Single-cell RNA sequencing of axolotl limb

Regeneration of a complex appendage structure such as limb requires hierarchical coordination of multiple types of cells. Given type of cell may sit at higher hierarchical position to control the activities of other cells. Muscles are one of the major cell masses in limbs. However, the subtle functional relationship between muscle and other cells in vertebrate complex tissue regeneration are still not well established. Here, we use Pax7 mutant axolotls, in which the limb muscle is developmentally lost, to investigate limb regeneration in the absence of skeletal muscle. We find that the pattern of regenerated limbs is normal in Pax7 mutants compared to the controls. Lack of muscles do not affect the early regeneration responses, specifically the recruitment of macrophages to the wound, as well as the proliferation of fibroblasts, another major population in limbs. Furthermore, using single cell RNA-sequencing, we show that, other than muscle lineage that is mostly missing in Pax7 mutants, the composition and the status of other cell types in completely regenerated limbs of Pax7 mutants are similar to that in the controls. Our study reveals skeletal muscle is not required for the guidance of other cells, as well the patterning in complex tissue regeneration in axolotls, and provides refined views of the muscle cell hierarchy in vertebrate appendage regeneration.