Limb blastema formation: How much do we know at a genetic and epigenetic level?

Abstract

Regeneration of missing body parts is an incredible ability which is present in a wide number of species. However, this regenerative capability varies among different organisms. Urodeles (salamanders) are able to completely regenerate limbs after amputation through the essential process of blastema formation. The blastema is a collection of relatively undifferentiated progenitor cells that proliferate and repattern to form the internal tissues of a regenerated limb. Understanding blastema formation in salamanders may enable comparative studies with other animals, including mammals, with more limited regenerative abilities and may inspire future therapeutic approaches in humans. This review focuses on the current state of knowledge about how limb blastemas form in salamanders, highlighting both the possible roles of epigenetic controls in this process as well as limitations to scientific understanding that present opportunities for research.

Keywords: cell signaling; epigenetics; morphogenesis; regeneration; stem cells.

Figure 1

Key stages of salamander limb regeneration. Upon amputation, the epidermal cells migrate over the amputated site forming wound epidermis. Within days, wound epidermis becomes innervated which then becomes the apical epidermal cap (AEC). Blastema-forming cells are attracted to the AEC resulting a blastema formation which will continue to undergo proliferation and differentiation until a newly regenerated limb is formed. Figure adapted from McCusker et al. 2015 (147).

Figure 2

Blastema formation with the migration of blastema-forming cells. Newly activated blastema-forming cells migrate towards the stump forming the blastema.

Figure 3

Proposed epigenetic regulation of blastema formation after wound healing. (Phase I, top) Innervation downregulates DNMT3a in the wound epidermis which then leads to initiation of FGF8 transcription by transcription factor SP9. FGF8 is then released to extracellular matrix. (Phase II, bottom) FGF8 present in the extracellular matrix initiates cell signaling through autocrine or paracrine signaling to induce HDAC1 contributing to blastema formation. DNMT3a, DNA methyltransferase 3a; FGF, fibroblast growth factor; HDAC, histone deacetylase.

Figure 4

Contribution map of various cells during limb regeneration.A, Schwann cells and epidermal cells both contribute to blastema formation. However, Schwann cells and epidermal cells are lineage restricted contributing to itself. B, dermal cells contribute to blastema and then behaves as bipotent progenitor cell giving rise to dermal and cartilage. C, periosteum-derived cells are multipotent which contributes to blastema then regenerates dermal, fibroblast-like, and skeletal cells (cartilage, bone, periosteum).

Figure 5

Positional memory is maintained in the cell. Cartoon showing that positional memory is maintained in each cell. (Top) Transplanting GFP⁺ distal blastema to proximal amputated region shows that GFP⁺ blastema cell contribution is limited to host hand. (Bottom) GFP⁺ proximal blastema transplanted to proximal amputated region shows that GFP⁺ blastema cells contribute to the entire host limb.