Intercalary and supernumerary regeneration in regenerating the mature limbs of Notophthalmus viridescens.

Abstract

AbstractWe describe the ability of advanced limb regenerates and mature limbs of Notophthalmus viridescens to undergo intercalary and supernumerary regeneration following grafting operations to confront normally nonadjacent cells. Although both of these types of regeneration are known to be exhibited by regenerating limbs up to the stage of early digits, this is the first investigation of these abilities in mature limbs of Notophthalmus viridescens using similar grafting procedures. The autopodia of mature limbs, or regenerates at the stage of late digits or older from a distal level of amputation (distal third of forearm), were transplanted to other limb sites so as to oppose either anterior and posterior, or dorsal and ventral positions of stump and transplant. Following transplantation of either the regenerate or the mature autopodium, supernumerary limbs were sometimes formed, and their handedness, position and orientation were, in those cases in which they could be accurately determined, similar to what has been found previously in experiments on younger regenerates. Unlike the results of the experiments on regenerating limbs, when mature autopodia were grafted ipsilaterally with 180° rotation, the handedness of the supernumerary limbs could seldom be determined, and their location appeared to be random. An enhancement of the incidence of supernumerary limb formation in mature limb grafts was noted when the grafts were made so as to oppose normally nonadjacent proximal‐distal as well as circumferential positions.When advanced regenerates were grafted ipsilaterally and without rotation to a more proximal limb level (mid upper arm), good intercalary regeneration of the distal portion of the humerus as well as the radius and/or ulna occurred in most, but not all cases. When similarly grafted mature autopodia were examined, it was found that none of the limbs formed a complete intercalary regenerate, but the majority formed partial intercalary regenerates by completing the humerus. When mature autopodia were grafted so as to oppose normally non‐adjacent circumferential as well as proximal‐distal positions, then the intercalary regenerate was more complete and frequently included a regenerated radius and/or ulna, normally in conjunction with one or more supernumerary limbs. The similarities between these findings and our earlier ones on intercalary and supernumerary regeneration in regenerating newt limbs are discussed in terms of a recently developed model for pattern regulation in epimorphic fields.