Investigating the Sources of Mesenchymal Cells that Contribute to Adult Murine Digit Tip Regeneration

While regeneration of appendages is relatively common among invertebrate, it is relatively restricted in mammals. Adult murine distal digit tip regeneration is one example of mammalian multi-tissue regeneration. Blastema formation is the critical step distinguishing regenerative from non-regenerative amputations. While the sources of the blastema cells in other regenerative vertebrates such as amphibians and zebrafish have been widely studied, the cellular origin and phenotypic identity of mammalian blastema cells are not well understood. In this thesis, using adult mouse digit tip regeneration as a model, I investigate the origin and nature of the mammalian blastema cells. First, I show that distinct Pdgfra-expressing mesenchymal cells in the uninjured distal digit tip contribute to the majority of the mouse blastema. Single cell profiling indicates that the Pdgfra-expressing blastema mesenchymal cells acquire a precursor-like transcriptional state distinct from both the uninjured and developing digit tip mesenchymal cells. Thus, digit tip regeneration occurs via a distinct adult mechanism. Second, I show that various differentiated Pdgfra-expressing mesenchymal cells in the uninjured digit tip, such as the bone lineage cells and nail bed dermal cells, upon contribution to the blastema, lose their differentiated phenotypes, re-enter the cell cycle and acquire the blastema transcriptional signature, suggesting dedifferentiation of cells during regeneration. Third, transplantation studies indicate that the regenerating environment dictates the blastema transcriptional state and confers mesenchymal lineage plasticity to some blastema mesenchymal cells. This allows these select cells to contribute to multiple mesenchymal tissues irrespective of their own tissue-of-origin. Fourth, I show that certain cells such as the nail bed dermal cells show spatial restriction in the regenerating blastema and regenerated digit tip suggesting that some cells might harbour positional memory during mammalian tissue regeneration. Finally, we show that the non-regenerating tissue acquires some aspects of the regenerative blastema transcriptional state. These findings lay the groundwork for further research investigating the differences between regenerative and non-regenerative responses with the hopes of enhancing mammalian tissue repair and regeneration.Ph.D