Positional information in the forelimb of the axolotl: properties of the posterior skin.

Two series of experiments were carried out to investigate the properties of the positional information carried by posterior skin of the axolotl forelimb. The skin was assayed by grafting it to the anterior side of a normal limb and then amputating through the graft region. The formation of a double posterior regenerate indicates that the grafted skin carried the posterior coding. In the first series, double posterior limbs were created by grafting posterior half limb rudiments to the flank of tailbud-stage embryos. The animals were reared for several months and then a half cuff of anterior skin, judged in relation to the whole body axes, was assayed by grafting to one of the host limbs. The results show that both sides of the double posterior limb carry the posterior coding and confirm our expectation that the visible anatomy is a good guide to the underlying codings. In the second series animals were prepared by embryonic grafts so that they bore an extra normal limb on the flank. This extra limb was marked by pigmentation and in some cases by triploidy. When the limbs had developed, posterior skin from the extra limb was grafted to the anterior side of a host limb. The host limbs were amputated at intervals ranging from 2 weeks to 1 year after the skin graft. The results show that the posterior coding carried by the graft is stable even in an anterior environment. Studies of the cellular composition of regenerates which had received triploid grafts showed that the graft epidermis was progressively replaced by that of the host. The dermis on the other hand retained triploid cells throughout.