Single-cell analysis uncovers convergence of cell identities during axolotl limb regeneration

Amputation of the axolotl forelimb results in the formation of a blastema, a transient tissue where progenitor cells accumulate prior to limb regeneration. Connective tissue (CT) – skeleton, periskeleton, tendon, dermis, interstitial cells – contributes the vast majority of cells that populate the blastema, however it is unclear how individual CT cells may reprogram their fate in order to rebuild the tetrapod limb. Here we use a combination of Cre-loxP reporter lineage tracking and single-cell (sc) RNA-seq to molecularly track, for the first time, adult CT cell heterogeneity and its transition to a limb blastema state. We uncover a multi-phasic molecular program where CT cell types found in the uninjured adult limb revert to a relatively homogenous progenitor state that participates in inflammation and extracellular matrix disassembly prior to proliferation, establishment of positional information, and ultimately re-differentiation. While the early regeneration transcriptome states are unique to the blastema, the later stages recapitulate embryonic limb development. Notably, we do not find evidence of a pre-existing blastema-like precursor nor limb bud-like progenitors in the uninjured adult tissue. However, we find that distinct CT subpopulations in the adult limb differentially contribute to proximal and distal portions of the regenerated limb. Together, our data illuminates molecular and cellular reprogramming during complex organ regeneration in a vertebrate. Single-cell transcriptomics and reporter lineage tracking was used to deconstruct cell composition, reconstruct lineage relationships, and trace connective tissue reprogramming during axolotl limb regeneration.