The effect of the zone of polarizing activity (ZPA) on the anterior half of the chick wing bud.

Removal of the posterior half of the chick wing bud between stages 17-22 results in failure of the anterior distal tissue to survive and differentiate. This observation has been interpreted in terms of a requirement by the anterior half of a factor supplied by the posterior half of the limb containing the zone of polarizing activity (ZPA). This relationship has been tested by grafting ZPA tissue to the posterior surface of the anterior half after posterior half removal. Grafts made proximally on the cut surface did not significantly improve survival and development, nor did the ZPA prevent the expansion of the cell death in the ANZ beyond its normal boundaries into the distal mesenchyme. However, when grafted distally the ZPA inhibited cell death in the apical mesenchyme and caused the anterior mesenchyme to change its normal prospective fate (radius and digit 2). In all these cases, in addition to digit 2, digit 3 and frequently also digit 4 differentiated. The anterior half went on to develop a full set of digits and zeugopod parts in almost 50% of cases, although no skeleton resulting from this regulation of the anterior half had totally size regulated. These results demonstrate a developmental 'rescue' effect by the ZPA, and further support the view that the ZPA has a central and unique function in normal limb bud development, controlling survival and differentiation of the mesenchyme along the anteroposterior axis.