Yokoyama H et al. (2018), Skin regeneration of amphibians: A novel model ...

XB-ART-55063

Dev Growth Differ 2018 Aug 01;606:316-325. doi: 10.1111/dgd.12544.

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Skin regeneration of amphibians: A novel model for skin regeneration as adults.

Yokoyama H , Kudo N , Todate M , Shimada Y , Suzuki M , Tamura K .

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Adult mammals do not regenerate the dermis of the skin but form a scar after a deep skin injury. Since a scar causes serious medical problems, skin regeneration, instead of formation of a scar, has been strongly desired from a clinical point of view. Recent studies have suggested multiple origins of myofibroblasts, which are scar-forming cells in skin wound healing of mammals. While amphibians have skin structures that are basically common to mammals as tetrapods, both urodele and anuran amphibians regenerate almost complete skin structures including the dermis and secretion glands without forming a remarkable scar after a deep skin injury. In skin regeneration of a metamorphosed Xenopus laevis, an amphibian, cells that resemble limb blastema cells accumulate under the epidermis after injury and cells from subcutaneous tissues (tissues underlying the skin) contribute to skin regeneration. The skin of urodele amphibians and that of anuran amphibians provide valuable models for studying skin regeneration as adults. Recent progress in transgenesis and genome editing techniques with whole genome sequencing in Xenopus and an axolotl have enabled comparative analyses by molecular genetics of mammal skin and amphibian skin. Such comparative analyses would enable direct comparison of scar-forming myofibroblasts in mammals and blastema-like cells that contribute to skin regeneration in amphibians, ultimately leading to realization of skin regeneration in adult mammals. Amphibian skin regeneration will also be useful for determining how to step up skin regeneration to a higher level of regeneration such as limb regeneration in the future.

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Species referenced: Xenopus laevis
Genes referenced: acta2 prrx1 prrx2

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	FIGURE 1 Vertebrate skin structures. (a) Teleost fish skin has a pluristratified epidermis but does not have a cornified (keratinized) layer. (b) Amphibian skin has a cornified layer (stratum corneum) in the epidermis and secretion glands (mucous glands and granular glands) in the dermis. (c) Mammal skin has multiple layers of keratinocytes in the epidermis, from undifferentiated stem cells of the stratum basale to terminally differentiated cells of the stratum corneum. The stratum lucidum is present only in thick skin. Illustrations for fish skin and amphibian skin were modified from Schempp et al. (2009)
	FIGURE 2 Healing processes after excisional deep skin injury in adult mammals. (a) Hemostasis. A clot is formed and red blood cells (shown as red color in the illustration), and platelets are trapped in the clot. The clot dries at the surface to form a scab. (b) Monocytes (which transform into macrophages, light blue) enter the wound space, and re-epithelization begins. (c) Proliferation. The original wound is almost entirely covered by the newly formed epidermis, and an extensive capillary network develops. Fibroblasts (purple) migrate to the wound, proliferate, and contribute to the formation of granulation tissue. Myofibroblasts (scar-forming cells, yellow), derived from multiple origins, differentiate in granulation tissue. (d) Maturation. The temporal collagen matrix degrades and maturates (is cross-linked) into thick bundles of collagen to form scar tissue