Zone of Polarizing Activity (ZPA)
The A-P axis is the 1st axis to be fixed in the presumptive limb mesoderm at a very early stage. On the basis of results of a series of current experiments on chick embryos it has been recommended that determination of the anterior-posterior pattern of wing parts is apparently within the control of a small block of mesodermal cells located at the posterior junction of the wing bud along with the body wall. This mesoderm has been termed as the Zone of Polarizing Activity (ZPA). The usual chick wing has three digits numbered II, III and IV in this anterio- posterior order. In stage 18 or little older embryos an extra ZPA was transplanted within the ectoderm on the anterior side of the bud near or in contact with the AER. This resulted in the creation of a duplicate set of digits in reverse order on the anterior side so the digital formula anterio-posteriorly turns IV, II, II- II, III, IV.
The second set was a mirror image of the original. The normal polarity was kept even in this set as the usually the most posterior digit IV was still nearest to the transplanted ZPA. It was recommended that ZPA secretes some type of substance, a morphogen, whose diffusion creates a gradient of decreasing concentrations along with the A-P axis. The digit IV may be formed in response to the highest concentration and digits III and II in response to in succession decreasing levels of this morphogen. A second such gradient would be established in reverse direction by the morphogen produced by the ZPA transplanted on the anterior side and hence the second set of digits in mirror image of the original set the above suggested role of ZPA is not generally accepted by most biologists. Identical additional set of digits in chick wing is induced by transplantation of mesoderm from another body regions, several different tissues, transplantation of an additional AER and as well by applying retinoic acid (a derivative of Vitamin A) to the anterior side of the bud. Furthermore, removal of ZPA from the posterior side of the bud does not prevent the creation of the normal set of digits while in an experimental situation ZPA does produce a polarizing effect, this role of ZPA in the normal development whereas it is in its usual posterior position in the wing bud is seriously doubled.