Skin wound healing in axolotls: a scarless process

Abstract

Urodele amphibians, such as the axolotl (Ambystoma mexicanum), have the unique faculty among vertebrates to regenerate lost appendages (limbs and tail) and other body parts (apex of the heart, forebrain and jaw) after amputation. Interestingly, axolotls never seem to form scar tissue at the site of amputation once regeneration is completed. Before now, very few studies were directly focused on the description of the events happening during wound healing after a skin injury in salamanders. In this paper, we directly investigated skin wound healing after excisional wounding which removed the epidermis, dermis and basement membrane in the axolotl. Axolotls were wounded with a 1.5-mm skin biopsy punch. Results show rapid re-epithelialization of the wound within 8 hrs after wounding. Histological analysis of wound healing confirmed the absence of tissue fibrosis throughout the process and shows that skin integrity is re-established by 90 days after wounding. Results also reveal the absence of neutrophils in the wound area, suggestive of a lack of or low inflammatory response. The expression of proteins central to wound healing seemed different than in mammals as α-smooth muscle actin was absent and transforming growth factor β-1 was only transiently expressed during wound healing in the axolotl. Finally, subcutaneous injections of bleomycin were performed to verify whether the induction of scar tissue was possible in axolotls. Surprisingly, results show that axolotls are not resistant to bleomycin-induced tissue fibrosis, but the resulting scar tissue does not seem to contain significant amounts of collagen.

Macrophages are required for adult salamander limb regeneration

“…More recent studies in the axolotl reported the recruitment of neutrophils in deep tissue wounds but not in regenerating limbs (8,31). By contrast, using a range of methods, the present study confirms the active involvement of macrophages within the regenerating axolotl limb.…”

“…S9). α-Smooth muscle actin (α-SMA) is an established marker for differentiated myofibroblasts displaying subepithelial localization (31)(32)(33) in several species. Macrophage-depleted animals showed increased numbers of α-SMA-positive cells compared with the normally low levels observed in control animals at 20 dpa (Fig.…”

“…Salamanders are also able to perform scar-free repair of deep tissue wounds after injury (8). Although the cellular mediators and immunological signaling necessary for regeneration in the salamander have not been described, recent reports suggest that inflammation may influence the initiation and completion of wound healing and regeneration (9)(10)(11)(12)(13)(14), as the cytokine microenvironment directly influences the time course of leukocyte infiltration, cellular proliferation, angiogenesis, and collagen remodeling of damaged tissues.…”

Activation of Smad2 but not Smad3 is required to mediate TGF-β signaling during axolotl limb regeneration

“…However, Chablais and Jazwinska (2012) did not look at p-Smad2 and they did not test Smad3-specific inhibitors, such as SIS3, to confirm that inhibition of heart regeneration was due solely to the loss of p-Smad3. In axolotl limb regeneration, scar formation is never observed (Levesque et al, 2007(Levesque et al, , 2010Denis et al, 2013). We observe low levels of p-Smad3 very early post-amputation (Figs 2 and 5).…”

Neuregulin-1 signaling is essential for nerve-dependent axolotl limb regeneration

“…3K,L). Axolotl tissue regeneration is a typically scar-free process that occurs with minimal collagen deposition (Levesque et al, 2010;Seifert et al, 2012), but extensive and aberrant collagen deposition was observed in the mesenchyme of mubritinib-treated limbs. The phenotype observed here after long-term ErbB2 inhibition indicates a disruption of these scar-preventing programs and resembles the phenotype observed in amputated limbs after total macrophage ablation (Godwin et al, 2013).…”