroposterior limb pattern in the to 2 embryo. We show that ta 2 posterior limb bud mesoderm is capable of respecifying the anteroposterior axis of a normal wing. However, the average duplication obtained after grafting a ta 2 polarizing region was significantly less than the average duplication formed after a graft of normal wing bud polarizing zone. Thus, polarizing activity appears to be weak in ta 2 . Grafts of normal polarizing zone to the posterior edge of ta 2 wing buds had no effect on the ta 2 phenotype. This result suggests that a weakly functioning polarizing signal does not account for the altered anteroposterior polarity in ta 2 limbs, and that normal polarizing zone activity is not sufficient for formation of normal limb bud cartilages. We demonstrated that transmission of a polarizing signal through the ta 2 limb mesoderm was normal. In addition, ta 2 anterior border mesoderm had no polarizing activity. We also assessed the ability of ta 2 limb bud mesoderm to respond to a polarizing signal. Either normal polarizing zone tissue or a bead containing retinoic acid was placed at the anterior edge of ta 2 wing buds at stages 18–23. Both polarizing zone and retinoic acid caused respecification of the ta 2 wing anteroposterior axis. The result was that a ta 2 ulna replaced the radius, and the most posterior digit was duplicated anteriorly. Limb cartilages with normal morphology never formed. When a bead containing retinoic acid was placed at the posterior edge of ta 2 wing buds, there was no effect on anteroposterior pattern. However, beads with retinoic acid always caused a reduction in the number of ta 2 wing digits which formed, whether the beads were placed at the anterior or posterior edge of the developing ta 2 limb.