Cell proliferation in the amputated limb of lizard leading to scarring is reduced compared to the regenerating tail

Effect of larval thermal conditions on limb regeneration in a ladybird beetle, Cheilomenes sexmaculata

In view of global environmental change, ecological factors especially temperature, affect development of the poikilotherms like insects. Since ladybirds are at risk of injury under mass-rearing conditions, their ability to regenerate injured limbs is highly crucial for their survival. Therefore, the effect of limb regeneration in relation to temperature forms the basis of the present study. The immature stages of insects, being more vulnerable to the surrounding temperature, were considered to study the effect of the prior thermal experience of larvae on regeneration. We exposed the early larval stages of the ladybird beetle, Cheilomenes sexmaculata, to different temperature conditions pre- and postamputation. Exposure of immature stages to extreme temperatures did not affect the ability to regenerate and regeneration occurred at given temperature conditions. However, the regenerated legs were smaller in size across given temperatures as compared to unamputated legs. Body weights in amputated treatments showed no difference and remained unchanged across temperatures when compared to unamputated treatments. Postamputation developmental duration, equivalent to recovery time postlimb amputation, was found to be affected by larval thermal conditions. Recovery was faster in larval treatments exposed to higher temperatures. Thus, larval thermal conditions though did not affect the ability to regenerate lost limbs directly, it does modulate the time taken to regenerate.

Microscopic observations on amputated and scarring lizard digits show an intense inflammatory reaction

The microscopic details of the failure of digit regeneration in lizards are not known. The present study reports some histological, ultrastructural and 5BrdU-immunohistochemical observations on healing digits after amputation in the lizard Podarcis muralis. At 7-12 days post-amputation, the stump of digits forms a multilayered wound epidermis covering a loose connective tissue that is invaded by granulocytes, macrophages and lymphocytes. In addition to macrophages also electron-pale multinuclear giant cells are seen underneath or penetrating the wound epidermis while osteoclasts are present in the degrading bone of the severed phalanges. Granulocytes and macrophages invading the wound epidermis indicate the formation of an intra-epidermal immune barrier beneath the scab where numerous bacteria remain entrapped. Immunofluorescence for 5BrdU reveals that few proliferating cells are present in the wound epidermis and the underlying connective tissue at 12 and 32 days post-amputation. Outgrowths of less than 1mm stop growing and at 32 days they appear scaling. Most of connective cells give rise to fibrocytes and large irregular collagen bundles, as is typical for scar tissue. In conclusion, like for the amputated limb, the intense inflammatory reaction and scarring here described after digit loss appears associated with immune cells invasion.

Immunolocalization of Wnts in the lizard blastema supports a key role of these signaling proteins for tail regeneration

A highly upregulated gene during tail regeneration in lizards is Wnt2b, a gene broadly expressed during development. The present study examines the distribution of Wnt proteins, most likely wnt2b, by western blotting and immunofluorescence in the blastema-cone of lizards using a specific antibody produced against a lizard Wnt2b protein. Immunopositive bands at 48-50 and 18 kDa are present in the regenerative blastema, the latter likely as a degradation product. Immunofluorescence is mainly observed in the wound epidermis, including in the Apical Epidermal Peg where the protein appears localized in intermediate and differentiating keratinocytes. Labeling is more intense along the perimeter of keratinocytes, possibly as a secretory product, and indicates that the high epidermal proliferation of the regenerating epidermis is sustained by Wnt proteins. The regenerating spinal cord forms an ependymal tube within the blastema and shows immunolabeling especially in the cytoplasm of ependymal cells contacting the central canal where some secretion might occur. Also, regenerating nerves and proximal spinal ganglia innervating the regenerating blastema contain this signaling protein. In contrast, the blastema mesenchyme, muscles and cartilage show weak immunolabeling that tends to disappear in tissues located in more proximal regions, close to the original tail. However, a distal to proximal gradient of Wnt proteins was not detected. The present study supports the hypothesis that Wnt proteins, in particular Wnt2b, are secreted by the apical epidermis covering the blastema and released into the mesenchyme where they stimulate cell multiplication.

Downregulation of lizard immuno-genes in the regenerating tail and myogenes in the scarring limb suggests that tail regeneration occurs in an immuno-privileged organ

Amputated tails of lizards regenerate while limbs form scars which histological structure is very different from the original organs. Lizards provide useful information for regenerative medicine and some hypotheses on the loss of regeneration in terrestrial vertebrates. Analysis of tail and limb transcriptomes shows strong downregulation in the tail blastema for immunoglobulins and surface B and T receptors, cell function, and metabolism. In contrast, in the limb blastema genes for myogenesis, muscle and cell function, and extracellular matrix deposition but not immunity are variably downregulated. The upregulated genes show that the regenerating tail is an embryonic organ driven by the Wnt pathway and non-coding RNAs. The strong inflammation following amputation, the non-activation of the Wnt pathway, and the upregulation of inflammatory genes with no downregulation of immune genes indicate that the amputated limb does not activate an embryonic program. Intense inflammation in limbs influences in particular the activity of genes coding for muscle proteins, cell functions, and stimulates the deposition of dense extracellular matrix proteins resulting in scarring limb outgrowths devoid of muscles. The present study complements that on upregulated genes, and indicates that the regenerating tail requires immune suppression to maintain this embryonic organ connected to the rest of the tail without be rejected or turned into a scar. It is hypothesized that the evolution of the adaptive immune system determined scarring instead of organ regeneration in terrestrial vertebrates and that lizards evolved the process of tail regeneration through a mechanism of immuno-evasion.