Divergent stem cell mechanisms govern the primary body axis and appendage regeneration in the axolotl

Exploring the fundamental mechanisms of organ regeneration is crucial for advancing regenerative medicine. The axolotl tail represents an opportunity to study regeneration of the primary axis including segmented muscle, vertebrae, and skin. During tail development, muscle stem cells (MuSCs) displayed expected specificity to the muscle lineage. Tail amputation, however, induced expansion of MuSC potential yielding clonal contribution to muscle, connective tissue including cartilage, pericytes, and fibroblasts. This expanded potential was not observed during limb regeneration, and cross-transplantation showed that these differences in potential are likely intrinsically determined. Single-cell RNA sequencing profiling revealed that tail MuSCs revert to an embryonic mesoderm–like state. Through genetic manipulation involving the overexpression of constitutively active transforming growth factor–β (TGF-β) receptors or Smad7 (antagonist of TGF-β signaling) in MuSCs, we demonstrated that the levels of TGF-β signal determine the fate outcome of MuSCs to connective tissue lineage or muscle, respectively. Our findings illustrate that variation in MuSCs may represent a fundamental difference between regeneration of primary axis versus appendage.