Control of maintenance and anteroposterior skeletal differentiation of the anterior mesenchyme of the chick wing bud by its posterior margin (the ZPA).

If the posterior half of the chick wing bud (stages 17-22) is excised, the anterior half which normally forms humerus (part), radius and digit 2, forms only a single skeletal element, either humerus or humerus fused with reduced radius. Beginning at 18 h after operation, and continuing to 48 h the anterior and distal mesenchyme in such anterior halves becomes necrotic and the AER regresses. By contrast, if the anterior half of the chick wing bud (stages 17-22) is excised, the posterior half develops as in the normal bud, and forms humerus (part), ulna and digits 3, 4 and 5. Such posterior halves develop no more mesenchymal necrosis than the normal contralateral wing buds and the AER remains healthy. Further, if the excision of the posterior part is made in such a way as to leave in place a part of the zone of polarising activity (ZPA), a normal wing with complete skeleton is formed. Thus in order to survive and differentiate, the anterior part of the wing bud needs a factor supplied by the posterior part containing the ZPA. These results support the view that the ZPA plays a role in controlling the anteroposterior differentiation of the normal wing bud.