SUMMARYEvolutionary modification has produced a spectrum of animal defense traits to escape predation, including the ability to autotomize body parts to elude capture1,2. Following autotomy, the missing part is either replaced through regeneration (e.g. urodeles, lizards, arthropods, crustaceans) or is permanently lost (mammals). While most autotomy involves the loss of appendages (e.g. leg, cheliped, antennae, tail), skin autotomy can occur in certain taxa of scincid and gekkonid lizards3. Here we report the first demonstration of skin autotomy in Mammalia (African spiny mice, Acomys). Mechanical testing revealed a propensity for skin to tear under very low tension and the absence of a fracture plane. Following skin loss, rapid wound contraction was followed by hair follicle regeneration in dorsal skin wounds. Surprisingly, we found regenerative capacity in Acomys extended to ear holes where they exhibited complete regeneration of hair follicles, sebaceous glands, dermis, and cartilage. Salamanders capable of limb regeneration form a blastema (a mass of lineage-restricted progenitor cells4) following limb loss, and our findings suggest that ear tissue regeneration in Acomys may proceed through assembly of a similar structure. This study underscores the importance of investigating regenerative phenomena outside of traditional model organisms and suggests that mammals may retain a higher capacity for regeneration than previously believed. As re-emergent interest in regenerative medicine seeks to isolate molecular pathways controlling tissue regeneration in mammals, Acomys may prove useful in identifying mechanisms to promote regeneration in lieu of fibrosis and scarring.

While considerable progress has been made towards understanding the complex processes and pathways that regulate human wound healing, regenerative medicine has been unable to develop therapies that coax the natural wound environment to heal scar-free. The inability to induce perfect skin regeneration stems partly from our limited understanding of how scar-free healing occurs in a natural setting. Here we have investigated the wound repair process in adult axolotls and demonstrate that they are capable of perfectly repairing full thickness excisional wounds made on the flank. In the context of mammalian wound repair, our findings reveal a substantial reduction in hemostasis, reduced neutrophil infiltration and a relatively long delay in production of new extracellular matrix (ECM) during scar-free healing. Additionally, we test the hypothesis that metamorphosis leads to scarring and instead show that terrestrial axolotls also heal scar-free, albeit at a slower rate. Analysis of newly forming dermal ECM suggests that low levels of fibronectin and high levels of tenascin-C promote regeneration in lieu of scarring. Lastly, a genetic analysis during wound healing comparing epidermis between aquatic and terrestrial axolotls suggests that matrix metalloproteinases may regulate the fibrotic response. Our findings outline a blueprint to understand the cellular and molecular mechanisms coordinating scar-free healing that will be useful towards elucidating new regenerative therapies targeting fibrosis and wound repair.

Why mammals have poor regenerative ability has remained a long-standing question in biology. In regenerating vertebrates, injury can induce a process known as epimorphic regeneration to replace damaged structures. Using a 4-mm ear punch assay across multiple mammalian species, here we show that several Acomys spp. (spiny mice) and Oryctolagus cuniculus completely regenerate tissue, whereas other rodents including MRL/MpJ ‘healer' mice heal similar injuries by scarring. We demonstrate ear-hole closure is independent of ear size, and closure rate can be modelled with a cubic function. Cellular and genetic analyses reveal that injury induces blastema formation in Acomys cahirinus. Despite cell cycle re-entry in Mus musculus and A. cahirinus, efficient cell cycle progression and proliferation only occurs in spiny mice. Together, our data unite blastema-mediated regeneration in spiny mice with regeneration in other vertebrates such as salamanders, newts and zebrafish, where all healthy adults regenerate in response to injury.