The talpid2 chick limb has weak polarizing activity and can respond to retinoic acid and polarizing zone signal

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Abstract
The talpid2 (ta2) chick mutant has wide, polydactylous wings and legs. Talpid2 limb cartilages have abnormal morphology and a very subtle anteroposterior polarity. Specifically, posterior ta2 limb structures are identifiable, while more anterior cartilages are less distinctive. Here, we investigate the development of anteroposterior limb pattern in the to2 embryo. We show that ta2 posterior limb bud mesoderm is capable of respecifying the anteroposterior axis of a normal wing. However, the average duplication obtained after grafting a ta2 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 ta2. Grafts of normal polarizing zone to the posterior edge of ta2 wing buds had no effect on the ta2 phenotype. This result suggests that a weakly functioning polarizing signal does not account for the altered anteroposterior polarity in ta2 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 ta2 limb mesoderm was normal. In addition, ta2 anterior border mesoderm had no polarizing activity. We also assessed the ability of ta2 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 ta2 wing buds at stages 18–23. Both polarizing zone and retinoic acid caused respecification of the ta2 wing anteroposterior axis. The result was that a ta2 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 ta2 wing buds, there was no effect on anteroposterior pattern. However, beads with retinoic acid always caused a reduction in the number of ta2 wing digits which formed, whether the beads were placed at the anterior or posterior edge of the developing ta2 limb.