Accessory Limb Model

Written by David M Gardiner

The steps leading to dedifferentiation and blastema formation are most directly demonstrated by the induction of ectopic (extra) limbs in the axolotl. By refining a series of classical experimental manipulations, we have established a novel assay for identifying the signals that induce regeneration. If a simple skin wound is created on the side of an axolotl arm, the epithelium heals within a few hours, and over the next several days the skin is regenerated without forming a scar. If however, a nerve is deviated to the site of the wound (Fig.1A), fibroblasts adjacent to the wound are induced to dedifferentiate, migrate to the wound site and proliferate to form an ectopic blastema (Fig2A).

Finally, when a piece of skin is grafted contralaterally (Fig. 1B) to the site of a nerve-deviated wound, the induced blastemas continue to grow and develop to give rise to an ectopic limb (Fig. 2B).
The induction of ectopic limbs thus illustrates that there are three critical signals that are required for limb formation. There are signals associated with wounding that induce a pathway of wound healing that does not involve scar formation. In the absence of any further signals, the skin is regenerated.There is a second signal, from nerves, that induces the morphogenetic events leading to blastema formation. Finally, interactions between blastema cells derived from progenitor fibroblasts from opposite sides of the limb drive continued outgrowth and pattern formation leading to the formation of a new limb. (Fig. 3)