Pattern regulation between hind- and forelimbs after blastema exchanges and skin grafts in Notophthalmus viridescens

Experiments were performed to ascertain whether cells from the forelimb and the hindlimb of newts share a common pattern-regulating mechanism during regeneration. In one series of experiments early digit hindlimb and forelimb blastemas were exchanged either ipsilaterally or contralaterally. In a second experimental series, a strip of forelimb skin was added to a slit in a homologous site on the hindlimb or in a site 180 degrees removed from the position of origin, and vice versa (hindlimb skin into forelimb). After contralateral blastema exchanges, supernumerary limbs developed in addition to the limb of graft origin, whereas after ipsilateral exchanges, a single limb of graft origin regenerated. Limbs in which skin grafts were inserted into homologous sites developed normal regenerates of the host type, while in contrast, limbs with supernumerary digits regenerated from limbs which had received skin grafts in positions 180 degrees opposite to the homologous position. The results of both experiments were dependent on the positional disparity between the graft and host tissues, and they suggest that the forelimb and hindlimb do indeed utilize a common mechanism for pattern regulation.