Regulation of proximal-distal intercalation during limb regeneration in the axolotl (Ambystoma mexicanum).

Intercalation is the process whereby cells located at the boundary of a wound interact to stimulate proliferation and the restoration of the structures between the boundaries that were lost during wounding. Thus, intercalation is widely considered to be the mechanism of regeneration. When a salamander limb is amputated, the entire cascade of regeneration events is activated, and the missing limb segments and their boundaries (joints) as well as the structures within each segment are regenerated. Therefore, in an amputated limb it is not possible to distinguish between intersegmental regeneration (formation of new segments/joints) and intrasegmental regeneration (formation of structures within a given segment), and it is not possible to study the differential regulation of these two processes. We have used two models for regeneration that allow us to study these two processes independently, and report that inter- and intrasegmental regeneration are different processes regulated by different signaling pathways. New limb segments/joints can be regenerated from cells that dedifferentiate to form blastema cells in response to signaling that is mediated in part by fibroblast growth factor.

Abnormal limb regeneration in the short-toes mutant of the axolotl, Ambystoma mexicanum: studies of age, level of amputation, and extracellular matrix.

Limb regeneration in the short-toes axolotl is impaired. Our goal was to characterize the regeneration process in this mutant by histological and immunocytochemical methods. Previous research indicates that age and a defective basement membrane may be instrumental factors in short-toes axolotl regeneration (Del Rio-Tsonis et al. [1992] Proc. Natl. Acad. Sci. U.S.A., 89:5502-5506). The present results show that limb regeneration can occur even in older (1-2-year-old) short-toes axolotls. The process was always significantly delayed, but the time required for complete regeneration varied. Even so, the basement membrane of short-toes regenerates showed no differences in thickness or shape compared with wild-type regenerates. Distally amputated short-toes limbs gave rise to more digits in the regenerate, indicating that regeneration may be somewhat dependent on the level of amputation. Since extracellular matrix (ECM) remodeling occurs extensively during regeneration, we compared the ECM of the short-toes and wild-type regenerates using monoclonal antibodies (mAbs) MT2 and ST1 (Tassava et al. [1996] Wound Rep. Reg., 4:75-81). The short-toes regenerates showed decreased reactivity to mAb MT2, which identifies type XII collagen, an ECM protein that is normally unregulated during regeneration, and increased reactivity to mAb ST1, which identifies a limb ECM component that typically undergoes breakdown in the distal stump. Thus, impaired regeneration in the short-toes axolotl is correlated with impaired ECM remodeling in the distal limb stump. This supports the view that ECM remodeling plays an important role in regeneration.