Abstract
Urodele amphibians can regenerate their limbs throughout life by forming a blastema of undifferentiated cells derived from the individual tissues comprising the region of amputation. The blastema forms subjacent to an apical thickening of the epidermis closing the wound called the apical epidermal cap (AEC), a critical signaling center for proximodistal growth of the blastema. Blastema cells are derived by the liberation of cells from tissues by histolysis and dedifferentiation, as well as via stem cells from muscle, but there are species and developmental differences in how myogenic cells are supplied for muscle regeneration. During formation of the blastema, its cells are arrested in G2 of the cell cycle. Breaking G2 arrest requires the synergistic action of mitogens produced by the AEC and communication between anterior and posterior blastema cells mediated by sonic hedgehog and fibroblast growth factor 2 (Fgf2). Candidates for AEC mitogens are Fgf2 and the anterior gradient protein. Production of these mitogens is induced by nerve factors. Candidate nerve factors are glial growth factor 2 and combinations of bone morphogenetic protein/Fgf8. Blastema cells are lineage-restricted in their regeneration of new limb tissues.
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